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F0|Gr1 TEXAS TECH UNIVERSITY

Natural Science Research Laboratory

Special Publications

{ > j* - Museum of Texas Tech University

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Front cover: Background photograph is the Merriami plot at Panther Junction, looking southwest towards the Chisos
Mountains (May 2011). Lower center inset: Perognathus merriami. Left insets: Chaetodipus eremicus and the Eremicus
plot (May 2008) looking southwest. Right insets: Chaetodipus nelsoni and the Nelsoni plot (May 2008) looking northwest
with Nugent Mountain on the left. The arrangement of the photographs represents the ecological distribution of the three
species of pocket mice in Big Bend. Chaetodipus eremicus commonly occupies flat, open, rock-free sandy or loamy areas,
while C. nelsoni is found most commonly in steep, rocky, and often densely vegetated habitats. Perognathus merriami
can occupy a wide range of habitats, but in the presence of the larger Chaetodipus species is most abundant in habitats
intermediate to those occupied by C. nelsoni and C. eremicus. Pocket mouse photographs were taken during the study by
R. D. Porter. Habitat photographs and cover design by C. A. Porter.

SPECIAL PUBLICATIONS

Museum of Texas Tech University

Number 58

Movements, Populations, and Habitat Preferences
of Three Species of Pocket Mice (Perognathinae)
in the Big Bend Region of Texas

Richard D. Porter

Calvin A. Porter, Editor

Layout and Design: Lisa Bradley

Cover Design: Calvin A. Porter

Production Editor: Lisa Bradley

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seum of Texas Tech University (nsrl.ttu.edu). The authors and the Museum of Texas Tech University hereby grant permission
to interested parties to download or print this publication for personal or educational (not for profit) use. Re-publication of any
part of this paper in other works is not permitted without prior written permission of the Museum of Texas Tech University.

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Printed: July 2011

Library of Congress Cataloging-in-Publication Data

Special Publicat ions of the Museum of Texas Tech University, Number 58
Series Editor: Robert J. Baker

Movements, Populations, and Habitat Preferences of Three Species of Pocket Mice (Perognathinae) in the Big Bend Region
of Texas

Richard D. Porter
Edited by Calvin A. Porter

ISSN 0169-0237
ISBN 1-929330-22-7
ISBN 13 978-1-929330-22-5

Museum of Texas Tech University
Lubbock, TX 79409-3191 USA
(806)742-2442

Table of Contents

Foreword (by the Editor) 3

Abstract 8

Introduction 9

Topography and Vegetation of the Study Area 9

Climate of the Study Area 11

Systematic Relationships of Pocket Mice 11

Materials and Methods 14

Preliminary F ield Work 16

The Habitat Plots 16

The Population Plots 17

Habitat Analysis Methods 22

Laboratory Examinations 26

Characteristics of the Population Plots 27

Ectoparasites 33

Altitudinal Distribution of Pocket Mice 35

Pocket Mice in Relation to Habitat 36

Distribution and Abundance of Pocket Mice in Relation to Slope 36

Distribution and Abundance of Pocket Mice in Relation to Soil Texture 44

Distribution and Abundance of Pocket Mice in Relation to Rock Content of the Substrate 46

Distribution and Abundance of Pocket Mice in Relation to Vegetation Density 47

Distribution and Abundance of Pocket Mice in Relation to Plant Species 49

Discussion and Comparisons of Habitat Preferences of Pocket Mice 50

Special Publications, Museum of Texas Tech University

Pocket Mouse Habitats in the Big Bend Region 52

Sex Ratios 62

Seasonal Activity Patterns 66

Seasonal Abundance 68

Breeding Habits 72

Breeding Season of Merriam’s Pocket Mouse 72

Breeding Season of Nelson’s Pocket Mouse 74

Breeding Season of the Chihuahuan Pocket Mouse 75

Discussion of the Extent of Breeding Season in Pocket Mice 76

Analysis of Numbers and Uterine Distribution of Embryos and Placental Scars 76

Incidence of Breeding in Juveniles 77

Longevity 77

Pelage Changes 79

Spatial Organization 83

Home Range 83

Shift in Center of Activity 87

Dispersal 88

Territoriality 88

Acknowledgments 93

Afterword (by the Editor) 94

Literature Cited 96

Appendix I 102

Appendix II 104

Appendix III

105

Porter—Ecology of Pocket Mice in the Big Bend Region

Foreword

Background of the Study

My father, Richard D. Porter, completed his B.S.
at the University of Utah in 1950 and his M.S. in 1952.
His thesis advisor was Dr. William H. Behle, and his
thesis was entitled The Hungarian Partridge in Utah,
with Special Reference to Ecology and Life History
(Porter 1951, 1955). He worked for two years as an
ecologist at Dugway Proving Grounds before enrolling
in a Ph.D. program at the Agricultural and Mechani¬
cal College of Texas (now Texas A&M University)
in 1955. His doctoral program was directed by Dr.
William B. Davis.

Upon completing his course work in June 1957,
my father moved with his family to Big Bend National
Park (BBNP) for more than two years, during which
time he conducted field work on the ecology of pocket
mice (Plate 1). He compiled more than 18,000 trap
nights and carefully recorded data about vegetation,
soil, habitats, movements, reproductive condition,
parasites, pelage, and other life history characteristics.
This ambitious effort was just a portion of the study
originally envisioned. The original research prospectus

also included proposed studies of systematics, diet,
and burrow architecture. “As the study progressed,”
he wrote, “the scope of the investigation was reduced
commensurate with the time available.” The result of
this “reduced” two-year field study was a 255-page
dissertation (Porter 1962) which is published here (in
edited form) for the first time. The study was funded by
the Texas Game and Fish Commission under contract
with the Texas Agricultural Experiment Station. In
addition to pocket mice and other mammals, my father
collected birds, reptiles, amphibians, and parasitic
arthropods (Wiseman 1959; Eads 1960; Degenhardt
1966; Wauer 1969, 1973a, c; Fleet and Dixon 1971),
and conducted population surveys of deer, all as part
of the larger project that funded the research. Speci¬
mens collected during the study are now deposited in
collections at BBNP, Texas A&M University, Brigham
Young University, the Smithsonian Institution, and
elsewhere. During the study, he collected two previ¬
ously undescribed species of fleas (Eads 1960), three
species of undescribed mites (discussed in this study),
and four undescribed species of avian chewing lice
(Wiseman 1959).

Plate 1. The author, Richard D. Porter, in Big Bend National Park, ca. January
1959. Photographer unknown.

Special Publications, Museum of Texas Tech University

An accomplished photographer, my father took
hundreds of photographs of the habitat, flora, and fauna
of BBNP. Many of his photographs were included in
the second edition of The Mammals of Texas (Davis
1960) and some still appear in the current sixth edition
(Schmidly 2004). Other Big Bend photographs were
published in Naturalist s Big Bend (Wauer 1973b, 1980;
Wauer and Fleming 2002) and Birds of Utah (Hayward
et al. 1976). Several were displayed for many years at
Park Headquarters.

While conducting field work, he resided with his
family in a trailer at Panther Junction. My mother, Lois
Gunderson Porter, was hired as the Park’s schoolteacher
(Tucker 2008), and taught grades 1-8 in a single class¬
room in the small San Vicente School at Panther Junc¬
tion. For much of her tenure, she was the only teacher
in the Park. My father was employed for $25 per
month as the school custodian. My mother frequently
assisted with his research, particularly with trapping,
substrate analysis, and recording data in the field. My
brothers Sanford (now a USD A research entomologist
in Gainesville, Florida) and Neil (who died at age 6 in
a tragic accident in August 1961) ranged in age from 2
to 5 during the study, and often accompanied my father
into the field while my mother was in the classroom. I
was bom in August 1959 as the field study concluded.
Because the Park had no physician on staff, and my
family was preparing to relocate to El Paso as my birth
approached, my mother and brothers traveled to Salt
Lake City where they stayed temporarily with family
until 1 was born. My father stayed at the K-Bar Ranch
in the Park for a few more weeks. He trapped the final
habitat plot (plot 101) during that time, and museum
records indicate he collected bats in Big Bend up to five
days before my birth. That he would still be collecting
mammals with his expectant wife 2,000 km away attests
to his dedication to research, and perhaps was a portent
of my becoming a mammalogist after him.

A few weeks later, as soon as I was old enough to
travel, my family settled in El Paso, Texas, where my
father taught at Texas Western College (now the Uni¬
versity of Texas at El Paso) for the 1959-1960 academic
year. We moved to Salt Lake City in June 1960, where
my father spent about a year working on analysis and
writing. In the fall of 1961, he took a teaching position
at what was then known as Wisconsin State College—

Whitewater. During his first year at Whitewater, he put
the finishing touches on the dissertation, and returned to
College Station in July 1962 to defend the dissertation.
The degree was awarded in August 1962

After leaving Whitewater in 1965, my father
taught biology/ at the New Mexico Institute of Mining
and Technology. In 1967, he began work for the U. S.
Fish and Wildlife Service, first at the Patuxent Wild¬
life Research Center near Laurel, Maryland, and then
(beginning in 1973) for the Denver Wildlife Research
Center, stationed in Provo, Utah. At Patuxent, he
performed research (e.g.. Porter and Wiemeyer 1969,
1972; Wiemeyer and Porter 1970) on the effects of
DDT and other pesticides on American Kestrels ( Falco
sparverius). His work at Provo involved ecological
surveys of raptors, particularly the Peregrine Falcon
(Falco peregnmis) in the western United States and
Baja California (e.g.. Porter and White 1973, 1977;
Porter et al. 1988). He retired from the Fish and Wild¬
life Service in 1980, and spent his retirement in Maple-
ton and Brigham City, Utah, and finally Gainesville,
Florida. He died in Gainesville on 2 October 2007 at
the age of 84.

Impact of the Study

Due to the demands of teaching and government
service, my father was never able to complete prepara¬
tion of the dissertation for publication. Although it has
not been entirely ignored, the impact of the work has
been narrowed by its unpublished status. Relatively
few authors (Davis 1966, 1974; Baccus 1971; Boeer
and Schmidly 1977; Schmidly 1977a, b; 2004; Davis
and Schmidly 1994; Best 1994; Best and Skupski 1994;
Wu et al. 1996; Yancey et al. 2006; Punzo 2007) have
directly referenced the dissertation, but some of the
basic findings of the study have made their way into
the pocket mouse literature via citations from these
secondary sources. In several cases, while working on
updating the literature citations, I encountered refer¬
ences which seemed to provide recent supporting data,
only to find upon tracing the reference to its source,
that the ultimate origin of the information was the very
dissertation I was updating!

The dissertation was cited a dozen or more times
each in the Mammalian Species accounts of Perog-

Porter—Ecology of Pocket Mice in the Big Bend Region

nathus merriami and Chaetodipus nelsoni (Best and
Skupski 1994; Best 1994). These citations include
references to field methods, identification, morphology,
molting, habitat, population, reproduction, life his¬
tory, ectoparasites, home range, territoriality, activity,
and community structure. The number and breadth
of these citations in the species accounts demonstrate
the importance of the study to knowledge of the biol¬
ogy of these species. Schmidly (1977b) referred to
Porter’s (1962) “excellent analysis of the influences of
substrates on local distribution and abundance of three
species of pocket mice.”

Taxonomy

My father disagreed with Wilson’s (1973) action
to synonymize Perognathus merriami with P. flavus.
According to correspondence in his files, the issue
came up as early as 1965 in conversations with Syd¬
ney Anderson at the annual meeting of the American
Society of Mammalogists in Winnipeg. Later that year,
Anderson sent my father excerpts from his forthcoming
manuscript on the mammals of Chihuahua (Anderson
1972) showing morphological differences between P.
flavus and P. merriami. Anderson agreed that the two
species were distinct and concluded that “it is possible,
of course, that some of the differences that seem to
distinguish the two species in Chihuahua would not do
so in some other part of their range, or that differences
might occur in some other area that do not occur in Chi¬
huahua. In any event, I am convinced that two distinct
species are involved” (S. Anderson, pers. comm, to R.
D. Porter, 3 August 1965). My father was pleased when
Lee and Engstrom (1991) published molecular evidence
supporting the validity of P. merriami.

Subsequent Activities

In December 1988 my parents and I visited the
study area in BBNP and found some wooden stakes
marking the trap sites (Plate 2). I visited the area
again with my mother in May 2008 to qualitatively
assess changes on the three population plots and map
the boundaries of the plots using GPS. I returned in
November 2008 and took additional photographs and
finished mapping the plots. On these trips, we located
at least one metal corner stake in each plot, along
with numerous wooden stakes that after 50 years, still

Plate 2. Richard D. Porter at the
Merriami plot, 30 December 1988.
Lone Mountain in the background.
Photograph by C. A. Porter.

marked the trap stations. As a result, we were able to
more accurately map the location and orientation of
the plots.

A total of 76 habitat plots (numbered non-consec-
utively from 1 to 101) were assessed during the study.
My father did not list the specific localities of all 76
habitat plots (Porter 1962), but stated that the full list
of localities would be deposited at BBNP and the Texas
A&M Department of Wildlife Management. In 2007
and 2008, I contacted personnel at BBNP and Texas
A&M; they were unable to locate the locality list. I
also was unable to find the list in a thorough search of
my father’s professional and personal files. Complete
field notes were available only for the first half of the
study, and the locality numbering system used in the
field notes did not correspond to that used in the dis¬
sertation. Specimens collected from the habitat plots
are deposited in the mammalogy collection at Brigham
Young University, but plot numbers were not recorded
on the tags. Localities for some of the habitat plots were

Special Publications, Museum of Texas Tech University

listed in the dissertation, and I was able to determine
some others from notations on photographs of the plots.
This information has been incorporated into the text
and figure legends.

Copies of my father’s field notes and other
documents related to this study along with my father’s
ornithological notes from his work at Big Bend, are
archived at BBNP. The original dissertation (Porter
1962) and additional photographs of study sites and
of the flora and fauna of the study can be accessed at
http://webusers.xula.edu/cporter/rdporter/.

Preparation of the Manuscript

Other than in the Foreword and Afterword,
which are signed by me, the author is writing in the
first person. Where I added or revised material in the
text, 1 did so in my father’s voice so as to maintain a
consistent narrative mode. The following is a summary
of revisions in the manuscript.

• I have incorporated modifications indicated
by the author in handwritten notations on his
copy of the dissertation and by revisions he
made in preparing the work for publication.

He made some corrections, revisions in word¬
ing, and in a few instances, added additional
information.

• Following recommendations of the referees,

I have shortened and omitted some sections
that are redundant or of lesser importance. 1
summarized the results of preliminary studies
and abridged some sections, instead citing the
original dissertation for more detailed analyses
and discussion. The sequence of some sections
has been rearranged. I have omitted some
tables and figures, combined some figures,
and converted the data in some tables into
graphs. 1 added locality information for some
habitat plots.

• I converted all measurements to metric units.

In some cases, I made a judgment call on the
precision of the measurement, and rounded
the metric equivalent accordingly. Trailing
zeros to the left of the decimal point should

not be considered significant unless the value
is used in association with other measurements
indicating greater precision.

• Nearly all figures have been redrawn, and in
some cases redesigned to more clearly depict
the data. The maps of the population plots
were redrawn and corrected based on held
work in 2008. Where needed, 1 converted the
scale of graphs to metric units. The photo¬
graphic plates of Porter (1962) were cropped
contact prints of 4x5 monochrome negatives
exposed with a Graflex Super D camera.
Although the original negatives are in my
possession, I substituted 35 mm Kodachrome
transparencies taken by my father at the same
time and from the same vantage points. I also
included some of my own photographs.

• The nomenclature of organisms has been
revised to reflect current taxonomy. I added
several paragraphs on heteromyid systemat-
ics and taxonomy, to describe changes that
occurred since the study was carried out.
Because two species have been moved out of
the genus Perognathns, 1 made the appropriate
revision in the title, and the collecting plots
originally referred to as “Perognathus plots”
are designated “population plots.”

• I added a description of changes observed on
the population plots in the fifty years since the
original study.

• I was able to greatly expand the section on
ectoparasites using information from the
published literature, my father’s notes and
correspondence, and my communication
with a number of parasitologists, The revised
parasite section includes additional informa¬
tion regarding undescribed species which my
father collected during the study, type locali¬
ties, the disposition of specimens, and records
of parasites collected from mammalian hosts
other than pocket mice.

• 1 have added more than 60 recent literature
citations pertinent to the work, but I have not

Porter—Ecology of Pocket Mice in the Big Bend Region

made an effort to comprehensively update all
citations. The original citations to literature
of the 1950’s and earlier remain in the text.
Although recent references might be used in
place of many of the original citations, T felt
that a complete modernization of the literature
citations would unnecessarily distance the
work from the context in which it was per¬
formed and written.

• I made changes in format to conform to mod¬
em conventions and the style of the Special
Publications. The original dissertation did not
include an abstract, so I used a modified form
of the abstract published by Porter (1963). 1
also corrected typographical and other errors,
and where appropriate, revised the wording to
be more clear and concise.

Acknowledgments

Although my contribution might be sufficient to
justify co-authorship, I have chosen not to dilute credit
for my father’s work. A case could also be made for
adding my mother to the author line for her effort in
the field. However, the majority of recognition for this
work belongs to my father for his two years of inten¬
sive field work and two years of analysis and writing.
My understanding of my father’s effort increased im¬
mensely when 1 visited the Nelsoni plot, located on a
steep grade more than 85% covered knee-deep in plants,
nearly all of them spiny. While nursing bloody lechu-
guilla wounds in my shins, 1 began to appreciate the
work involved in setting up a grid of784 trap sites, then
routinely traversing this difficult terrain while moving,
setting, and checking traps, surveying vegetation, and
sampling rocks and soil during the trapping period of
more than a year. On some occasions when the dirt
road was impassable, he hiked more than a kilometer
on foot to reach the plot. The work on the Nelsoni plot
was just a fraction of the effort involved in this study,
for which due credit should be given the author.

Numerous people assisted with preparing the
manuscript for publication. Lois G. Porter, an invalu¬
able participant in the original field research, provided
field notes, catalogs, correspondence, and many other
documents related to the study. She provided extensive
information, answered many questions, helped to iden¬
tify collecting sites, and provided funds to cover field
work and publication costs. Field work in 2008 was
authorized by the National Park Service (Permit B1BE-
2008-SC1-0025). Raymond Skiles authorized the work
and Susan Simmons assisted with accommodations at
the K-Bar Ranch. Lois G. Porter and Mark A. Porter
assisted in the field.

Christine L. Hice, Thomas E. Lee, Sanford D.
Porter, and David J. Schmidly reviewed the manu¬
script and provided valuable suggestions. Lisa C.
Bradley provided helpful editorial assistance. John
O. Whitaker, Jr. and Brianne Walters of Indiana State
University and D. A. Crossley, Jr. of the University of
Georgia provided information and advice regarding the
ectoparasites collected during the study. Ann M. Porter
provided advice and assistance in preparing figures and
in revising the text. The following people assisted with
locating documents, specimens, and other information:
Jeffery Bennett and Sue Buchel (BBNP); Debra Creel
and Ronald Ochoa (USDA Systematic Entomology
Laboratory); James R. Dixon, Thomas A. Lacher, Jr.,
and Heather Prestridge (Texas A&M University); Bruce
D. Eshelman (Wisconsin State University, Whitewater);
Jerry Louton (National Museum of Natural History ),
Barry M. OConnor and Priscilla Tucker (University of
Michigan); Steven Platt (Sul Ross State University);
Eric Rickart (University of Utah); Duke S. Rogers
(Brigham Young University); Cal Welbourn (Florida
Department of Agriculture & Consumer Services);
and Franklin D. Yancey, II (State Center Community
College).

I thank all of these people in addition to those
mentioned in the author’s Acknowledgments.

Calvin A. Porter

Department of Biology
Xavier University of Louisiana

Special Publications, Museum of Texas Tech University

Abstract

Population characteristics and habitat preferences of Merrianrs pocket mouse ( Perognathus
merriami), Nelson’s pocket mouse ( Chaetodipus nelsoni), and the Chihuahuan pocket mouse
(C. eremicus ) in the Big Bend region of Texas were studied intensively over 26 months. Steep
slopes limited the distribution and abundance of Chihuahuan pocket mice but not the other two
species. Merriam’s pocket mice normally were not present on steep slopes because of the usual
occurrence of tall, dense vegetation on these sites, but they did occur there when the understory
vegetation was sparse and short and there were either large boulders or fine gravels. Nelson’s
pocket mice were most abundant on slopes >20%, P. merriami on slopes 3-10%, and C. eremicus
on slopes <2%. C. eremicus was most abundant on deep, rock-free (<5% gravel) sands, loams,
and sandy loams, and was rarely found on shallow, rocky, sandy, and sandy clay loam soils. P.
merriami was common on deep sandy loams and sandy clay loams covered with erosion pave¬
ment (rocks usually <7.5 cm in diameter accumulated on the surface) and usually containing
40-60% gravel. Nelson’s pocket mouse was most abundant on shallow sandy loam or sandy
clay loam soils of the mountain slopes containing cobbles and boulders (70% rocks at least 7.5
cm in diameter and frequently much larger). It was rarely found on rock-free, deep loams or
sandy loams. The three species segregated themselves in the habitat according to the number
and size of the rocks and the density and height of the understory vegetation. The three spe¬
cies of pocket mice were not restricted to specific plant associations, although each species had
preferences. Nine habitat types were identified in the Big Bend area, with C. nelsoni commonly
found in rocky, often steep and densely vegetated habitats, and C. eremicus in flat, rock-free,
sparsely vegetated habitats. Merriam’s pocket mouse, which has a wide range of habitat toler¬
ance throughout its geographic range, reached its peak of abundance in habitats intermediate
to and not preferred by the two species of Chaetodipus. It is believed that despite its greater
range of habitat tolerance, P. merriami is better adapted to these intermediate habitats, and that
because of its smaller size, it further tends to be crowded into this habitat by population pres¬
sures from Chaetodipus. During colder years, males of P. merriami and C eremicus emerged
from hibernation earlier in the spring than females. These two species showed a much stronger
tendency to hibernate than did C. nelsoni , which was generally active throughout the winter.
Reproductive activities of C. nelsoni started earlier in 1959 than those of the other two species,
probably due to a lesser tendency of C. nelsoni to hibernate. The principal period of reproduc¬
tion for pocket mice was in spring with a smaller fall peak. Juvenile female P. merriami molted
into adult pelage before pregnancy occurred; juvenile females of the other two species appeared
more sexually precocious. The incidence of pregnant juveniles of C. eremicus was higher than
that of the other two species. Based on sperm production, juvenile males of P. merriami were
more precocious sexually than juvenile males of Chaetodipus. Although the general progression
of molt among the three species was similar, details were different. In general, adults moved
greater distances than juveniles (except in September). P. merriami moved significantly shorter
distances between captures than the other species. Analysis of the shift in center of activity
from one period of capture to the next revealed that these pocket mice are relatively sedentary
animals. Adult females of all three species showed a stronger tendency toward territoriality than
did adult males. Several species of arthropod ectoparasites were collected from pocket mice
and associated rodents, including some species not previously described.

Key words: Big Bend, Chaetodipus , ectoparasites, habitat, molting, Perognathus , pocket
mice, population dynamics, reproduction

Porter—Ecology of Pocket Mice in the Big Bend Region

Introduction

Three species of pocket mice were studied in
the Big Bend region of Texas: MerrianTs pocket
mouse, Perognathus merriami , Nelson’s pocket mouse,
Chaetodipus nelsoni , and the Chihuahuan pocket
mouse, Chaetodipus eremicus. These species occupy
adjacent and often overlapping habitats in the Trans-
Pecos region of Texas (Fig. 1). Two additional species,
C. hispidus (Denyes 1956; Schmidly 1977a; Yancey
and Jones 2000) and C. intermedins (Yancey and Jones
2000), are found in nearby regions of the Trans-Pecos
but are not known from the study area. Field work in
Big Bend National Park (BBNP) was performed almost
continuously from June 1957 to August 1959. The
study emphasized population dynamics, home range,
and habitat preference, with special reference to sub¬
strate composition. Observations were made of body
weights, molts, ectoparasites, and reproduction.

Mammalian taxonomy follows Wilson and
Reeder (2005), with revisions recommended for het-
eromyids by Anderson et al. (2007) and for ground
squirrels by Helgen et al. (2009). Herpetological no¬
menclature follows recommendations of Crother et al.
(2000), including revisions of nomenclature ofwhiptail
lizards by Reeder et al. (2002). Names of mites were
updated with reference to Whitaker and Wilson (1974)
and Whitaker et al. (1993). The botanical names and
identifications in this report came fromMcDougall and
Sperry (1951), with taxonomy as updated by Jones et
al. (1997).

Topography and Vegetation of the Study Area

The Big Bend is that region where the south¬
western boundary of Texas, formed by the Rio Grande,
resembles a large pocket (Figs. 1-2). The rugged to¬
pography is marked by isolated rocky mountains and
mesas. These mountain ranges extend up to 1,500 m
above the desert floor. Elevations range from about
520 m at the Rio Grande below Boquillas Canyon to
2,387 m in the Chisos Mountains 40 km to the west
(Fig. 2).

The Chisos Mountains consist of a Tertiary igne¬
ous mass intruding through Cretaceous sedimentary
formations. In local areas in the Chisos and Chilicotal

mountains. Burro Mesa, Grapevine Hills and elsewhere
throughout the region, there are other exposed intru¬
sions of igneous rock. Some barely reach the surface
and produce a rugged and rocky terrain. Others form
high cliffs with steep talus slopes of large boulders.
The central area of the Park is a sunken block of ig¬
neous intrusions flanked on the east by the Sierra del
Carmen and on the west by the Mesa de Anguila; both
are Cretaceous sedimentary formations (Maxwell et
al. 1955), which are predominately limestones. The
foothills of the Chisos Mountains have steep, rocky
slopes, with many igneous outcrops extending through
shallow soil. The steeper slopes grade into more gentle
outwashes of cobbles and coarse gravels extending
from the mouths of the canyons. As the slope dimin¬
ishes, fine gravelly outwashes become prevalent. On
the shrub-studded desert flats and rolling lowlands,
gravels become scarcer and in some areas almost
non-existent. Washes and arroyos of sand and gravel,
originating in the mountains, interrupt the continuity
of the foothills and plains. In many locations along
the Rio Grande, and adjacent to some of the larger
creeks and dry washes, are broad flood plains made
up of deep, rock-free, fine loamy soils. Steep foothills
therefore, tend to have boulders, cobbles, and coarse
gravels, which grade into finer gravels and ultimately
sandy and loamy rock-free soils as the slopes diminish
farther from the mountains. Rodents such as pocket
mice are thus presented with a variety of substrates in
which they may construct their burrows.

Scrubby vegetation decreases in density as
it extends into the desert lowlands from the lower
mountain peaks. Mesquite ( Prosopis glandulosa )
was predominate on the deep loamy soil of the flood
plains, but further from the flood plain, where silts and
clays give way to sands and fine gravelly sandy loams,
creosotebush ( Larrea tridentata) was the dominate
shrub. These areas of creosotebush and particularly
mesquite are usually characterized by a scant under¬
story of grasses and herbs.

Closer to the foothills, perennial grasses and
lechuguilla (Agave lechuguiUa ) become abundant
and produce an understory' denser than the overstory.
Influenced by the nature of the parental material, the

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I P. merriami

Figure 1. Geographic distribution of three species of
pocket mice, modified from Best (1994), Best and Sk-
upski (1994), and Mantooth and Best (2005b). The star
indicates the Big Bend region of Texas.

Figure 2, Map of Big Bend National Park, Texas, showing the major
topographical features and the locations of die three 2.2-ha popula¬
tion plots. Dotted line represents an unpaved road.

Porter—Ecology of Pocket Mice in the Big Bend Region

soil of some fine gravelly outwashes becomes more
compact in areas where the fine gravels merge with
the coarse gravels and cobbles of the foothills. Under
these conditions, creosotebush is gradually replaced
by tarbush ( Flourensm cernua ), mariola (Parthenium
incanum) and other, less common shrubs. Here, fluff
grass (Dasyochloa pulchel/a) and lechuguilla are
among the common understory plants.

In other localities creosotebush extends to the
base of the foothills where it is replaced abruptly by
sotol (Dasylirion Jeiophyllum ), ceniza, ( Leucophylhm ),
feather dalea ( Daleaformosa ), prickly pear ( Opuntia ),
and other shrubs characteristic of the steeper slopes.
The understory of such slopes consists principally of
lechuguilla, chino grass ( Bouteloua breviseta ), and
other perennial grasses. Higher in the mountains, low
desert scrub gives way to woodlands of pine {Finns),
oak ( Quercus ), and juniper {Jumperns), which fre¬
quently extend down the canyons on dry creek beds in
response to the greater seasonal availability of water.

Climate of the Study Area

Big Bend is in the arid zone of Thornthwaite
(1948) with a moisture index of -60 to -40. The area
is characterized by diy mild winters and hot sum¬
mers (Figs. 3-4), During late spring and summer at
lower elevations (near the Rio Grande ) the mean daily
maximum temperature usually is well over 38°C (Fig.
4) and frequently exceeds 43°C. In the Chisos Moun¬
tains a much cooler climate prevails. There, the mean
temperature during late spring and summer generally
is <32°C and seldom exceeds 38°C. Temperatures
along the Rio Grande seldom drop much below freez¬
ing during the winter (Fig. 4) and snow is uncommon.
However, snow falls more frequently in the mountains
where sub-freezing temperatures are common. Figure
5B shows months that were above or below average
monthly temperatures during the study.

Precipitation was infrequent, but often of near
cloud-burst magnitude, at lower elevations. At Bo-
quillas Ranger Station, near the Rio Grande, annual
precipitation was usually less than 25 cm. In contrast,
the mountainous regions usually received >25 cm an¬
nually, and 40-75 cm or more during wet years (Fig.
6). On average, rainfall in Big Bend was more frequent

during spring, summer and early fall than during late
fall, winter and early sprmg (Fig. 3).

Precipitation in the study area at Panther Junction
during the investigation was above average (Fig. 6).
The monthly mean precipitation was above average
during the fall of 1958 and 1959 and during spring 1957
and 1959 (Fig. 5 A). Precipitation was well below aver¬
age in April and May 1958, August of all three years,
July 1957 and during January 1959.

Systematic Relationships of Pocket Mice

Pocket mice belong to the family Heteromyidae, a
clade of 59 species (Patton 2005) of burrowing, but not
fossorial, rodents. Heteromyids have proven valuable
as models for ecological studies of terrestrial vertebrates
(Brown and Harney 1993). Three extant subfamilies
and five living genera are currently recognized in the
family (Anderson et al. 2007): Heteromyinae, (includ¬
ing the spiny pocket mice, Heteromys ); Dipodomyinae
(including the kangaroo rats, Dipodomys, and kangaroo
mice, Microdipodops ); and Perognathinae, (two genera
of pocket mice, Perognathus and Chaetodipus). Earlier
workers (Wood 1935, Hall and Kelson 1959) included
Microdipodops in the subfamily Perognathinae, but
more recent classifications (Hafher and Hafiier 1983;
Williams et al. 1993; Patton 2005, Anderson et al. 2007)
have indicated an association between kangaroo mice
and kangaroo rats in the subfamily Dipodomyinae, an
arrangement first proposed by Reeder (1957).

Members of the genus Perognathus are usually
small (100 mm) to medium (200 mm) in total length.
The pelage varies from fine and silky to coarse, but
never includes distinct spines or bristles. The tail may
be short (about equal to body length) and tuftless, long
and tuftless, or long and tufted. The soles of the hind
feet are more or less hairy. Species of Chaetodipus
are usually medium (150 mm) to large (230 mm) in
size with a harsh pelage. Many of them have distinct
spines or bristles that extend from the rump or sides,
whereas others have no grooved spines. The soles of
the hind feet are usually naked.

In the time since the study was carried out, the
taxonomy of these species has been in flux. The three
species studied in this investigation were regarded as

Special Publications, Museum of Texas Tech University

Mean precipitation in cm

Figure 3. Hythergraphs of the average monthly mean temperature and average
monthly precipitation for two weather stations in the Big Bend area. The black dots
indicate the temperature and precipitation for February. Other months are repre¬
sented by letters.

Daily low Daily high Daily high

Panther Junction

Figure 4. The average number of days during a month in which the maximum
temperature exceeded 32°C or 38°C, and the number of days in which the
minimum temperature fell below freezing Gray areas of each bar indicate
days in which none of these conditions occurred.

Precipitation in cm s> ^ Deviation from monthly Deviation from monthly

Porter—Ecology of Pocket Mice in the Big Bend Region

ure 5. Monthly deviation from the mean at Panther Junction during the study
iod for (A) precipitation; and (B) temperature.

Figure 6. Annual precipitation for three weather stations in the Big Bend region of
Texas. The inset bar graph indicates the annual deviation at Panther Junction dur¬
ing the study period from the long-term annual precipitation.

Special Publications, Museum of Texas Tech University

congeneric by Porter (1962), following Hall and Kel¬
son (1959) and other authorities. The two currently-
recognized genera of pocket mice were considered
subgenera of the single genus Perognathus until Hafiier
and Hafner (1983) elevated the subgenera to the rank
of genus, moving P. nelsoni and P. penicillatus to the
genus Chaetodipus.

Based on morphological analysis, Wilson (1973)
concluded that P. merriami was conspecific with P.
flavus. This arrangement was accepted by a number of
workers including Schmidly (1977a) and Hall (1981),
but not by Davis (1974), who noted morphological
differences between the species. Subsequently, an
allozymic study by Lee and Engstrom (1991) and a
DNAstudy by Coyneretal. (2010) showed that/! mer¬
riami was behaving genetically as a distinct biological
species, and those authors recognized P merriami as
a species. Lee and Engstrom ? s (1991) taxonomy was
accepted by Williams et al. (1993), Nowak (1999),
Baker et al (2003), Patton (2005), and Manning et al.
(2008) (though not by Yancey 1997 or Yancey et al.
2006), thus reverting to the nomenclature originally
used for this species by Porter (1962). Although the
two species cannot be consistently identified on the
basis of morphology alone, there are significant mor¬
phological differences between them (Anderson 1972;
Brant and Lee 2006).

Perognathus merriami (Plate 3) is one of the
smallest rodents, with an average total length of around
115 mm (Best and Skupski 1994; Schmidly 2004).
It has a short tail which is usually <50% of the total
length (Best and Skupski 1994). P. merriami can be

distinguished from the species of Chaetodipus by size
alone. The fine silky pelage is yellowish or ochraceous
buff with white postauricular and subauricular spots
(Plate 3).

The nomenclature of C. nelsoni has remained
unchanged since Hafiier and Hafiier’s (1983) revision,
but Lee et al. (1996) divided C. penicillatus into two
species based on mitochondrial DNA data. Findley
et al. (1975) had earlier suggested that there were two
species. The eastern (Chihuahuan Desert) species is
now recognized by Lee et al. (1996) and subsequent
authorities (Mantooth and Best 2005a, b; Baker et al.
2003; Patton 2005; Manning et al. 2008) as a distinct
species* Chaetodipus eremicus. Patton (2005) desig¬
nated this species in the vernacular to be the Chihua¬
huan pocket mouse.

Chaetodipus eremicus (Plate 4) and C. nelsoni
(Plate 5) are both medium sized rodents, with C. er¬
emicus an average of 153-192 mm in length (Wilkins
and Schmidly 1979; Mantooth and Best 2005b, Yancey
1997), and C. nelsoni 156-210 mm (Wilkins and
Schmidly 1979; Best 1994). Both species of Chaetodi¬
pus have a long tail which is well over 50% of the total
length (Best 1994). The presence of white subauricular
spots, well defined grooved bristles on the rump and
dusky colored soles of the hind feet definitely separate
C. nelsoni in the field from C. eremicus which lacks
these characteristics (see Plates 4-5), Although young
C. nelsoni lack rump spines, they still may be recog¬
nized easily from adult or young Chihuahuan pocket
mice by the presence of white subauricular spots and
dusky plantar surfaces of the hind feet.

Materials and Methods

This investigation was divided into four major
phases: (1) prelim inary field work, (2) habitat selec¬
tion, (3) home range and population dynamics and (4)
dissection of pocket mice to determine reproductive
condition.

Galvanized iron Sherman live traps, 25 cm long
and 7.5 cm square in cross section, were used for
trapping. The traps were baited with chicken scratch

composed of maize, wheat, and barley. Live traps were
used because snap traps are ineffective for capturing
Perognathus merriami (Bailey 1905) and traps were
needed that would capture all three species of pocket
mice with equal efficiency. During the summer, traps
were set in shade. In the absence of natural cover,
cardboard covers were used. Traps were inspected and
animals removed before 1000 h. Each individual was
marked by clipping a combination of two toes.

Porter—Ecology of Pocket Mice in the Big Bend Region

Plate 3. Merriam's pocket mouse, Perognathus merriami, from Big Bend, 1958. Fluff grass ( Dasyochloa pulchella )
in the background. Photograph by R. D. Porter.

Plate 4. Chihuahuan pocket mouse, Chaetodipus eremicus , from Big Bend, 1958. Ground cholla ( Opuntia schottii)
in the background. Photograph by R. D. Porter.

Special Publications, Museum of Texas Tech University

Plate 5. Nelson's pocket mouse, Chaetodipus nelsoni , from Big Bend, 1958. Note the prominent rump spines.
Lechuguilla (Agave lechuguilla) in the background. Photograph by R. D. Porter.

Preliminary Field Work

Preliminary field investigations were conducted
to determine the most efficient trap interval and best
means for eliminating “trap addiction” of individual
mice. Tests indicated that traps placed at 10-m or 12-m
intervals gave equally good indications of population
size, whereas a 15-m interval with a resultant smaller
number of traps gave a poorer estimate of the popu¬
lation. An analysis of the number of sites at which
individual mice were trapped indicated that a distance
of 15 m approached the maximum distance between
traps for the purpose of determining the movements
of pocket mice, and that a 10-m interval was best for
collecting data on home range and movements under
the prevailing conditions (Porter 1962).

During the trap-interval tests it was noted that
many of the mice were recaptured at the same sites.
This “addiction” to certain traps not only restricted the
movements of the mouse so that the extent of its home

range could not be determined (Chitty 1937), but also
excludes other individuals from being captured at the
site (Chitty and Kempson 1949; Miller 1958). To avoid
these difficulties, a system of rotating the traps was de¬
vised and tested. The rotation system proved effective
in eliminating trap addiction (Porter 1962).

The Habitat Plots

To compare habitat preferences and obtain
specimens of the three species of pocket mice, 76
small “habitat plots” of 0.4-0.8 ha were live-trapped in
a variety of vegetation and substrate types. For most
plots, the distance between traps was approximately
12 m. The number of traps per plot ranged from 14 to
85, with an average of 42. A total of 17,382 trap nights
were compiled on the plots. Each plot was trapped for
an average of 6.3 nights. The pocket mice captured on
these plots were weighed and their reproductive tracts
were removed for study.

Porter—Ecology of Pocket Mice in the Big Bend Region

The Population Plots

To study home range and population dynamics,
three plots 148.5 m square (2.2 ha) were constructed at
three of the sites analyzed as habitat plots. Each plot
contained 784 trapping stations 5.5 m apart in a 28 x
28 grid (Fig. 7). The plot corners were staked with
rebar and each of the remaining 780 trapping stations
was marked with a wooden stake. Throughout the plot,
each stake was marked alternately with orange or red
tape in one row, and white or yellow tape in the next
row (Fig. 7). Each 2.2-ha plot was located in a habitat

which supported predominately one of the three species
of pocket mice. The plot for the study of Chihuahuan
pocket mouse populations was west of the Marathon
highway, 1.6 km south of the upper Tornillo bridge at
an elevation of 864 m (Figs. 2, 8; Plate 6). Population
studies of Merriam’s pocket mice were performed at
a plot west of the Marathon highway, at Panther Junc¬
tion and southeast of Tone Mountain at an elevation of
1,127 m (Figs. 2, 9; Plate 7). The third plot, selected
for the study of Nelson’s pocket mouse, was located
west of the Glenn Spring road 1.6 km south of the
junction with Boquillas highway at an elevation rang-

■148.5 m-

: w

i 0

; w

://

5.5 m

W 7

W 6

Y 10

°3

RlS

0 2

Rl4

W 5

y 9

w 8

y 12

R"13

0 4

^16

22 m

5.5 m
O = orange
R = red
W = white
Y = yellow

Figure 7. Layout and trap rotation scheme for the population plots. The left square shows the entire 2.2-ha plot. Let¬
ters represent the 784 trapping sites each marked in the field by a stake with a colored flag. Only 49 traps were set
on a given night and each site was trapped for a single night during the 16-day trapping period. The placement of the
49 traps for the first night of the trapping period is marked on the figure by a trap at an orange stake. On subsequent
nights of the trapping period, each trap was systematically rotated through the 16 trap sites delimited by the dashed
lines marked on the grid, always maintaining a distance of 22 m between traps. Subscript numbers 1-16 in the en¬
larged section represent the rotation sequence of a single trap through 16 nights, beginning with the four orange sites,
followed in sequence by the white, yellow, and red trap sites.

Special Publications, Museum of Texas Tech University

Figure 8. The Eremicus plot in relation to surrounding features. The plot is indicated by the square. Black circles
indicate soil sample locations. Circle graphs show percentages of sand, silt, and clay in each sample, with soil
classifications indicated The number enclosed in the square indicates the percentage gradient at that location,
and the arrow indicates the direction of die uphill slope. Contour intervals represent approximately 1 m elevation,
beginning with the level of the plot. Elevated areas in the plot are sandstone outcrops about 1 to 1.5 m above the
general elevation of the plot. UTM coordinates of corner stakes located in 2008 are as follows: southeast comer
13-67998IE 3253996N; southwest corner 13-679835E 3254029N; northwest corner 13-679865E 3254171N.

Porter—Ecology of Pocket Mice in the Big Bend Region

A B

~ ^

Plate 6. Eremicus plot, rock-free flats habitat. A. Looking southeast towards the bluffs just beyond
the southern edge of the plot. Yucca data is in the left foreground. Creosotebush (Larrea tridentata ) is
the dominant plant in the foreground; ocotillo (Fouquieria splendens) in the background. B. Looking
north along the boundary of the gravelly and non-gravelly portions of the plot showing the area of clay
loam soil (Fig. 8). The vegetation (mostly creosotebush) in the middle distance marks the location of
the wash. C. Rock-free sandy loam soil characteristic of the majority of the Eremicus plot. Opuntia
schottii , the most abundant understory species, is visible in the photograph. The scale shown in this
and other plates is graduated in tenths of feet (3.0 cm). D. Surface rocks at the northern edge of the
plot. E. Southwestern corner of the plot looking northeast. Photographs A and D by R. D. Porter,
Spring 1960. Photographs B, C, and E by C. A. Porter, November 2008.

Special Publications, Museum of Texas Tech University

Figure 9 The Merriami plot in relation to surrounding features. The boundaries of the plot are indicated
by the square. The easternmost comer of the plot is at UTM 13-674237E 3245884N. Since only one
comer stake was located, the orientation of the plot is approximate, but is inferred from physical evidence
at the site, and from the description in Porter (1962). Black circles indicate soil sample locations. Circle
graphs show percentages of sand, silt, and clay in each sample, with soil classifications indicated. The
number in the square indicates the percentage gradient, and the arrow indicates the direction of the uphill
slope. The “deep” wash is approximately 45 cm deep.

Porter—Ecology of Pocket Mice in the Big Bend Region

Plate 7. Merriami plot, fine gravelly foothill habitat. See front and back cover for an additional view of this
plot. A. Looking northwest towards Lone Mountain. Engelmann's prickly pear ( Opuntia engelmannii ) is
visible to the right. B. Looking east; Lechuguilla ( Agave lechuguilla ) in left foreground; Torrey’s yucca
(Yucca torreyi ) in background; tarbush (Flourensia cernua ) in between. Note the very small clumps of
fluff grass ( Dasyochloapulchella ) in open areas that result in a high density of understory plants while still
maintaining minimal ground cover. C. Surface rocks. D. Merriami plot at Panther Junction seen from
the ridge of Lone Mountain looking south-southeast. The structures beyond the plot are the Park Visitors
Center and Headquarters buildings. Photographs A-C by R. D. Porter, Spring 1960. Photograph D by C.
A. Porter, November 2008.

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ing from 959 to 977 m (Figs. 2, 10; Plate 8). The three
plots were used from March 1958 through July 1959
in a mark-and-recapture study of population dynamics
and home range size. Hereafter these three plots will
be referred to collectively as the “population plots”
and individually as the “Eremicus,” “Merriami”, or
“Nelsoni” plot.

In 1958 the population plots were trapped simul¬
taneously for four periods of 16 days each (4 March
to 8 April; 5-29 July; 4 September to 7 October; and
11-29 December). In May and July 1959, they were
trapped for only eight days each period (12-24 May
and 7-13 July) except for the Nelsoni plot which was
trapped for 16 days in May (12-31 May). The 16 or
8 days were not always consecutive and the interval
between trap nights was not the same for each period.
The March-April and September-October trapping
periods will hereafter be referred to simply as March
and September periods.

On each population plot, one-sixteenth (49) of the
784 trap sites was trapped each night, with the 49 traps
being rotated systematically to a new site each night
of the 16-day trapping period, until all 784 sites were
trapped for one night each. Within the plot, each trap
was rotated through a 4 x 4 grid of 16 trap sites as shown
in Fig. 7. For any one night, the distance between traps
was always 22 m and each trap was rotated at least 5.5
m and usually 15,6 m or 11 m from where it had been
placed on the previous night (Fig. 7). Consequently,
each of the 784 sites was sampled only once during each
16-day trapping period. Therefore, it was impossible
for an individual to be captured more than once at a
unique trap site during a single trapping period.

Miller (1958) used a rotational system which dif¬
fered from mine in that he selected sites randomly rather
than systematically. Under conditions of this study,
random selection of sites would have complicated the
setting of traps to such an extent that the amount of
time involved in moving traps would have rendered the
method impractical. Furthermore, if all traps had been
placed randomly, a set distance between traps could not
have been maintained.

This rotational method of trapping has the fol¬
lowing advantages over methods of stationary trap

arrangement: (1) Because mice are not captured at the
same trap site twice, the extent of home ranges may be
ascertained with fewer captures. (2) A more complete
coverage of the plot is accomplished. (3) Abetter esti¬
mate of the population size is possible. Hayne (1949)
reported that trap addiction tends to underestimate the
size of the population when population estimates are
based on the Lincoln Index because traps which con¬
sistently capture the same annuals have a much higher
probability of capturing a particular animal than do
other traps. (4) Stickel (1960) has shown that during
periods of low population density the home ranges
of animals increased in size and with an increase in
population the reverse was true. She pointed out that a
trapping area may be large enough to enclose the ranges
of several mice when home ranges are small and of only
one mouse when home ranges are large. As a result
she believes that it may be necessary to set traps closer
together when ranges are small than when ranges are
large. Stickel (1960) also pointed out that with larger
intervals between traps (with a resultant decrease in
number of traps) a higher percentage of animals was
captured in only one trap. She also indicated that the
scarcity of traps probably prevented capture of some
mice. The rotational system tends to alleviate these
problems by eliminating trap addiction and by bringing
about a more random distribution of captures. Thus
it probably lessens the need to change the distance
between traps to correspond with the size of the home
range. (5) A more random capture of animals is ac¬
complished by the use of this method since mice have
to search out the new location of the trap each night if
they do not encounter it accidentally.

Habitat Analysis Methods

Analysis of Vegetation .—Substrate and vegeta¬
tion were studied on the three 2.2-ha population plots
and on selected habitat plots chosen to include those
having predominately one species of pocket mouse
and also those plots having combinations of two or
three species.

The point-centered quarter method (Cottam
and Curtis 1956) was used for vegetational analysis.
Cottam and Curtis (1956) found that this method was
generally superior to the other distance methods stud¬
ied. Although the method normally is not applicable

Porter—Ecology of Pocket Mice in the Big Bend Region

3<e e / /o 0

'/ O fL

\ ' V f.j \

/ Saifid^ —•• ' &

/ loarn i

Sandy^

loam

Figure 10. The Nelsoni plot in relation to surroundmg features. The plot is marked by die large square.
Black circles indicate soil sample locations. Circle graphs indicate percentages of sand, silt, and clay in
each sample, with soil classifications indicated. The numbers in squares indicate the percentage gradient,
and the arrow indicates the direction of the uphill slope. The shaded areas in the plot designate substrate
areas 2 and 3 (see text). The present location of a backcountry campsite is indicated. Metal stakes marking
the south (UTM 13-679447E 3237900N) east (13-679559E 3238000N), north (13-679457E 3238098N),
and west (13-679345E 3238004N) corners of the plot were located during field work in 2008.

Special Publications, Museum of Texas Tech University

Plate 8. Nelsoni plot, coarse stony mountainside habitat. A. Lechuguilla (Agave lechuguilla ) and chino
grass {Bouteloua breviseta) in foreground. Nugent Mountain is in background. B. Area of Nelsoni plot
which is underlain by shale (Area 3). Note relative paucity of understory vegetation in this area and presence
of ocotillo (Fouquieria splendens). C. Surface rocks. D. Looking southeast from the northern comer of
the plot. Area 2 is in the foreground; Area 1 is in the distance. The white lines beyond the plot show the
location of the Glenn Spring Road and the present entrance to the Nugent Mountain backcountry campsite.
Photographs A-C by R. D. Porter, Spring 1960. Photograph D by C. A. Porter, November 2008.

Porter—Ecology of Pocket Mice in the Big Bend Region

to non-randomly distributed vegetation such as usually
occurs in the desert, sufficient randomness seemed to
prevail empirically (Table 1). Additionally, less time
was involved in collecting vegetational data by use
of this method than by more conventional methods
because dimensional plots were not needed. Wallrno
(1960) used this method to determine the density of
desert scrub in the Big Bend area and believes that it
gives a reasonable estimate of plant density in desert
scrub.

At selected points, measurements were made to
the closest understory, overstory and annual herbaceous
plant in each of four quarters around the point. A device
designed to randomize the compass position of the
quarters was driven through the loop of the measuring
tape at each point and the bisector whirled and allowed
to come to a stop. Illustrations of the device and its use
are given in Porter (1962).

On the 2.2-ha population plots, plants which
were of a height that the canopy did not inhibit move¬
ments of pocket mice were regarded as overstory.
Hence prickly pear (subgenus Platyopuntia ), krameria
(Krameria) and leather plant ( Jativpha dioica) were
considered understory plants on these plots (Appendix
I), but as overstory on the habitat plots. The popula¬
tion plots were analyzed a second time in conjunction
with the habitat plots, but the second time these three
types of pla nts were considered as overstory plants for
comparison. There was little change in the density of
vegetation and in species composition as a result of the
shift in classification.

The average measurements of each of these
categories were converted to plants/acre by dividing

43,560 by the square of the average distances (Cottam
and Curtis 1956). The resulting plant density values
were then converted to plants/m 2 , which is the unit
reported in this publication. The relative density and
dominance of plant species were determined by the
following formulae (Cottam and Curtis 1956).

Number of individuals of this species

Relative density = - x 100

Number of individuals of all species

Total basal area of the species

Relative dominance = - x 100

Total basal area of all species

Basal area is the average area covered by an
individual of the species (Appendix 11); total basal
area is the area covered by all individuals on the plot.
On each habitat plot 16 points were established, 30.5
m apart, with a total of 64 measurements for each
category or a total of 128 measurements for each plot.
On the population plots, 49 points were established
every 22 m at a staked trap site (Fig. 7). Hence 196
measurements were made of each category on each
2.2-ha population plot.

Cottam and Curtis (1956) regarded a standard
error of less than 10% of the mean as satisfactory for
most biological work and that the distance measure¬
ments needed to be squared before density could be
determined. Hence, they considered that the standard
error of the distance measurements be such that when
they were converted to densities the plus or minus fig¬
ures be within 10% of the mean density. They found
that a standard error of <4.65% of the mean distance
met this condition. In a hardwood forest in Wisconsin

Table 1. Statistical tests of the point-centered quarter method of vegetational analysis in desert scrub for 196
samples . The parenthetical number represents the value for 195 samples, with one unusually large distance
measure omitted.

Standard Error

Coefficients of Variation for 196 samples

Overstory a

Understory

Combined

Overstory

Understory

Combined

Merriami Plot

160

6.86

5.2

57.2

96.0

73.6

Nelsoni Plot

196

7.5 (5.8)

5.9

63.3

80.6

82.7

Eremicus Plot

160

4.97

58.1

95.6

69.6

a Number of measurements needed to obtain a standard error of 4.65% of the mean distance for the overstory.

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Cottam and Curtis (1956) found that 40 random mea¬
surements were sufficient to give a statistically valid
analysis of the density of a forest. Table 1 gives the
number of measurements necessary to get a standard
error of 4.65% of the mean for the two categories on
the population plots. It was found that in desert scrub
vegetation at least 190 measurements of overstory
plants and well over 196 measurements of understory
plants were needed to get a standard error as low as
4.65% of the mean.

Cottam and Curtis (1956) reported coefficients of
variation of their samples in Wisconsin in the range of
24-29%. The samples I took in desert scrub vegetation
were more highly variable than those taken in homo¬
geneous hardwood forests (Table 1). Desert scrub is a
heterogeneous mixture of species which apparently are
less randomly d istributed than are trees in a hardwood
forest. Hence, the coefficients of variation in desert
scrub were much higher than those obtained by Cottam
and Curtis (1956). Additionally, samples taken of the
overstoiy vegetation were less variable (more random)
than those of the understory (Table 1). Although the
standard errors in the analyses of desert scrub (Table
1) exceeded the arbitrary limits of Cottam and Curtis
(1956), they were small enough that the differences
could be distinguished between the habitats considered
(Tables 2-3).

Analysis of Substrate .—The upper regolith was
analyzed for soil texture (percentage of sand, silt and
clay), rock content (percentage rock of various sizes),
and surface rock (percentage of rock of various sizes
accumulated on the surface). One or more samples
(approximately one liter) of soil, 52 in all, were taken
from 43 habitat plots to determine soil texture. Six or
seven samples were collected from each population plot
for the same purpose (Figs. 8-10). These samples were
analyzed by the Bouyoucos (1936) hydrometer method.
Soil texture was determined from a U. S. Department
of Agriculture (1951) Guide for Textural Classification.
For purposes of this study, particles <5 mm in diameter
were regarded as soil; larger particles were considered
to be rock. Rocks less than 7.5 cm were regarded as
gravel; rocks at least 7.5 cm and less than 40 cm in lon¬
gest dimension were considered cobbles; and rocks >40
cm were regarded as boulders. This terminology will
be followed hereafter, although the dimensions differ
somewhat from standard geological definitions.

Rock content was determined by digging a hole
approximately 40 cm in diameter and deep enough (ap¬
proximately 15 cm) to remove sufficient soil and rock
to fill a 20-liter can. Samples were taken at plots where
the greatest number of each species of pocket mice
was captured, 60 cm from the flag marking a trapping
station. The sites to be sampled were selected using a
table of random numbers. From the 30 habitat plots 65
samples of soil and rock were taken; 51 samples were
taken from the 2.2-ha population plots, 17 from each.

The soil was separated from the rock and weighed.
To determine percentage of water a 500-gram sample
of soil was removed and dried in a pan over a flame,
stirring constantly until no moist soil remained attached
to the spoon. The soil fraction of the total sample was
then corrected for dry weight by subtracting the total
weight of the water from the total weight of the sample.
The rock fraction of the sample was weighed, but the
water content was not determined. The dry weight of
the soil fraction plus the weight of the rock fraction was
the total weight of the sample. For the population plots
the rock was passed through eight sieves of 5, 10, 13,
19,25, 38, 50, and 75 mm. For the habitat plots the 10
and 19-mm sieves were not used. The rocks retained
on each sieve were weighed and their percentage of the
total sample ascertained.

Surface samples were taken by using a 930-cm 2
square frame at sites selected in the same manner as the
soil samples mentioned above. The surface samples
were sorted with the sieves mentioned previously.
Sixty samples of surface rock were taken from each
2.2-ha population plot and 164 samples from 28 habitat
plots. The point-centered quarter method (Cottam and
Curtis 1956) was used to analyze the abundance of the
boulders >40 cm in diameter on the Nelsoni plot and
on four of the habitat plots.

Laboratory Examinations

A total of 633 specimens of the three species of
pocket mice (P. merriami, 216; C. nelsoni , 196; C.
eremicus , 221) was removed from the habitat plots. In
the laboratory the mice were placed alive in a plastic
bag and weighed on a balance calibrated to 0.1 g. Cot¬
ton soaked in chloroform was placed in the bag and
ectoparasites were brushed from the animals and sent
to the Texas Department of Health for determination.

Porter—Ecology of Pocket Mice in the Big Bend Region

Table 2. Comparative density and mean height of plant cover on the population plots. Understory
plants are those for which the canopy does not inhibit the movement of pocket mice.

Plant Density (plants/m 2 ) Height (cm)

Overstory

Understory

Annual Herbs

Overstory

Understory

Merriami Plot

0.489

0.854

25.628

Nelsoni Plot

0.242

4.047

1.779

Eremicus Plot

0.164

0.227

0.546

Table 3. Comparative dominance based on basal area of plant cover on the popula¬
tion plots.

Percent of Area Covered

Overstory

Understory

Merriami Plot

5.8

46.8

Nelsoni Plot

2.8

82.8

Eremicus Plot

3.0

4.3

Standard mammalogical measurements were made of
each specimen prior to autopsy. An analysis of weights
and measurements with regard to molting and reproduc¬
tive condition is given in Porter (1962).

The reproductive tract was removed and, except
for the right testes of the males, placed in a solution of

Characteristics of

Merriami Plot. —The Merriami plot was located
on an alluvial bench near Lone Mountain (Fig. 9; Plate
7) and had a gradient of only 4%. Young cottontails
(Sylvilagus audubonii) were trapped infrequently on the
Merriami plot, and pocket gophers ( Thomomys bottae)
were present. The soil was shallow, moderately per¬
meable, and consisted of a brown, fine gravelly sandy
clay loam about 35-45 cm deep overlying a gravelly
outwash. Two shallow washes (<50 cm in depth) ran
through the plot. The soil in and adjacent to these
washes was sandy loam, deposited there by the action
of flood waters (Fig. 9). 56% of the substrate material,
by weight, was gravel and cobbles (Fig. 11; Plate 1C),

formalin-alcohol-acetic acid. The right testis of each
male was cut in half and a smear made on one half of
a microscope slide. A smear of the right epididymis
was made on the other half and the presence or absence
of spermatozoa was noted. Uteri were examined for
embryos and placental scars before being placed in the
preservative.

the Population Plots

with 0.9 cobbles (rocks 7.5 to 40 cm in diameter) per
20-liter substrate sample, and 0.6 surface cobbles per
square meter.

Neither the plants of the overstory nor those of
the understory were dense on the Merriami plot (Plate
7; Table 2). Although understory plants were nearly
twice as numerous as overstory plants, the former
covered about eight times more area (Table 3). The
most notable species of the overstory included tarbush
(Flourensia cernua ), tasajillo ( Opuntia leptocaulis),
and Torrey’s Yucca ( Yucca torreyi ), with yucca being
less abundant (Appendix I) but more dominant based

Special Publications, Museum of Texas Tech University

A—Soil and rock content of substrate

Figure 11. Average composition of substrate samples from each 2.2-
ha population plot. Rocks between 7.5 and 40 cm in diameter are
regarded as cobbles. Part A shows surface and subsurface cobbles,
gravel, and soil to a depth of approximately 15 cm. Part B shows
surface rocks only. The bar graphs show the average proportions of
each of eight size classes of rocks expressed as a percentage of the
total rock weight. Circle graphs show the average percentage of rocks
by dry weight of substrate, including the gravelly and non-gravelly
portions of the Eremicus plot.

Porter—Ecology of Pocket Mice in the Big Bend Region

on basal area (Appendix II). Lechuguilla, fluff grass,
krameria and Engelmann’s prickly pear ( Opimtia en-
gelmcmnn ) were the most abundant understory species
(Appendix I). Overall, the most abundant plants on the
plot were lechuguilla, fluff grass, and krameria (Table
4). As a result of heavy precipitation during the fall of
1958 and the spring of 1959 (Figs. 5-6) an unusually
large number of short herbaceous annuals, particularly
bladderpod ( Lesquerella argyraea), invaded the plot
(Table 5).

Nelsoni Plot. —The Nelsoni plot, situated on the
lower east-facing slopes of foothills of Nugent Moun¬
tain varied in gradient from about 5 to 28% (Fig. 10;
Plate 8). Pocket gophers {I bottae) were present on
the plot. This plot had three distinct substrate types
(Fig. 10; Plate 8): (Area 1) very shallow, stony sandy
loam underlain by igneous parental material (Brewster
stony loam); (Area 2) grayish-brown gravelly sandy
loam, slope materials, 15-76 cm deep, overlying igne¬
ous parental material, and with a thin layer of calcium
carbonate caliche several centimeters below the sur¬
face; (Area 3) white-colored, weathered red shale lying
over a deposit of nearly pure reddish-brown calcareous
shale. This portion of the plot is characterized by the
presence of ocotillo and by a less dense understory
(Plate 8B). Laboratory analyses indicated the soil in
Area 3 to be a silt loam (Fig. 10). Igneous rock frag¬
ments were present on the surface (Plate 8C).

Over 70% by weight of the substrate on the
Nelsoni plot was gravel and stones 5 mm or larger.
Cobbles 7.5 cm in diameter or larger made up 40%
of the weight of the substrate rock and 60% of the
samples of surface rocks (Fig. 11). An average of four
cobbles per 20-liter sample; about 617 boulders (40 cm
or larger) per hectare (that is, one boulder per 16 m 2 );
and 8.15 cobbles per square meter of surface occurred
on the Nelsoni plot.

The Nelsoni plot was densely vegetated (Tables
2-3) with understory plants (Table 4; Plate 8 ) such as
lechuguilla (Plate 9) and chino grass (Plate 8D). Density
of the overstory, however, was sparse (Tables 2-3). Four
species of plants of the overstory were about equally
abundant (Appendix 1), namely, ceniza ( Leucophyllum)
feather dalea, sotol, and Gregg's coldenia ( Tiquilia
greggii). Although chino grass was the most abundant

species on the plot, it accounted for only 8% of the
total area covered by plants. The ratio of understory
to overstory plants was nearly 17:1. In addition, plants
of the understory accounted for 29 times more area
covered than did plants of the overstory. Plants of the
overstory encompassed only about 3% of the total area
of the plot, whereas understory plants covered almost
83% (Table 3 ). Hence almost 86% of the surface area
of the Nelsoni plot was covered with vegetation. The
several species of annual plants recorded on this plot did
not contribute appreciably to the density of the cover
(Table 5). A small prostrate spurge (Euphorbia) was the
most abundant annual. Plants of the understory and the
overstory averaged 38 and 71 cm in height, respectively
(Table 2). The average height of chino grass was >50
cm; that of lechuguilla, 32 cm (Appendix II).

EremicusPlot. —The Eremicus plot was situated
in a flat area (slightly over 1% gradient) surrounded by
shale and sandstone bluffs 6-12 m in height (Fig. 8;
Plate 6). Coyotes ( Cants latrans) and kit foxes ( Vulpes
macrotis) were known to be present. Soil of the south¬
ern portion of the plot was composed of rock-free, pale
brown calcareous loamy sands (Plate 6C). That of the
northern half consisted primarily of pale brown calcare¬
ous sandy loams (Fig. 8). Awash 1.5 m wide and 1 m
deep that cut through the northern edge of the plot was
bordered by an area of fine gravelly sandy loam (Fig. 8;
Plate 6B, D). These gravels were apparently deposited
by mass wasting, as the gravelly portion of the plot is
slightly elevated over the surrounding rock-free areas
(Plate 6B). The soil was azonal and was underlain by
clays to a considerable depth. A small area of clay loam
soil had been exposed near the wash (Fig 8; Plate 6B)
by water action. The gravelly region of the plot (Plate
6B, D) is 61% gravel (Fig. 11 A) and abruptly gives
way (Plate 6B) to the sandy rock-free portion of the
plot (Plate 6A, C) which is <1% fine gravel (Fig. 11 A)
and makes up the majority of the plot.

Both the overstory and the understory were sparse
(Plate 6A, E) and both were about equal in abundance
and dominance of the plant species (Tables 2-3). Less
than 10% of the surface area of the plot was covered
by plants. The most abundant plant of the overstory
was creosotebush; the most dominant plant (in area
covered), mesquite (Table 4; Appendix II). Although
ground cholla ( Opuntia schottii; Plates 4, 6C) was

Special Publications, Museum of Texas Tech University

Table 4. Percentage composition of selected species ofperennial plants on the
population plots, based on numbers of plants. The values do not total 100%
because not all species are included. See Appendix I for the complete species
list.

Merriami Plot

Nelsoni Plot

Eremicus Plot

Ephedra

0.5

1.5

Bouteloua breviseta

41.0

0.5

Bouteloua eriopoda

1.0

Bouteloua trifida

1.5

Tridens muticus

1.0

Dasyochloa pulchella

21.0

4.0

2.0

Dasylirion leiophyllum

0.5

1.0

Yucca torreyi

1.0

Agave lechuguilla

22.0

34.0

4.0

Senna bauhinioides

5.0

Dalea formosa

1.5

Krameria

16.0

1.0

Prosopis glandulosa

1.0

3.0

Larrea tridentata

2.0

17.0

Jatropha dioica

3.0

Ziziphus

1.0

Echinocereus stramineus

0.5

2.0

Mammillaria

1.0

Opuntia engelmannii

4.0

6.0

Opuntia leptocaulis

6.0

0.5

4.0

Opuntia schottii

2.0

37.0

Tiquilia greggii

0.6

Flourensia cernua

1.5

0.5

Viguiera stenoloba

1.0

Unidentified grass

0.5

Unidentified shrubs

5.0

3.0

Porter—Ecology of Pocket Mice in the Big Bend Region

Table 5. Percentage composition of the annual herbaceous plants on the 2.2-ha
population plots.

Merriami Plot

Nelsoni Plot

Eremicus Plot

Zephyrcmthes longifolia

4.0

Eriogonum rotundifolium

2.0

Allionia incarnata

4.0

Lesquerella argyraea

80.0

Nerisyrenia camporum

15.0

9.0

Oligomeris Hnifolia

28.0

Dalea wrightii

2.0

Linum

0.5

0.9

Polygala scopariodes

13.0

Croton pottsii

6.0

Argythamnia neomexicana

3.0

0.5

Euphorbia

0.5

41.0

6.0

Nama

4.0

Verbena halei

3.0

Bahia absinthifolia

3.0

0.5

Bahia pedata

6.0

0.5

6.0

Bailey a multiradiata

0.5

3.0

24.0

Dyssodia

4.0

10.0

Iva ambrosiifolia

3.0

1.0

Melampodium leucanthum

0.5

7.0

Parthenium confertum

1.0

Psilostrophe

2.0

Unidentified

0.5

7.0

Special Publications, Museum of Texas Tech University

Plate 9. Lechuguilla (Agave lechuguilla) is a
common understory plant in areas occupied by
Perognathus merriami and Chaetodipus nelsoni.
Photograph by C. A. Porter, May 2008.

most abundant in the understory (Appendices I-II),
Engelmann’s prickly pear was the most dominant (Ap¬
pendix II). The most abundant species were ground
cholla and creosotebush (Table 4). The density of
annual herbs was only slightly greater than that of
perennial shrubs (Table 2). Desert marigold (Baileya
multiradiata) and Oligomeris linifolia were the most
abundant annuals (Table 5).

The Population Plots, 1958-2008. —Heavy graz¬
ing was permitted in the Park area until 1 January 1945
and the vegetation in many areas became severely de¬
pleted (Baccus 1971). Beginning in 1945, the National
Park Service made successful efforts to aid recovery
of native vegetation (Maxwell 1985). My original
(1958-1959) analysis of plants on the population
plots was performed less than 15 years following the
establishment of the national park and the subsequent
vegetational recovery. According to Warnock (1970),
the effects of overgrazing in desert scrub can persist
for decades and the processes of succession during
recovery can occur slowly over many years.

The population plots were qualitatively assessed
by L. G. Porter and C. A. Porter in May and November
2008. The brief qualitative observations performed in
2008 were not sufficient to assess the extent of veg¬
etational succession on the plots. However, the data

on vegetation presented in this study provide a valu¬
able baseline for evaluating the successional changes
that have occurred over the past half century on the
population plots. The data already collected would
make these plots an excellent location to pursue studies
of long-term ecological change in the Big Bend area
(Leavitt 2010).

During the 2008 survey, at least one corner stake
on each plot was located, along with many of the
wooden stakes which marked the trap sites (Figs. 7-10;
Plate 2). A large number of wooden stakes were found
on the Eremicus plot, and relatively few on the Nelsoni
plot, perhaps due in part to the difficulty of locating
stakes in the dense vegetation. On the Merriami plot,
stakes were found only in the northern half of the plot,
and some stakes apparently had been dislodged and
washed north of the plot by heavy runoff. One of the
southernmost stakes located on the Merriami plot had
been partially burnt, suggesting that physical evidence
of the southern portion of the plot may have been oblit¬
erated by efforts to control invasive vegetation growing
along the highway.

Baccus (1971) reported increasing ground cover
throughout BBNP during the period 1956-1969. How¬
ever, based on data and photographs from my original
study, L. G. Porter and C. A. Porter did not note sig¬
nificant changes in the topography or vegetation of the
population plots during the 50 years since my initial
evaluation. The soil was not examined in 2008, but the
size, quantity, and distribution of surface rocks matched
my descriptions and photographs (Porter 1962) in this
study. The plants found to be predominating on the
plots in the late 1950’s are still abundant.

The plots were not trapped in 2008, but based on
the apparent stability of the habitat, there is no reason
to suspect that population levels of pocket mice have
changed substantially on the plot. Lizards of the genus
Aspidoscelis were observed in 2008 on the Merriami
and Eremicus plots. In November of that year, cot¬
tontails were seen on the Merriami plot, and collared
peccaries (Pecari tajacu ) were observed adjacent to
the plot, but no other mammals or reptiles were seen
on the plots in 2008.

Porter—Ecology of Pocket Mice in the Big Bend Region

Ectoparasites

The collection of ectoparasites was incidental to
other phases of the study. Specimens collected were
sent to Dr. Richard B. Eads and Dr. John S. Wiseman,
Texas Department of Health, for determination. Chig-
ger mites were sent by them to Richard B. Loomis and
D. A. Crossley, Jr. for identification.

Fleas. —I collected fleas from all three species
of pocket mice, Merriam’s kangaroo rat ( Dipodomys
merriami ), the white-ankled mouse ( Peromyscus
pectoralis), and a ringtail ( Bassariscus astutus). The
occurrence of fleas was most noticeable during the
winter months. Yancey (1997) did not observe fleas
on any pocket mice, although he collected throughout
the winter at Big Bend Ranch State Park. However, he
did note the presence of fleas on Dipodomys merriami
in the Big Bend area (Yancey 1997)

1 collected the stick-tight flea (Echidnophagagal-
linacea) from C. nelsoni but not from any other rodents.
This flea is a common ectoparasite of gallinaceous
birds, but it exhibits little host preference (R. B. Eads,
pers. comm., 16 December 1958). Chaetodipus has
not been previously noted as a host of E , gallinacea.
Whitaker et al. (1993) reported this flea from three
species of kangaroo rats (D. merriami , D. ordii, and
D. spectabilis), but not from any other heteromyids.
I also found E. gallinacea on a ringtail (Bassariscus
astutus) at Green Gulch. Eads and Wiseman (R. B.
Eads, pers. comm,, 10 January 1958) found a raccoon
(Procyon lotor) 16 km west of Marathon (Brewster
Co.) to be heavily infested with E. gallinacea. Both
the ringtail and the raccoon also were parasitized by
the flea Pulex irritans.

I collected a new species of flea ( Meringis agilis
Eads, 1960) from all three species of pocket mice.
Fleas of the genus Meringis are primarily nest inhabit¬
ants, visiting the host only to feed (R. B. Eads, pers.
comm.. 18 March 1959) and are common parasites
of kangaroo rats (Eads 1960; Whitaker et al 1993).
Although pocket mice and kangaroo rats were both
abundant in the habitats sampled, T collected at least
26 specimens of M. agilis from pocket mice and none
from Dipodomys. However, this flea has since been
found on six individuals of D. elator from northern

Texas (Thomas et al. 1990). Because M. agilis was
not collected on Dipodomys in the Big Bend region,
Whitaker et al. (1993) suggested that the species dis¬
plays some host specificity, and Eads (1960) believed
that pocket mice are the normal host of this flea. 1 col¬
lected the holotype, allotype, and all 24 paratypes of M.
agilis during February 1958 and February 1959 (Eads
1960). The type locality of M. agilis is the foothills of
the Chisos Mountains at Panther Junction, though the
locality was previously reported (Eads 1960, Adams
and Lewis 1995) as simply Panther Junction. The type
host is P. merriami and the host of the type specimen
is deposited in the mammalogy collection at Brigham
Young University (BYU 6502). Although Eads (1960)
did not state the depository for the type series of M. agi¬
lis , Adams and Lewis (1995) indicate that the holotype
(USNM 65455) and allotype are deposited in the U. S.
National Museum at the Smithsonian Institution.

I also collected Meringis vitabilis Eads 1960,
another previously undescribed flea, from Dipodomys
merriami. Eads (1960) had earlier examined one
specimen of this flea collected in 1955 by Sherman
Minton from a kangaroo rat in Big Bend. Minton’s
specimen, together with 26 individuals I collected,
form the type series for M. vitabilis (Eads 1960). Eads
(1960) deposited the type specimens of M. vitabilis
in the U. S. National Museum The holotype of M.
vitabilis is USNM 65454 (Adams and Lewis 1995).
The type locality (not previously published in full) is
Tornillo Flat, 3.2 km south of Tornillo Creek along the
Marathon Highway.

Eads (I960) did not indicate which species of
kangaroo rat was the host of the paratype of M. vitabilis
collected by Minton, but Minton informed Eads (R.
B. Eads, pers. comm., 28 October 1957) that the flea
was collected from D. ordii near the “pipeline house”
on the Olin Blanks Ranch at the northern base of the
Rosillos Mountains, and that the host was identified by
the University of Michigan Museum of Zoology. The
host data provided by Minton correspond exactly to
specimen UMMZ 103322 in the University of Michigan
mammal collection. As recently as February 2008, this
specimen remained identified as D. ordii; however Pris¬
cilla Tucker (pers. comm, to C. A. Porter, 5 February

Special Publications, Museum of Texas Tech University

2008) examined the specimen, and found that it was
Dipodomys merriami. Two other kangaroo rats in the
UMMZ collected at Big Bend by Minton also are also
D. merriami. It is clear, therefore that Minton’s speci¬
men was actually collected from D. merriami , and thus
Merriam’s kangaroo rat is the host of the entire type
series. Eads et al. (1987) did report M. vitabilis from
D. ordii collected in 1950 from Motley County, Texas,
though they mistakenly credited me as the collector.

I collected six specimens of the flea Malaraeus
sinomus from Peromyscus pectoralis at Green Gulch.
This flea is uncommon in Texas (Eads and Dalquest
1954), but is known from a variety of hosts elsewhere
(Whitaker 1968; Whitaker et al. 1993).

Sucking Lice .—The only louse found on mam¬
mals during the study was Fahrenholzia pinnata, col¬
lected from four specimens of Merriam’s kangaroo rat
in the vicinity ofTornillo Flat. This louse is common
on many species of kangaroo rats and has been recorded
from some pocket mice (Thomas et al. 1990; Whitaker
et al. 1993; Light and Hafner 2007). Light and Hafner
(2007) reported lice of this genus from P. merriami
(in Coahuila) and from C. eremicus (southern New
Mexico). No lice of the genus Fahrenholzia have been
reported from C. nelsoni , and the pocket mice examined
in this study did not host any louse species. Light and
Hafner (2007) found evidence of cryptic species in lice
identified as F. pinnata.

Ticks and Mites .—The larvae and nymphs of ticks
(Dermacentorvariabilis) were taken h orn all three spe¬
cies of pocket mouse and from Merriam’s kangaroo rat.
Adult stages of this tick usually parasitize carnivores
(R. B. Eads, pers. comm., 18 March 1959). Whitaker
et al. (1993) report P. parvus as the only known het-
eromyid host of this species.

Several species of mites were also taken. The
large mite, Androlaelaps grandiculatus, was originally
described from P. merriami (Eads 1951), and was by
far the most common parasite found on pocket mice
during this study. This species was taken from all three
pocket mice and from Merriam’s kangaroo rat. Other
mites collected in this study include Androlaelaps
fahrenholzi (from C. nelsoni and Dipodomys merriami )
and Echinonyssus incomptis (from C. nelsoni). Three

mites from Merriam’s kangaroo rat were identified by
Eads as an undescribed species of Hirstonyssus (genus
Echinonyssus in current taxonomy). The specimens
were sent to Richard W. Strandtmann (Texas Tech
University) for further study, but only one specimen
was an adult, and it was determined that a larger se¬
ries would be needed to describe the species. Yancey
(1997) found unidentified mites on all three species
of pocket mice at Big Bend Ranch, and indicated that
mites were typically found on the tail in Chaetodipus.
Baccus (1971) reported that chiggers in P. merriami
were usually concentrated in the soft tissues of the
cheek pouches.

I collected chigger mites from several mammalian
and avian species in BBNP. The chiggers taken from
pocket mice include Kayella lacerta and Euschoengas-
toides hoplcii (my specimens were reported by Loomis
and Crossley 1963). Other chigger specimens from
pocket mice were identified (R. B. Eads, pers. comm.,
3 April 1959; Porter 1962) as representing two unde¬
scribed species of the genera Trombicnla (subgenus
Trombicula) and Euschoengastia. These specimens
were sent by Eads to Richard Loomis (University of
Kansas), and if the specimens are still in existence, they
would likely be included in the extensive Loomis col¬
lection housed at the Field Museum of Natural History.
There is no record that species were described from
these particular specimens. Extensive subsequent stud¬
ies of Big Bend chiggers have been reported by Loomis
and Crossley (1963), Baccus (1971), Loomis et al.
(1972), Loomis and Wrenn (1972,1973), Whitaker and
Easterla (1975 ), and Wrenn et al. (1976), and it is likely
that the undescribed species I collected are among the
species reported and described by these authors.

Euschoengastia ch iso sens is was reported from
Big Bend by Loomis et al. (1972) and described by
Wrenn et al. (1976). Loomis et al. (1972) also reported
an undescribed species of Pseudoschoengastia from all
three species of pocket mice in Big Bend. Although E.
chisosensis has only been reported from Peromyscus ,
either of these species could have been identified in
1959 (Brennan and Jones 1959) as an undescribed
species of Euschoengastia, and are possible candidates
for the undescribed Euschoengastia I collected from
pocket mice.

Porter—Ecology of Pocket Mice in the Big Bend Region

Several species of chiggers have been collected
from Big Bend that were undescribed in 1959 (Brennan
and Jones 1959; Loomis 1971) and plausibly could have
been identified at that time as an undescribed species
of Trombicula ( Trombicula). These include Hexidionis
breviseta (Loomis and Crossley 1963; Loomis and
Wrenn 1972), Otorhinophila baconsi (Loomis et al.
1972; Loomis and Wrenn 1973), Euschoengastoides
arizonae, E. neotomae, E. loom isi, and an undescribed
species of Euschoengastoides (Baccus 1971; Loomis
etal. 1972). The undescribed Euschoengastoides was
provisionally named “similis” by Baccus (1971) and
Loomis et al. (1972), but the species was not formally
described, and since the name did not include a diag¬
nosis, it remains a nomen nudum. In addition, three
species of Trombicula were reported (Loomis and
Wrenn 1972) from the bat Mormoops megalophylla
in BBNP.

Loomis et al. (1972) and Loomis and Wrenn
(1972) report 51 species of chiggers collected from in
or near BBNP. These were collected from amphibian,
reptilian, avian, and mammalian hosts, and include 21
specimens (reported by Loomis and Crossley 1963)

of Neoschoengastia americana that I collected from a
rock wren ( Salpinctes obsoletus) in Oak Creek Canyon.
Loomis and Crossley (1963) also report 15 specimens
of Sasacarus whartoni that I collected from Peromy-
scus eremicus (specimen BYU 6440) at Tomillo Flat,
10 km north of Panther Junction. Baccus (1971) and
Loomis et al. (1972) report the same chigger species
from both Peromyscus eremicus and P. pectoralis in
Big Bend. Chiggers reported (Porter 1962; Loomis
and Crossley 1963; Baccus 1971; Loomis et al. 1972;
Loomis and Wrenn 1972, 1973) from pocket mice in
or near BBNP include Euschoengastoides arizonae,
E. hoplai, E. loom isi, E. n. sp. (“similis”), Hexidionis
allredi, Hyponeocula arenicola, Hyponeocula n. sp.,
Leptotrombidium panamense, Kayella lacerta, O. bac-
cusi, Pseudoschoengastia n. sp. (from all three species
of pocket mice), Euschoengastoides neotomae (from
both species of Chaetodipus ), Hexidionis harveyi (from
P. merriami and C. eremicus ), Fonsecia gurneyi, Pseu¬
doschoengastia farneri , Hexidionis breviseta (from
P. merriami ), Pseudoschoengastia hungerfordi (from
C. nelsoni), and Euschoengastoides imperfectus , and
Hexidionis sp. (from C. eremicus ).

Altitudinal Distribution of Pocket Mice

The altitudinal distribution of pocket mice is
correlated with the occurrence of suitable habitat.
Hence, at a specific elevation they may be abundant
in one locality and uncommon in another, depending
on the nature of the habitat. In BBNP, C. eremicus
apparently is limited in its upward distribution by the
absence of deep sandy or loamy soils. Merriam’s and
Nelson’s pocket mice, on the other hand, appear to be
limited in their upward distribution by vegetational
influences. The maximal altitude of P. merriami and C.
nelsoni coincide with the ecotone between desert scrub
vegetation and the pine-oak-juniper woodlands. For
example, both of these species were trapped in small
numbers at the upper end of Green Gulch at an elevation
of 1,675 m on a grassy alluvial bench surrounded by
sparse stands of pine, oak, and juniper where Sigmodon
was abundant.

In the study area, Merriam’s pocket mouse ranges
from 580 m near the Rio Grande to 1,675 m in Green
Gulch. It is most abundant on alluvial soils in the

foothills of the Chisos Mountains between 1,075 and

l, 220 m. Baccus (1971) reported a similar altitudinal
range in BBNP. To my knowledge, the maximal alti¬
tudinal record for P. merriami is 1,905 m in Guadalupe
Mountains National Park, Texas (Genoways et al.
1979). Baker (1956) reported P. m. merriami from as
low as 250 m in Coahuila.

In Big Bend, I found that the altitudinal range of
Nelson’s pocket mouse extended from 580 m to 1,675

m, but was most abundant on the steeper slopes of the
Chisos foothills near 915 m. Judd (1967) also collected
the species up to 1,675 in in rocky habitat at Blue Creek
Ranch, and Baccus (1971) found C. nelsoni in the same
general attitudinal range in Big Bend, but collected
the species as high as the Chisos Basin Campground
at 1,700 m. Blair (1940) reported three specimens of
C. nelsoni at 1,460 m in Lunpia Canyon about 1.6 km
north of Fort Davis, Texas. In Coahuila, Baker (1956)
collected specimens of C. nelsoni canescens at eleva¬
tions up to 1,460 m and of C. nelsoni nelsoni up to 2,060

Special Publications, Museum of Texas Tech University

m. In Durango, Baker and Greer (1962) reported C. n.
nelsoni between 1,400 and 1,860 m.

In the study area, C. eremicus reached its great¬
est abundance above 760 m, but it was common up to
about 910 m. It was taken in small numbers at about
1,220 m in the foothills of the Chisos Mountains. Borell
and Bryant (1942). trapped Chihuahuan pocket mice
at elevations up to 1,220 m in the Big Bend area, but

they found them to be most plentiful below 1,070 m.
Baccus (1971) reported C. eremicus in BBNP mostly
at 560-870 m, but found specimens as high as 1,075 m.
Genoways et al. (1979) collected the species up to 1,646
m in the Guadalupe Mountains. Baker (1956) took C.
eremicus at elevations as low as 400 m and as high as
1,585 m in Coahuila In Durango, the species was col¬
lected at 1,140-1,265 m (Baker and Greer 1962).

Pocket Mice in Relation to Habitat

It has long been recognized that the various char¬
acteristics of substrate and topography influence the
distribution of plants and animals. The close relation¬
ship between the soil and its plant cover is shown by
the fact that some species of plants may be indicators
of certain soil characteristics (Clements 1920; Shantz
1938). The relationship between the soil and verte¬
brates, however, is more indirect and complex than
that between soil and plants, since the distribution of
many vertebrates is related to both soil and plant cover.
It was noted early in this investigation that each of the
three species of pocket mouse had its peculiar require¬
ments of substrate and vegetation. Hence, the research
was directed so as to delineate these requirements, and
habitat plots were sampled in a variety of habitat types
(Plates 6-8; 10-16).

Hardy (1945) analyzed the influence of types
of soil on the local distribution of small mammals in
southwestern Utah and determined that depth of soil,
degree of slope, and the chemistry and texture of soil
directly influenced the distribution of certain species.
These factors along with moisture content of the soil
indirectly influenced the distribution of other species.
Fossorial and burrowing rodents appear to be influenced
by soil type more than those that live on the surface.
Davis (1938), for example, found a direct relationship
between soil texture, soil depth and altitude and the
size of pocket gophers ( Thomomys) in various parts of
the western United States. Davis et al. (1938) found
that the distribution of pocket gophers ( Geornys) in
Texas is correlated with the distribution of fine sandy
loams over 10 cm in depth. Grinnell (1932) found a
direct correlation between the distribution of Panoche
fine sandy loam in California and the kangaroo rat
Dipodomys heermanni .

The distribution of some non-burrowing species
of rodents also is influenced directly by soil type. The
canyon mouse ( Peromyscus crmitus), the brush mouse
(P. truei ), and the bushy-tailed woodrat ( Neotoma
cinerea ) are all associated with rocky habitats. The
relationship between soil and habitat preference is cor¬
related with the nesting characteristics and food storage
habits of the mice. Plants influence the distribution of
vertebrate animals in that they provide food, nesting
sites, shelter from the elements and escape cover.

Distribution and Abundance of Pocket Mice in
Relation to Slope

In southwestern Utah, Hardy (1945) found that
steeper slopes usually have soils of a coarser texture
than those of more nearly level areas. In Big Bend the
relationship between soil texture and gradient is not
always apparent because slopes are often interrupted
by local differences in topography and parental soil
materials. Nevertheless, the soils of the flood plains
adjacent to the Rio Grande with a gradient of 1 % or less
are fine loams that frequently contain 40-50% silt and
more than 20% clay. Most other soils in the Big Bend
area are sandy loams and sandy clay loams, depending
upon their locations in relation to topographic features
such as mountains, hills, rock outcrops, etc.

Hardy (1945) reported that in southwestern Utah
in stony soils, topsoil particles of granule gravel varied
in amount according to the gradient if the parental
materials were similar. In the present study a similar
correlation was noted in the cobbles and gravel present
at different gradients (Fig. 12).

Porter—Ecology of Pocket Mice in the Big Bend Region

Plate 10. Sandy wash habitat (Plot 76 near Tomillo
Creek and Rio Grande). Chihuahuan pocket
mouse habitat. Note the rocky ground which
distinguishes this from the rock-free flats habitat.
Photograph by R. D. Porter, Spring 1960.

Plate 11. Rock-free flats habitat. The Chihuahuan pocket mouse was common in these areas. See also
Plate 8 (Eremicus plot) for another example of this habitat. A. Plot 77, Lower Tornillo Creek, near the
bridge. Sandy loam soil. Creosotebush (Larrea tridentata ) in foreground; Sierra del Carmen of Coahuila,
Mexico in background. B. Plot 78, Boquillas Village. Loam soil. Note large clumps of mesquite (Prosopis
glandulosa ) surrounded by extensive areas devoid of understory plants. C. Plot 71, Upper Tornillo Creek.
Loam soil. Note clumps of mesquite surrounded by areas devoid of understory vegetation. Chisos Mountains
in background. D. Plot 32, near San Vicente. Photographs by R. D. Porter, Spring 1960.

Special Publications, Museum of Texas Tech University

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Porter—Ecology of Pocket Mice in the Big Bend Region

Plate 15. Coarse stony sandy hillside habitat. A. Plot 40. Note the paucity of understory cover. Nelson’s and Merriam’s pocket mice
were equally common on this plot. B. Surface rocks on Plot 40. C. The cactus mouse, Peromyscus eremicus , reached its peak of
abundance in this habitat. D. Tasajillo (Opuntia leptocaulis) is a common overstory species in this habitat. E. Though the understory is
sparse, the most abundant understory species in this habitat was chino grass (Bouteloua breviseta) as in other coarse rocky habitats (Plates
7, 13, 14, and 16). Photographs A-B by R. D. Porter, Spring 1960; C-D, 1958. Photograph E by C. A. Porter, November 2008.

Special Publications, Museum of Texas Tech University

Plate 16. Rough-broken mountainside habitat (Plot 75).
A. Note large boulders and the density of understory
contrasted with the coarse stony sandy hillsides shown
in Plate 15. Nelson’s pocket mouse was the only pocket
mouse trapped on this plot. B. Surface rocks. Note
Engelmann’s prickly pear ( Opuntia engelmannii ) in the
upper right corner. Photographs by R. D. Porter, Spring
1960.

There was correlation between slope and plant
density. Slopes with a gradient of <3% had the fewest
plants per square meter, whereas slopes with gradients
of 3-6% and 20-30% had the most cover (Fig. 12).
The high plant density on sites with gradients of 3-6%
was due to an abundance of fluff grass and other small
plants which covered little surface area (Appendix
II). An increase in slope brought about an increase
in the amount of surface area at ground level covered
by plants. Steeper gradients also showed an increase
in the number of plant species (Fig. 12). There was a
relationship between slope and certain plant species.
For example, mesquite and creosotebush were com¬
monest on slopes of 1% or less. Plants abundant mostly

on steeper slopes were chino grass, Parry’s ruellia
(RueIlia parryi), and leather plant (Jatropha dioica ).
Fechuguilla was common in all areas except where the
gradient was zero to 1%, but it was most abundant on
slopes of 20-26%.

The distribution of the three species of pocket
mice was correlated with gradient (Fig. 13). Merriam’s
pocket mouse was found on a wide range of slopes,
most commonly on gradients of 3-30% and reached
its peak of abundance on slopes of 3-5% (Fig. 13). It
occurred commonly on steep slopes providing the fol¬
lowing conditions prevailed: (1) there was not a high
percentage of large rocks (i.e., cobbles and boulders)
either on the surface or in the soil, and/or (2) the under¬
story vegetation was sparse with rather extensive bare
areas interspersed with the plant cover. For example,
Plot 40 (Plate 15A-B) had a gradient of 47%, relatively
sparse understory (0.47 plants/m 2 ), and large boulders
(1,852 boulders per hectare). This plot had relatively
low numbers of C. nelsoni and P. merriami (0.8 and
0.6 individuals, respectively, per 100 trap nights). On
the other hand, on Plot 69 (Plate 13C-D), which had
a gradient of 20%, a sparse understory (0.52 plants/
m 2 ), and few large rocks, Merriam’s pocket mouse
was relatively abundant (3.7 individuals per 100 trap
nights). Plot 62 (Plate 13A-B; Table 6), which had
similar characteristics of substrate and slope (26%) but
a denser understory (9.04 plants/m 2 ), also supported a
large population of P. merriami (5.4 individuals per
100 trap nights).

Nelson’s pocket mouse attained its peak on gra¬
dients of 30-40% (Fig. 13). I did not find C. nelsoni
on slopes of <1%, but it was occasionally captured
on slopes of 2-10% if large rocks or dense stands of
vegetation, or both, were present. For example, Plot 82
(Plate 14B, E) had a gradient of 5% and an estimated
430 boulders >40 cm in diameter per hectare. This plot
yielded 5.6, 2.2 and 1.1T! merriami , C. nelsoni and C.
eremicus , respectively, per 100 trap nights (Table 6).
In August 1959, another plot (Plot 101, east of Dugout
Wells; Plate 12A, E) with a gradient of only 3% and
fine gravelly loam (few rocks >38 mm) yielded 5.6,
10.0 and 1.1T! merriami , C. nelsoni and C. eremicus ,
respectively, per 100 trap nights. The understory veg¬
etation of this plot, chiefly lechuguilla and chino grass,
was sparse (0.47 plants/m 2 ). The high population of C.

Porter—Ecology of Pocket Mice in the Big Bend Region

Figure 12. Percentage gradient in the habitat plots plotted against (1) plant density; (2)
percentage gravel; (3) average number of cobbles (rocks 7,5 to 40 cm in diameter) per 20-liter
sample of substrate; and (4) average number of plant species. Circled values indicate the
number of plots sampled for each range of gradient.

P. merriami

Figure 13. Abundance of three species of pocket mice compared
with slope on the habitat plots.

Special Publications, Museum of Texas Tech University

Table 6. Characteristics of habitat plots having 3.7 or more under story plants per square meter.

Understory
Plants per
m 2

Percentage Composition

Individuals per 100 Trap Nights

Plot

Substrate

Slope

(%)

Lechuguilla

Chino

Grass

Fluff

Grass

merriami

nelsoni

eremicus

Gravel

3.72

3.7

Cobbles

4.43

0.6

3.3

Boulders

5.28

3.3

Gravel

7.97

3.9

2.2

Gravel

9.04

5.4

0.6

Cobbles and Boulders

10.35

5.6

2.2

1.1

Gravel

13.35

2.3

0.9

nelsoni at Dugout Wells may be attributed either to the
relative abundance of chino grass and lechuguilla or
to population pressures which may have caused them
to migrate from their normal habitat. Ten years after
my study, Baccus (1971) found similar conditions at
Dugout Wells.

The Chihuahuan pocket mouse (C. eremicus)
was most abundant where the slope was <2% (Fig. 13).
None were taken where the gradient was >9%. This
species apparently does not favor steep slopes since
none were collected on slopes containing fine powdery
soils with fine gravels and sparse vegetation (Plot 69;
Plate 13C-D) or slopes with soft sandy soils, sparse
vegetation and larger rocks (Plot 40; Plate 15A-B),
immediately adjacent to sandy loam flats containing
high populations of Chihuahuan pocket mouse. That
individuals of this species may occasionally venture
onto steep slopes, however, is illustrated by one marked
animal from the Eremicus plot which was trapped once
on the bluffs surrounding the plot.

Distribution and Abundance of Pocket Mice in Rela¬
tion to Soil Texture

Perognathus merriami. —Although this species
was captured on all soil types (Fig. 14A) in the Big Bend
area, it attains it peak of abundance on sandy loams, but
was also common on the more compact soils containing
a relatively high percentage of clay. For example, the
soils of the Merriami plot were primarily sandy clay

loam (Fig. 9). These clayish soils are tighter and less
easily dug by rodents than the looser sandy loams.

Borell and Bryant (1942) trapped only four
specimens of this rodent in Big Bend, from a sand
flat near Mariscal Mountain. Although they trapped
in many sandy areas, they were unable to catch other
specimens. They concluded that tracts of hard rocky
soil probably are probable barriers to the dispersal of
P. merriami.

Denyes (1951) stated in her unpublished thesis
that P. merriami is restricted to soft soils, though in the
published account (1956) she recorded this mouse on
both hard and soft soils. For example, she listed 20
individuals trapped on the gravelly and stony loams
(Reeves series ) of the Creosote-Ocotillo-Mesquite as¬
sociation, which is a greater number than she reported
for any other association. She indicated that P. mer¬
riami was abundant in this association, although the
soil was fairly hard.

Denyes (1954) showed experimentally that P.
merriami can construct burrows in hard, dried clay loam
soils by chewing its way through the hard outer crust.
She believes that the reason P. merriami “are seldom
taken on hard soils is probably a result of the oppor¬
tunity to select more pliable soils in nearby habitats in
relation to the balance between other factors which may
be used as keys to selection.” Baker (1956) apparently
misinterpreted Denyes (1954) when he stated that she

Average number of individuals captured per 100 trap nights

Porter—Ecology of Pocket Mice in the Big Bend Region

per 20 liters of substrate per 20 liters of substrate

Figure 14. Abundance of three species of pocket mice compared with (A) soil texture, (B) average density of cobbles
(rocks ranging in diameter from 7.5 to 40 cm) exposed on the surface, (C) percentage of rocks in the substrate, and (D)
average number of cobbles per 20-liter substrate sample. Circled values indicate number of habitat plots sampled.

Special Publications, Museum of Texas Tech University

had shown experimentally that P. merriami is unable
to make burrows in hard soils. Using the statements
of Denyes (1954) and Borell and Bryant as evidence,
Baker stated that heavy, rocky soils are barriers to the
dispersal of this species in Coahuila, Mexico.

Results of my investigation reveal that the types
of soil which Borell and Bryant (1942), Denyes (1951),
and Baker (1956) indicated are barriers to the dispersal
of this species are, in fact, its preferred types in the Big
Bend area. Blair (1952) states that in southern Texas
P. merriami shows no apparent preference for soil type
and occurs in abundance on soils ranging from tight
clays and caliche to deep sands. Baccus (1971) reported
P. merriami in BBNP primarily from clay-loam soils,
but also from sand.

Snap traps are ineffective in capturing P mer¬
riami , which may account for the small numbers of
the species captured by other investigators (Borell and
Bry ant 1942) in the Big Bend area. Although Denyes
(1956) took specimens of P. merriami on nearly all
types of soil, most individuals apparently were taken
on silty clays and sandy loams. Baker (1956) believes
that in Mexico this mouse probably occurs on alluvial
soils bordering arroyos.

Chaetodipus nelsoni. —Nelson’s pocket mice
were not present on either deep sand or loam soils
but were most abundant on shallow sandy loams and
sandy clay loams (Fig. 14A). Denyes (1956) took
them on loams, silt clay loams, fine sandy and silt
limestones, clay loams and silt loams. Baker (1956)
rarely collected C. nelsoni on sandy or other fine soils,
and Borell and Bryant (1942) obtained no spec miens
from sandy washes. Baccus (1971) reported the spe¬
cies from sand, loam, and clay loam, when rocks and
gravel were present.

Chaetodipus eremicus. —This mouse reached its
peak of abundance on sands and loams and especially
on the deep loams of the Rio Grande flood plain and
the wide, dry, sandy washes of the arroyos running
into the Rio Grande. Boeer and Schmidly (1977)
found C, eremicus to be extremely abundant along the
Rio Grande. Figure 14A indicates that C. eremicus
reached its peak of abundance on sand. This, however,
is a biased estimate because it represents the number
of animals captured on a single plot. Denyes (1956)

trapped C. eremicus in greatest numbers on loams,
sandy loams and clay loams, and Baccus (1971) found
the species in sandy soils.

Distribution and Abundance of Pocket Mice in Rela¬
tion to Rock Content of the Substrate

As shown previously, the distribution and size of
rocks in the soil are directly correlated with degree of
slope. The distribution of pocket mice is better corre¬
lated with the number and size of rocks than with soil
texture (Fig. 14B-D).

Merriam’s pocket mouse is intermediate between
C. nelsoni and C. eremicus in its preference for stony
substrates (Fig. 14B-D; compare Plate 7C with Plates
6C and 8C). It was infrequently encountered on sub¬
strates containing either a large percentage of rocks (81-
90%) or on those free of rocks. Although P. merriami
reached its peak of abundance in substrates composed
of 50-60% rocks, it was common on the coarse cobble
outwashes (coarse gravelly foothills) extending from
the canyons of the Chisos Mountains. Usually 500 or
more boulders >40 cm in diameter per hectare were
present in these areas (Fig. 15). Nelson’s pocket mouse
was equally abundant on these sites; the Chihuahuan
pocket mouse was also present, but in smaller numbers
(Fig. 16). Judd (1967), Baccus (1971), and Yancey et
al. (2006) confirmed the occurrence of P. merriami
in gravelly substrates in Big Bend. Wu et al. (1996)
collected P merriami from rock-free substrates in the
Trans-Pecos, and reported a similar low density of P.
merriami (<1 mouse per 100 trap nights) as I found
(Figs. 14, 16) in rock-free habitats in Big Bend. The
vegetation and substrate of Wu et al.’s (1996) study
sites, while not its most preferred habitat, are well
within the broad habitat tolerances of Merriam’s pocket
mouse as identified in this study.

Chaetodipus nelsoni reached its maximum abun¬
dance on substrates containing 80-90% rock (Fig. 14C;
Plate 8C) with 3-5 cobbles per 20-liter sample (Fig.
14D) and 10-22 cobbles per square meter of surface area
(Fig. 14B). On some sites, such as talus slopes (rough-
broken mountain) and coarse gravelly outwashes, there
were 5,000 or more boulders >40 cm in diameter per
hectare (Fig. 15). Some boulders, especially on talus
slopes, had diameters of >1.5 m (Fig. 15; Plate 16B).

Porter—Ecology of Pocket Mice in the Big Bend Region

Figure 15. Density of boulders (>40 cm in diameter), mean boulder size, and the percentage
gradient compared with four habitat types in the Big Bend region.

The Chihuahuan pocket mouse reached its peak
of abundance on deep soils free of rocks or nearly so
(Fig. 14B-D; Plate 6A, C, E). It was noticeably less
common where rocks comprised more than 50% (Fig.
14C) of the samples. Though less common, its range
extended into the coarse rocky foothills (outwashes of
the higher foothills) where loose sandy gravelly areas
were interspersed with cobbles and small boulders (Fig.
16). On such sites all three species of pocket mice were
occasionally encountered.

Distribution and Abundance of Pocket Mice in Rela¬
tion to Vegetation Density

Features of the plant community which influence
the distribution of pocket mice most are (1) propor¬
tion of understory plants to those in the overstory,
(2) relative density and dominance of these plants in
relation to the amount of bare ground and (3) general
physiognomy of the plants, especially their height and
their basal diameters.

Merriam’s pocket mouse reached a peak of abun¬
dance in areas with a relatively sparse plant understory.

0.25-0.75 plants/m 2 (Fig. 17), and with at least 50%
of the surface area free of plant cover (Merriami plot,
Table 3; Plate 7). This species reached a second peak of
abundance on some plots with high understory density.
However, on plots with high populations of.P. merriami
and with 8-15 understory plants/m 2 (Fig. 17A), fluff
grass was the most numerous plant. Apparently these
short plants, each of which covers a relatively small
surface area (Appendix II; Plates 3,7), do not influence
the distribution of this mouse as adversely as do taller
plants with a similar density. Plots 74, 13, and 6 had a
relatively dense understory composed largely of fluff
grass (Table 6). All three plots had high populations
of P. merriami. The large catch of P. merriami on
Plot 62 (with a 26% gradient) was probably due to the
predominance of small gravel in the soil (Plate 13B).
In spite of the high relative density of lechuguilla and
fluff grass, ground cover on this plot was sparse.

Nelson’s pocket mouse attained a maximum
density in areas where the understory was dense and
the average plant height was 30-63 cm (lechuguilla
and chino grass, respectively). In addition, understory
plants outnumbered overstory plants nearly 18:1. On

Average number of Individuals captured per 100 trap nights

Special Publications, Museum of Texas Tech University

Figure 16. Average number (individuals per 100 trap nights) of five species of rodents captured on nine habitat types
in the Big Bend area. Circled numbers are the number of plots represented in the sample.

Porter—Ecology of Pocket Mice in the Big Bend Region

Figure 17, Abundance of pocket mice captured on the habitat plots compared with density of individual plants in the
understory (A) and overstory (B). Circled numbers represent the number of plots sampled.

the habitat plots where the relative density of lechu-
guilla and chino grass was greatest (plots 81, 29, 82
and 13), populations of C. nelsoni were also high
(Table 6).

The Chihuahuan pocket mouse reached its
maximum abundance in habitat which had little ground
cover (usually fewer than 0.25 understory plants/m 2 ;
Fig. 17A) and extensive areas (usually much more than
50%) of bare ground. On the Eremicus plot, under-
story plants covered less than twice as much surface
as the overstory. On most plots having an abundance
of Chihuahuan pocket mice, the overstory appeared to
cover more surface area at ground level than did the
understory (Plate 11).

Distribution and Abundance of Pocket Mice in rela¬
tion to Plant Species

There were fewer plant species on plots where
large numbers of C. eremicus were captured than on
those where captures were few. The reverse prevailed
where Nelson's pocket mouse was abundant. Plots
where Merriam’s pocket mouse was taken assumed an
intermediate position (see Figure 18 in Porter 1962).
The physical aspects of the understory appear to exert

a greater influence on the distribution of pocket mice
than do those of the overstory.

In Porter (1962) I present a detailed analysis of
the influence of selected plant species on pocket mouse
distribution and how the various plants are used by
pocket mice. Sotol, tasajillo, prickly pear, and tarbush
were not strongly correlated with pocket mouse distri¬
bution. Other plant species were commonly found in
areas favored by certain species of pocket mice, but
it is probable that the distribution of pocket mice is
influenced by the entire complex of vegetation, soil,
slope, exposure and climate. There was an inverse
or negative correlation between abundance of chino
grass (Plate 15E) and that of Merriam's pocket mouse.
The denser the stands of chino grass, the smaller was
the population of P. merriami. Chihuahuan pocket
mice seldom were present in chino grass areas, but C
nelsoni often reached maximum abundance in dense
stands of this grass.

Lechuguilla (Plate 9) was present in the habitats
of all three species of pocket mice, but in varying
degrees of density and dominance. Populations of C.
nelsoni frequently attained maximum density in heavy
stands of lechuguilla if other habitat requirements were

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suitable. Although lechuguilla apparently is a preferred
understory plant for C. nelsoni , it is by no means a
necessary one, since dense stands of chino grass or any
species of plant with a similar physiognomy, such as
false agave ( Hechtia texensis ) and wax plant ( Euphor¬
bia antisyphilitica ), may serve as a substitute.

The Chihuahuan pocket mouse normally did not
occur in dense stands of lechuguilla. However, it was
abundant on Plot 24 which had a high relative density
(45%) of lechuguilla, but there were only 0.18 under¬
story plants/m 2 on this plot, 45% of which consisted
of lechuguilla, mainly restricted to one edge of the
plot, and 17% of leather plant. The overstory density
was 0.43 plants/m 2 , of which 45% were creosotebush.
Hence, in the areas of the plot where C. eremicus
was captured there were large areas of powdery, fine,
gravelly sandy loam nearly devoid of ground cover.
Adjacent to the plot was a dry wash and a slope leading
to the wash containing boulders as large as 50-75 cm in
diameter The presence of the slope, the large rocks, and
the lechuguilla probably accounted for the large number
of C. nelsoni taken on this area of the plot. Lechuguilla
was a common plant in most areas inhabited by P. mer-
riami, but this mouse usually was not common where
lechuguilla formed a dense ground cover.

Discussions and Comparisons of Habitat Prefer¬
ences of Pocket Mice

Chaetodipus nelsoni. —Nelson’s pocket mouse
attained its peak of abundance on the steeper rocky
slopes (30-40% gradient) containing sandy loam
and sandy clay loam soils. The substrate, which was
frequently shallow with numerous boulders and rock
outcrops breaking through the surface, contained 80-
90% rock. There were three to four cobbles per 20-liter
sample and 10.8 to 21.5 cobbles per square meter of
surface area. In areas of maximum abundance of C.
nelsoni there were 500 to 14,300 or more boulders >40
cm in diameter per hectare (i.e., a maximum of nearly
1.5 boulders perm 2 ).

Ground cover in areas of maximum abundance
of Nelson’s pocket mouse usually was dense (well
over 8 plants/m 2 ). Over 60% of the surface area usu¬
ally was covered at ground level with vegetation. The

major understory plants (lechuguilla and chino grass)
in habitats preferred by C. nelsoni were usually 30-50
cm in height.

Habitats of C. nelsoni generally contained many
more plant species than habitats where C. eremicus
was abundant. The dominant understory plants were
lechuguilla and chino grass. Overstory species were
variable, but sotol and prickly pear generally were pres¬
ent. Steep slopes with sparse cover and fine gravel or
with fine gravels and dense cover yielded small popula¬
tions of Nelson's pocket mouse. This mouse was also
common on steep slopes with large rocks and sparse
cover. Thus rock size is probably of more importance in
determining the abundance and distribution of Nelson’s
pocket mouse than slope or density of vegetation. C.
nelsoni was not restricted to steep slopes; it occurred
commonly on slopes with 3-6% gradient provided
large rocks or dense vegetation, or both, were present.
Nelson’s pocket mouse was rarely found on the deep
sandy loam and loam flats occupied by C eremicus,
but it was occasionally taken with C. eremicus on deep
fine gravelly sandy loams (few cobbles, loose powdery
soil) when vegetation consisted of sparse stands of
lechuguilla.

Chaetodipus eremicus. —This species reached its
greatest abundance on deep sandy loam or loam flats
with less than 2% gradient, usually containing <1%
rock by weight. C. eremicus was abundant in areas
where the understory plants were sparse (frequently
<0.25 plants/m 2 ), and the overstory vegetation was
generally denser than the understory. Extensive areas
of bare ground were exposed between the sparse clumps
of vegetation, which usually covered less than 25% of
the total surface area.

Habitats where C. eremicus was most abundant
had fewer plant species than habitats preferred by the
other two species. Short (<12 cm) understory plants,
such as ground cholla and fluff grass which clumped
together so that considerable bare ground was exposed
between dumps, apparently did not interfere with the
distribution and abundance of this mouse. Although
not restricted to any particular plant association, C.
eremicus was most abundant in habitats containing
creosotebush and mesquite.

Porter—Ecology of Pocket Mice in the Big Bend Region

The Chihuahuan pocket mouse was seldom
found on steep slopes even though conditions of soil
and vegetation appeared suitable. The combination of
steep slopes (over 9% gradient), large rocks and dense
vegetation appears to limit severely the distribution
and abundance of this mouse. However, it was taken
in small numbers in coarse rocky outwashes from the
canyons of the Chisos Mountains in areas of >500
boulders per hectare. Yancey et al. (2006) found C.
eremicus primarily on rock-free sandy soils in BBNP,
though they reported the species occasionally in grav¬
elly habitats.

Perognathus merriami. —This species reached its
peak abundance on slopes with a gradient of about 5%
having fine gravelly, sandy loam soils. Usually 50-60%
of the substrate consisted of gravel with rocks seldom
>7.5 cm in diameter. The sparse understory vegeta¬
tion on preferred sites usually had 0.25-0.75 plants/
m 2 if the ground cover were 25 cm or greater in height
(lechuguilla) and as many as 7.4 or more plants/m 2 if
the understory were less than 13-15 cm in height (fluff
grass). This mouse attained its maximum abundance
in habitats in which 50-60% of the total surface area
was covered by vegetation at ground level.

The most frequent understory cover in preferred
habitats was lechuguilla and fluff grass. The most
frequent overstory plants were tarbush and mariola.
These sites usually had more plant species than were
found in habitats preferred by C. eremicus. MerrianTs
Pocket Mouse occurred uncommonly in habitats having
sparse plant density (<0.25 plants/m 2 ) and deep loam
and sandy loam soils. At the other extreme, it was
also uncommon on steep slopes where large rocks and
dense (>2.2 plants/m 2 ), tall (at least 25 cm) vegetation
were predominant. Steep slopes (10-30%) did not limit
either the abundance or the geographic distribution of
this mouse when soil and vegetation were suitable.
For example, it was present in small numbers on steep
slopes containing large rocks when the density of the
vegetation was sparse and the soil deep.

Perognathus merriami had a wide range of habitat
tolerance in the Big Bend area (Fig. 16), which seems
to be typical in other parts of the range. Blair (1940)
likewise noted that it occupied a wide range of habitat
conditions in the Davis Mountains of western Texas,
and in southern Texas. In the Panhandle of Texas, Blair

(1954) observed a similarly large range of tolerance,
and Judd (1967) indicates broad habitat preferences in
Big Bend. The greater range of habitat tolerance dis¬
played by P. merriami is illustrated by the following:
(1) Each of the three species was taken exclusively on
a roughly equivalent number of habitat plots (P. mer¬
riami exclusively on eight plots, and C. eremicus and C.
nelsoni exclusively on 10 plots each). (2) P. merriami
and C. nelsoni were captured together exclusively on
21 plots and P. merriami and C. eremicus were found
together exclusively on 19 plots. (3) C. nelsoni and C.
eremicus were taken together on only four plots. (4)
All three species occurred together on nine plots. The
information published by Denyes (1956) points to the
same conclusion. She recorded P. merriami in many
more plant associations in Brewster County, Texas,
than either C. nelsoni or C. eremicus. Some P. mer¬
riami usually occurred in preferred habitats of each of
the other two species. Baccus (1971) found P. mer¬
riami to be sympatric with both Chcietodipus species
in BBNP, but found C. nelsoni and C. eremicus to be
ecologically allopatric.

In the study area, the preferred habitat of P.
merriami was intermediate between that of C. nelsoni
and that of C. eremicus with respect to slope, density
of vegetation and substrate characteristics (Figs. 13,
14, 16, 17). Because the Chihuahuan pocket mouse is
better adapted to deep soft (powdery ) sandy loam and
loam soils and sparse understory vegetation, it is un¬
able to compete successfully with P. merriami on the
harder clayish soils where the understory vegetation is
denser. Similarly, at the other extreme, C. nelsoni is un¬
able to compete successfully with P. merriami in those
intermediate areas because the understory vegetation is
neither sufficiently tall nor dense and the soil lacks large
rocks. Hence, these data indicate that P. merriami has
occupied the intermediate habitat because it is better
adapted than the other two species. Merriam’s pocket
mouse probably is crowded out of habitats preferred
by the other two species by population pressures and
because of its small size. Because P. merriami has a
wide range of habitat tolerance it is able to occupy
habitats not preferred by the other two species. In
New Mexico, Bailey (1931) noted that although the
habitat preferred by P. merriami was similar to that of
P flavus , P. merriami was more often found on stony or
hard ground than in the mellow sandy valley bottoms
occupied by P. flavus.

Special Publications, Museum of Texas Tech University

Pocket Mouse Habitats in the Big Bend Region

In previous sections it was shown that pocket
mice in the Big Bend area have specific habitat prefer¬
ences. Density and height of the understory, amount
of surface area free of plant cover, size and density of
rocks on the surface and below the surface of the soil,
and distribution and spatial arrangement of surface
rocks are among the most important factors influencing
local distribution and abundance of pocket mice. Fac¬
tors of lesser importance include gradient, soil texture,
height and density of canopy vegetation, and species
composition of the plant cover.

Neither the soil textural types nor the vegetation
are sharply delineated in the area of study, but grade
gradually from one type to another. The study area
included nearly every conceivable combination of
plant associates. Few if any rodents in the Big Bend
area are restricted to any specific plant association or
soil textural type, although they do attain their greatest
abundance in those areas where characteristics of the
habitat are optimum for survival.

Characteristics of the habitat in the Big Bend
region differ from those in the desert areas of western
Utah where Vest (1955) described several distinct bi¬
otic communities. There, Vest found several sharply
delineated soil types and that each type supported a
distinct complex of plant and animal associates. He
also found that many species of plants and rodents were
restricted almost entirely to a specific combination of
soil and vegetation.

In the Big Bend area, however, where there is a
gradation of soil types and plant associations, and where
rodents are not restricted either to soil type or to plant
associations, it was difficult to discern definite biotic
communities. Hence, the study area was classified into
more or less distinct habitat types for the purpose of
comparing the habitat requirements of desert rodents.
These are based chiefly on the characteristics of the
substrate in relationship to the dominance of a particular
species of pocket mouse, and to a lesser extent on slope,
density and species composition of the vegetation, and
other architectural characteristics of the habitat. Since
plant associations seemingly are of lesser importance

in the distribution of pocket mice, each habitat type
discussed below may include one or more of the plant
associations described by Thompson (1953), Tasmitt
(1954) and Denyes (1956).

In estimating populations of Nelson’s pocket
mouse and Merriam’s kangaroo rat ( Dipodomys mer-
riami ), captures for every month were utilized, but,
because of winter periods of inactivity of P. merriami
and C. eremicus, capture records for the months of
November through January were omitted from the
calculations for these two species. Only the capture
records from November through June were used in
calculating population densities of the cactus mouse
(.Peromyscus eremicus).

Rock-free Flats (Plate 6A, C, E; Plate 11)

Altitude and Location. —550-850 m. (1) Rio
Grande flood plains (Plate 1 IB, Plot 78), (2) flood
plains of the Tornillo Creek, (Plate 11C, Plot 71), and
(3) sandy loam flats below, and derived from, igneous
volcanic capped sandstone cliff near upper Tornillo Flat
(Plate 6A, C, E, Eremicus plot), and the lower portion
of Tornillo Creek near the bridge (Plate 11 A, Plot 77),
in the area of San Vicente, and other localities.

Plots.— 16, 32, 34, 35, 39, 41, 71, 77, 78, 93,
100, Eremicus plot.

Gradient. —Average, 1%; range, 0% on Rio
Grande flood plain to 2.5% (Fig. 18A).

Substrate .—Deep loam, containing a high per¬
centage of silt in the flats of the Rio Grande and Tornillo
Creek flood plains; deep sandy loam below sandstone
cliffs. It is a soft, powdery, pliable, easily dug soil
that frequently develops a rather hard surface crust.
Average percentage composition of soil for six plots:
sand, 38; silt, 41; clay, 21. No surface or soil gravel
was present in the deep loams near the Rio Grande and
Tornillo Creek. Less than 1% of the substrate mate¬
rial by weight consisted of gravel in the steeper sandy
loam areas, where rocks >2.5 cm were rare (Fig. 19;
Plate 6A, C).

Porter—Ecology of Pocket Mice in the Big Bend Region

Figure 18. A. Average percentage gradient of several habitat types in the
Big Bend region. B. The total number of plant species recorded on each
of several habitat types in the Big Bend region. Data are derived from the
habitat plots.

Density of Vegetation, —Generally sparse with
<0.5 perennial plants/m 2 . Overstory usually more
dense than understory, but with <0.25 plants/m 2 (Plates
6A, E; 11). Understory usually has <0.13 plants/m 2
(Fig. 20).

Plant Species Composition. —Mean percentage
plant species composition for all six plots studied is
given in Appendix III In the deep loams of the Rio
Grande flood plains (plots 35,71, and 78), mesquite, in
the form of large clumps (up to 9 m or more in crown
diameter) was the predominant plant (Plate 11B-C).
These clumps were surrounded by extensive areas of
bare ground. The understory plants were so sparse on
plots 71 (Plate 11C) and 35, for example, that it was

impractical to measure or record them. Plots located on
sandy loam (32, 77, and the Eremicus plot) contained
principally creosotebush (Plate 12B). Eleven under¬
story and 17 overstory species were recorded on these
plots (Fig. 18B; Appendix III).

Reptilian Associates. —The Common Side-
blotched Lizard ( Uta stansburiand) and the Marbled
Whiptail ( Aspidoscelis marmorata) were the most
abundant reptiles in this habitat where Uta attained
its maximum abundance. The Greater Earless Lizard
(Cophosaurus texanus) was present, and the Desert
Spiny Lizard ( Sceloporus magister) was found on the
loamy soils of the Rio Grande flood plains. A rattle¬
snake {Cmtalus) was recorded on the Eremicus plot.

Special Publications, Museum of Texas Tech University

F igure 19. Soil and rock composition of nine habitat types in the Big Bend region based on the habitat plots.
Cobbles as defined in tins study include rocks 7.5-40 cm in diameter. A. Average number of cobbles per
square meter of surface area, and in a 20-liter sample of substrate. Stars in the fine gravelly foothills and
the coarse stony mountainside represent cobbles in the Merriami plot, and the Nelsoni plot, respectively.
The Eremicus plot is a rock-free flats habitat containing no cobbles, B, Percentage of the total average
dry weight of soil and assorted sizes of rocks in a 20-liter substrate sample. Minimum diameter of each
size class is shown to the right.

Porter—Ecology of Pocket Mice in the Big Bend Region

1—200

v\> V \\

Figure 20, Plant density on nine habitat types in the Big Bend area. Stippled area indicates
the mean proportion of fluff grass (which contributes very little to ground cover) in the
understory.

Rodent Associates. —The predominant rodent in
this habitat was the Chihuahuan pocket mouse (Fig.
16). MerrianTs kangaroo rat ( Dipodomys merriami )
was next in abundance, especially on sandy loam
soils. Kangaroo rats were not common on the deep
alluvial soils adjacent to the Rio Grande. MerrianTs
pocket mouse and the cactus mouse were uncommon
and C, nelsoni was absent. The spotted ground squir¬
rel ( Xerospermophilus spilosoma ), the Texas antelope
squirrel ( Ammospermophilus interpres), and southern
plains woodrat ( Neotoma micropus ) were present in
small numbers.

Remarks. —On the Eremicus plot, P. merriami
utilized holes in the sandstone outcroppings surround¬
ing the plot. Nelson’s pocket mouse was not trapped
in this habitat type, although one individual was taken
about 6 m up on a ledge of the sandstone butte sur¬
rounding the Eremicus plot (Fig. 8; Plate 6). This

butte was entirely surrounded by habitat preferred by
C. eremicus. The paucity of the cactus mouse in this
habitat is likely a reflection of the sparseness of the
vegetation and its accompanying lack of nesting sites.
Lechuguilla and sotol seem to be important as nesting
sites of this mouse.

Sandy Washes and Arroyos (Plate 10)

Altitude and Location. —Most frequent at lower
elevations (600 m) but some sandy washes occur in the
foothills at elevations of 1,100-1,300 m.

Plots.— 36, 76, 95.

Gradient. —2% or less (Fig. 18A).

Substrate. —Sand, loamy sand, and sandy loam.
Average percentage composition of soil on three plots:

Special Publications, Museum of Texas Tech University

sand, 80; silt 10; clay, 10. Sandy washes in the foothill
areas have deep, tine textured, moderately permeable,
brown gravelly soils which sometimes occupy depres-
sional drain areas within the Pinal and other soil series
and develop over gravelly outwash materials from the
Chisos Mountains. Gravels frequently were present
in the central portion of the wash where the main in¬
termittent stream flow occurred. In these areas, gravel
constituted as much as 50% of the total weight of the
substrate material (Fig. 19B). Some cobbles were
present in these sections (Fig. 19A), and vegetation
was sometimes completely absent. On each side of
the central gravelly section were deep sandy overflow
areas.

Density of Vegetation. —Density of the vegetation
in this habitat type was similar to that of the rock-free
flats in its sparseness (Fig. 20). The overstory was also
denser than the understory. Sandy washes usually had
little if any cover at ground level.

Plant Species Composition. —Considerable
variation was found in the plant species on sandy
washes. In areas near mountains, species characteristic
of mountains may predominate. The vegetation was
analyzed only at one plot (76; Plate 10) near the Rio
Grande. Plant cover on this plot consisted mainly of
Porophyllum scoparium , huisache (Acacia minuata ),
creosotebush, and tasajillo. But this is not representa¬
tive because huisache is very uncommon in the general
area. The most abundant understory plants were sand
dropseed (Sporobolus cryptandrus ) and fluff grass
(Appendix III.),

Reptilian Associates. —The Marbled Whiptail
was observed in this habitat.

Rodent Associates.—Chaetodipus eremicus was
the most abundant rodent. Merriam’s pocket mouse
was uncommon and Nelson’s pocket mouse absent.
Merriam’s kangaroo rat and the cactus mouse were
both present in small numbers (Fig. 16).

Fine Gravelly Plains (Plate 12)

Altitude and Location. —Widespread, covering
many square kilometers of desert lowlands from the
flood plains (600 m) to the foothills (1,000 m).

Plots— 8 , 9, 10, 14, 22, 23, 24, 25, 26, 45, 46,
63,64, 65, 66, 68, 101.

Gradient. —Average, 3%; range, 2-10% (Fig.

18 A).

Substrate. —Powdery sandy loam, relatively soft,
pliable, and easily dug by rodents. Average percentage
composition of soil samples on nine plots: sand, 65;
silt, 22; clay, 12. Approximately 50% of the substrate
was rock with a relatively small proportion of cobbles
(Fig. 19; Plate 12E, Plot 101).

Density of Vegetation. —Plant cover of this habitat
was slightly denser (>0.75 plants/m 2 ) than on sandy
washes (Fig. 20; Plate 12A). On average, understory
plants were slightly denser than overstory. Both the
overstory and the understory of this habitat usually
have >0.25 plants/m 2 .

Plant Species Composition. —The overstory
(Appendix 111) was principally creosotebush (Plate
12B). In some areas, leather plant was common; in
others it was absent. The understory was principally
lechuguilla (Plate 9; Appendix Ill). Lechuguilla (Plate
9) and ehino grass (Plate 15E) may be present, and the
strawberry pitaya ( Echinocereus stramineus: Plate 12C)
frequently occurs in these areas. Only 4 understory and
13 overstory species were recorded in this habitat type
(Appendix III; Fig. 18B).

Reptilian Associates. —Marbled Whiptails and
Common Side-blotched Lizards occurred in this habitat
in small numbers.

Rodent Associates. —The cactus mouse occurred
in this habitat and the kangaroo rat ( Dipodomys mer-
riarni) reached its maximum abundance here (Fig. 16;
Plate 12D). All three species of pocket mice were
present, with C. eremicus the most abundant and C.
nelsoni the least. Woodrats ( Neotoma ) and spotted
ground squirrels occurred in small numbers.

Remarks. —The deep pliable soil and sparse un¬
derstory enable C. eremicus to compete successfully
with P. merriami in this ha bitat and the absence of large
rocks, coupled with a sparse plant understory, allows it
to compete successfully with C. nelsoni.

Porter—Ecology of Pocket Mice in the Big Bend Region

Fine Gravelly Foothills (Plate 7)

Altitude and Location. —About 1,100 m eleva¬
tion in the region of Lone Mountain and in some areas
along the base of the Chisos Mountains, It was usually
sandwiched between the canyon outwashes and the line
gravelly plains. In areas where fine gravelly foothills
were absent, the line gravelly plains habitat joined di¬
rectly either with the coarse gravelly foothills, with the
coarse stony mountainsides, or with the rough-broken
mountainsides habitats.

Plots.— 6 , 11, 12, 13, 44, 49, 57, 58, 59, 60, 73,
74, Merriami plot.

Gradient. —Average for 14 plots, 5%; range 3-8%
(Fig 18A).

Substrate. —Soil in this habitat contained more
clay than the line gravelly plains habitat; consequently,
it was more compact. It ranged from clay loam to
sandy clay loam and sandy loam. Average percentage
composition of four plots: sand, 61; silt, 19; clay, 20.
Except for a slightly higher percentage of rocks (Fig.
19) 2.5 cm or larger, the gradation of rocks in this habi¬
tat (Plate 7C) was similar to that of the fine gravelly
plains (Plate 12E, Plot 101).

Density of Vegetation. —The understory density
ranged from 0.85 to 13.35 plants/m 2 (Tables 2,6), with
an average of almost 6.5 plants/m 2 (Fig. 20). The high
average density of the understory was due in part to a
high proportion of fluff grass (Fig. 20; Tables 4,6; Ap¬
pendix III), which as noted previously, covers relatively
little surface area (Appendix II).

Plant Species Composition. —This habitat had a
larger number of plant species than usually occurred in
the fine gravelly plains habitat (Fig. 18B). Tarbush and
mariola were the two most abundant shrubs recorded
here (Appendix III). Fluff grass, lechuguilla, and black
grama ( Bouteloua eriopoda) were the most abundant
understory species.

Reptilian Associates. —The Marbled Whiptail
was the most abundant reptile (28 were caught in live

traps). Other lizards included Aspidoscelis exsanguis,
A. inornata and Uta stansburiana.

Rodent Associates. —MerrianTs pocket mouse
not only acquired its greatest abundance in this habitat,
but it also was the most abundant rodent. Nelson's and
Chihuahuan pocket mice were present in small num¬
bers (Fig. 16). P. merriami was common in the coarse
gravelly foothills, but much less so on the fine gravelly
plains (Fig. 16). Presumably because of the paucity of
large rocks and the absence of a tall, dense understory,
C. nelsoni was uncommon except where this habitat
intergraded with the coarse gravelly foothills. In this
transition zone there were occasional areas of large
rocks which were preferred by C. nelsoni. Because
of the harder clayish soils and the denser understory
vegetation of the fine gravelly foothills, populations of
C. eremicus were low in this habitat. C. nelsoni was the
least common pocket mouse probably because of the
relative sparseness of the vegetation in this habitat type
compared with that usually inhabited by C. nelsoni.
Hence the lessened competition between P. merriami
and the other two species of pocket mice undoubtedly
accounts for the almost exclusive presence of P. mer¬
riami in this habitat.

The cactus mouse was abundant in this habitat.
White-throated woodrats ( Neotoma albigula ; Plate
14C), spotted ground squirrels, and Texas antelope
squirrels were also present. The presence of lechuguilla
and sotol may account for the abundance of cactus mice
in this habitat.

Remarks. —This habitat differs from the fine
gravelly plains in its more compact soil, and its denser
and more diverse vegetation, particularly in the under¬
story (Figs. 18B, 20). However the denser understory
in the fine gravelly foothills does not result in extensive
ground cover because of the preponderance of fluff
grass. Note the small clumps of fluff grass visible in
open areas of the fine gravelly foothills (Plate 7A-B),
which are absent in the fine gravely plains (Plate 12A).
Although the Merriami plot had a high proportion of
fluff grass, other plots representing this habitat had
even higher densities of fluff grass, resulting in a very
large mean understory density for the fine gravelly
foothills (Fig. 20).

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Fine Gravelly Hillsides (Plate 13)

Altitude and Location. —Distributed uncom¬
monly throughout the study area at elevations of 600-
1,100 m.

Plots.— 1,31,62, 69, 94.

Gradient. —Average for four plots, 20%; range,
14-26% (Fig. 18A).

Substrate. —The soil on two plots was sandy
loam; on a third, sandy clay loam (approaching sandy
loam); on a fourth, sandy loam (approaching sandy
clay loam). Average percentage composition of the soil
on four plots: sand, 65; silt, 18; clay, 17. The gravel
was similar to that of the fine gravelly plains and fine
gravelly foothills (Fig. 19; Plate 13B, D).

Density of Vegetation, —Of two plots sampled
for vegetation in this habitat type, Plot 69 was similar
(Plate 13C, overstory 0.15 plants/m 2 , understory 0.52
plants/m 2 ) to that of the gravelly plains habitat (Plate
12), whereas Plot 62 (Plate 13A, overstory, 1.47 plants/
m 2 and understory, 9.04 plants/m 2 ) was much denser
and similar to the vegetation on the coarse stony moun¬
tainside habitat.

Plant Species Composition. —There was con¬
siderable variation in the plant cover in this habitat
type. Plot 62 was near the foothills and had vegetation
consisting of components from both the coarse stony
hillside habitat (such as sotol, 13%; chino grass, 9%;
Dalea formosa , 14%; and Acacia angustissima var.
texensis , 9%) and the fine gravelly foothill habitat (such
as Gutierrez ia, 18%; Dasyochloa pulchella, 19%; and
Gymnosperma , 16%; Appendix III).

The overstory vegetation on the second plot
(69) was chiefly creosotebush and mesquite, which
were the principal components of the fine gravelly
plains and the rock-free flats. Purple-tinged prickly
pear ( Opuntia macrocentra; Plate 14D) and tasajillo
(Plate 15D) were present. The understory vegetation
was sparse and consisted of false grama ( Cathestecum
erectum), 38%; drop seed ( Sporobolus wrightii ), 20%;
and Bouteloua tnfida , 11%.

Reptilian Associates. —The Collard Lizard (Cro-
taphytus collar is ) and Marbled Whiptail were present.
A Long-nosed Snake ( Rhinocheilus lecontei) was taken
from a trap on Plot 62.

Rodent Associates. —Merriam’s pocket mouse
and the cactus mouse were the two most abundant
rodents in this habitat (Fig. 16). One C, eremicus
was trapped in a wash cutting through the hill on Plot
94, and two Merriam’s kangaroo rats were taken on a
small road-like terrace which wound up the hill from
the flat below. Only three specimens of C. nelsoni and
two species of ground squirrels ( Ammospermophilus
interpres and Xerospermophilus spilosoma) were taken
in this habitat.

Remarks. —The abundance of Merriam’s pocket
mice in this habitat indicates that slope alone does not
adversely influence the distribution and abundance of
this mouse. The fact that Plot 62 had a rather dense
plant understory (Table 6; Plate 13A) would indicate
that the absence of large rocks on this plot and the
presence of deeper soils were the major reasons for the
abundance of Merriam’s pocket mouse and the uncom¬
mon occurrence of C. nelsoni. The absence of large
rocks on Plot 69 (Plate 13D) undoubtedly accounts for
the paucity of C. nelsoni. Flence, it is probable that
the apparent preference of C. nelsoni for steep slopes
(Fig. 13) is associated with the usual occurrence of
large rocks and dense understory vegetation on steep
slopes (Fig. 12), rather than with the degree of the
slope per se. Data indicate that these two factors were
also responsible for the usual paucity of P. merriami
on steep slopes. The absence of C. eremicus on these
slopes despite the absence of large rocks, the sparse¬
ness of the vegetation and the presence of a “pliable”
soil (Plot 69), suggests that slope alone may restrict its
distribution and abundance.

Coarse Gravelly Foothills and Outwashes (Plate
14)

Altitude and Location. —Around the base of the
Chisos Mountains (1,000-1,700 m). In some places
it extended from the mouths of the canyons onto fine
gravelly foothills and fine gravelly plains. In these
localities the major intermittent streams had cut below

Porter—Ecology of Pocket Mice in the Big Bend Region

the main level of the wash, leaving adjacent secondary
overflow areas or low terraces on which large rocks had
been deposited. These course gravelly washes gradu¬
ally graded into sandy washes.

Plots.— 2, 3, 4, 5, 7, 38, 47, 48, 50, 51, 53, 54,
56, 82.

Gradient. —Average for 14 plots, 5%; range,
3-7% (Fig. 18A).

Substrate. —Sandy loam (approaching loamy
sand), and sandy clay loam. Average percentage com¬
position of samples: sand. 68; silt, 19; clay, 13, similar
to the texture of line gravelly plains. The coarse grav¬
elly foothills had a high percentage of rocks, a relatively
high average number of cobbles per 20-liter sample,
and an average of 5.4 surface cobbles per square meter
(Fig. 19A). There were about 430 boulders (>40 cm)
per hectare (Plot 82). Average diameter of boulders was
46 cm; the largest was 84 cm (Fig. 15; Plate 14E).

Density of Vegetation. —Recorded density of the
understory in this habitat is not a valid indication of
the amount of surface area covered because many of
the plants were species which cover very little surface
area at ground level as compared to lechuguilla and
chino grass. This habitat had a denser overstory than
other habitats (Fig. 20).

Plant Species Composition. —This habitat had
a large variety of species of overstory and understory
plants (Fig. 18B). Grasses (13 species) made up the
bulk of the understory (Appendix 111); lechuguilla and
chino grass were not abundant. The overstory, likewise,
was composed of a great variety of species of shrubs
many of which were dominants in other habitats. Be¬
cause of the great variety of species, it was difficult
to designate any one or two species as dominants in
the understory or the overstoiy. In the mouth of Pine
Canyon, this habitat type was composed chiefly of bear
grass ( Nolina erumpens ), grama grasses ( Bouteloua ),
lechuguilla and sotol.

Reptilian Associates. —The following lizards
and snakes were recorded in this habitat: Coleonyx
variegatus , Crotaphytus collar is, Cophosaurus texanus,
Sceloporus poinsettii , Phynosoma modestum , Aspidos-
celis marmorata , A. exsanguis. Diadophis punctatus

regalis, Masticophis flagellum, M. taeniatus, Salva¬
dor a graham iae, Arizona elegans, Pituophis catenifer,
Lampropeltis getnla, Hypsiglena torquata , Crotalus
molossus , C. lepidus and C. scutulatus.

Rodent Associates .—Merrianvs and Nelson’s
pocket mice were about equally abundant (Fig. 16).
The Chihuahuan pocket mouse was only about 25% as
abundant as the other two, and it was taken on only 5
of the 14 plots. White throated woodrats (Plate 14C)
and cactus mice (Plate 15C) were both common in this
habitat, in which the former probably attains its peak of
abundance in this region. Dipodomys merriami (Plate
12D) and two species of ground squirrel (A spilosoma
and A. interpres) were also present.

Remarks. —This habitat had a greater diver¬
sity of reptiles and mammals than any of the others.
Four of the 14 plots trapped in this habitat yielded
all three species of pocket mice. The soil and gravel
characteristics (Fig. 19; Plate 14E) were intermediate
between those of the coarse stony mountainside (Plate
8C) and the fine gravelly foothills (Plate 7C). It was
frequently characterized by alternating areas of coarse
and fine gravels (see Plate 14B), and by intermediate
characteristics of the vegetation. When this situation
prevailed, C. nelsoni was usually found in the areas
of cobbles and coarse gravel whereas P. merriami
was more inclined to occur in the fine gravels. A few
shallow sandy washes and the accompanying sparse
vegetation probably accounted for the presence of C.
eremicus on some of the plots. Sotol, Torrey’s yucca
and prickly pear provided nesting sites for the white-
throated woodrat and cactus mouse.

Coarse Stony Sandy Hillsides (Plate 15)

These hillsides were derived from sandstone hills
with igneous volcanic caps. The surface is covered
with rock fragments which have weathered from the
igneous cap.

Altitude and Location. —At sites throughout the
area 850-1,100 m: upper Tornillo Flat Lower Tornillo
bridge, near Glenn Spring, Glenn Spring Road, 12.7
km west of Panther Junction on the Basin Junction-
Maverick Junction Flighway.

Special Publications, Museum of Texas Tech University

Plots.— 30, 40, 67.

Gradient —Average of three plots, 33%; range,
21-47% (Fig. 18A).

Substrate. —Soil material was calcareous, rang¬
ing from sandy loam (approaching loamy sand) to
sandy clay loam; deep soft and not forming a hard-
baked crust, hence easily dug by pocket mice. Aver¬
age percentage composition of three plots: sand, 66;
silt, 19; clay 15. Over 65% of the substrate material
sampled in this habitat type was rocks, of which over
30% by weight was cobbles (Fig. 19B). There were
>2.5 cobbles per 20-liter sample, and >8 cobbles per
square meter of surface area (Fig. 19 A). Over 4,200
boulders were recorded per hectare on Plot 40 (Fig. 15;
Plate 15B), for an average of one boulder per 2.4 m 2 .
The average boulder was 66 cm in longest dimension;
the largest recorded was 107 cm.

Density of Vegetation. —One feature of this
habitat was the sparsity of vegetation (Fig. 20; Plate
15A). In this respect it was similar to the fine gravelly
plains habitat.

Plant Species Composition, —This and the fine
gravelly hillside habitat (Plot 69) were the only steep
slopes on which creosotebush was noted. Creosotebush
(Plate I2B) and chino grass (Plate 15E) were the most
abundant overstory and understory plants, respectively,
in this habitat (Appendix III). Ocotillo ( Fouquieria
splendens), narrow leafed moonpod ( Selinocarpus
angustifolius ), and false grama were present. Tasajillo
(Plate 15D) reached its maximum abundance in this
habitat.

Reptilian Associates. —The Greater Earless Liz¬
ard was observed in this habitat.

Rodent Associates. —Only three species of ro¬
dents were taken in this habitat: C. nelsoni, P. merriami
and Peromyscus eremicus. Nelson’s pocket mouse
was nearly five times as abundant as P. merriami (Fig.
16), and the cactus mouse (Plate 15C) reached its peak
abundance here. Kangaroo rats were absent from this
habitat and others with an abundance of large rocks
(Figs. 16, 19), as is typical of bipedal heteromyids
(Brown and Harney (1993).

Remarks. —The presence of cobbles and boulders
undoubtedly accounted for the common occurrence of
C. nelsoni , despite the sparseness of the understory.
This indicates that a dense overstory is not necessary
for C. nelsoni if large rocks are present. It was shown
previously that slope does not limit the distribution or
abundance of P. merriami when large rocks are absent
and the soil deep. The presence of small numbers of
P. merriami in this habitat suggests that large rocks do
not restrict its distribution if the understory vegetation
is sparse and the soil deep, but they may influence
its abundance, at least when in competition with C.
nelsoni.

Coarse Stony Mountainsides (Plate 8)

Altitude and Location. —Along the base of the
Cliisos Mountains (1,000 m) where rocky outcrops
predominate without extensive cliffs, it is present along
the base of Nugent Mountain (Nelsoni plot, Plate 8) and
near the Glenn Spring and Pine Canyon roads.

Plots. —27, 81, Nelsoni plot.

Gradient. —Average of three plots, 26%; range,
15-43% (Fig. 18A).

Substrate. —Loamy sand, sandy and silt loam.
Average percentage composition: sand, 64; silt, 26;
clay, 10. Fine gravels and stony materials 15-75 cm
deep overlie thin-bedded, igneous parental material.
The nature of the rocks is shown in Plate 8C and
Figures 15 and 19. Rocks comprised nearly 80% by
weight of the substrate. There was a greater number
of cobbles in this habitat than in any other (Fig. 19A).
The larger rocks were fragments of the outcropping of
igneous parental material, and usually they were long
thin fragments (Plate 8C). There were on average,
about 500 boulders per hectare in this habitat type
(Fig. 15), for an average of one boulder for every 20
m 2 . The average longest boulder diameter was 56 cm;
the largest, 94 cm.

Density> of Vegetation. —The overstory of this
habitat was not much denser than that of the rock-free
flats and fine gravelly plains, but the understory veg¬
etation was much denser (Fig. 20). A comparison of
the percentage of surface area covered by understory

Porter—Ecology of Pocket Mice in the Big Bend Region

plants in this habitat with that of the fine gravelly
foothills (compare the Nelsoni plot with the Merriami
plot. Appendix IT), and the coarse gravelly foothills
indicates that more of the surface area of the coarse
stony mountainside was covered at ground level with
vegetation. The main reason tor this is that fluff grass
in the fine and coarse gravelly foothills covered less
surface area than did chino grass and lechuguilla in the
coarse stony mountainsides.

Plant Species Composition. —The principal
overstory plants were leather plant, prickly pear (three
species. Appendix 111), ceniza. Parry's ruellia ( Ruellia
parry i), and sotol. The dominant understory species
were chino grass and lechuguilla (Appendix III) which
formed a dense understory 25-50 cm high.

Reptilian Associates.—Aspidoscelis marmorata,
A. exsanguis , Eumeces obsoletus , Diadophis puncta-
tus regalis , Crotalus molossns and C. scutulatus were
observed in this habitat.

Rodent Associates.—Chaetodipus nelsoni and the
cactus mouse were the most abundant rodents (Fig. 16).
P. merriami was not common. Only one white-throated
woodrat was captured even though the nests of this
species were fairly common under dead sotols.

Remarks. —Merrianf s kangaroo rat and adult C.
eremicus were not recorded in this habitat (Fig. 16).
One juvenile C. eremicus was collected. Apparently
steep slopes, large rocks, and dense vegetation were
factors which prevented C. eremicus from occupying
coarse stony mountainside. Probably the dense, tall
understory vegetation (chino grass and lechuguilla)
and the cobbles and boulders were the principal factors
limiting the number of P. merriami. The abundance of
cactus mice may be related to the presence of lechu¬
guilla and sotol.

Rough-broken Mountainsides (Plate 16)

Altitude and Location. —Characterized by large
cliffs and boulder strewn talus slopes, at elevations
ranging from 600 m (Rio Grande) to 1,370 m (foothills

of the Chisos Mountains). Numerous mountainous
areas in the Big Bend region have this type of habitat.
Some are of limestone origin (Dead Florse Mountains);
others are igneous (Grapevine Hills, Chilicotal Moun¬
tain, and Burro Mesa).

Plots. —21, 28, 29,61,70, 75.

Gradient. —Average six plots, 39%; range 21-
60% (Fig. 18 A).

Substrate. —Sandy loam sandy clay loam (ap¬
proaching clay loam), loam. Average composition of
soil samples: sand 56; silt, 19; clay, 25. Nearly 80%
by weight of the substrate in this habitat was rock (Fig.
19B). There were more than four cobbles per 20-liter
sample (Fig. 19A) and more than 6,700 boulders per
hectare (i.e., one boulder per 1.5 m 2 ; Fig. 15). The
average boulder was 66 cm in the greatest dimension;
the largest, 229 cm (Fig. 15; Plate 16B)

Density of Vegetation. —A dense understory was
characteristic of this habitat (Fig. 20). More than 3.9
plants/m 2 were recorded in the understory; 0.7 in the
overstory.

Plant Species Composition. —A greater variety
of plant species was found in this habitat than in any
of the other habitats studied (Fig. 18B; Appendix HI).
It was difficult to designate a dominant overstory
plant because several species were equally abundant,
particularly Ruelliaparryi , Carlowrightia linearifolia,
and Ephedra (Appendix III). The dominant species of
the understory were chino grass and lechuguilla. False
grama and cotton top ( Digitaria californica) were com¬
mon on some plots but not on others.

Reptilian Associates. —The following squamates
were observed in this habitat: Eumeces obsoletus , Cro-
taphytus collar is. Sceloporus merriami\ and Crotalus
molossus.

Rodent Associates.—Chaetodipus nelsoni and the
cactus mouse were equally abundant (Fig. 16). Neither
P. merriami nor C. eremicus was captured here.

Special Publications, Museum of Texas Tech University

Sex Ratios

The data on rodents collected from the 2.2-ha
population plots are reported in Tables 7-10. Sex ratios
on the population plots approached 1:1 for adults of all
three species of pocket mice during every trapping in¬
terval except March when males predominated. Among
Merriam’s pocket mice the March prevalence of males
over females was highly significant (0.01 probability
level) when tested for chi-square (Table 22 in Porter
1962). The lopsided sex ratios in March were not
significant for the other two species but the chi-square
values in some instances were nearly significant. Dur¬
ing December 1958 all three P. merriami and both C.
eremicus trapped on the Merriami plot were males.

Data for the habitat plots give a similar picture. On
those plots, the preponderance of adult male over adult
female P. merriami became highly significant during
February. A preponderance of males over females was
recorded for juvenile Merrianrs pocket mice taken
on the habitat plots during May. This discrepancy in
numbers of male and female juveniles was also highly
significant (Porter 1962). There was no divergence
from a 1:1 ratio for either adults or juveniles of the
other two species during any of the months in which
samples were taken. Dixon (1959) also reported an
early spring excess of males of C. nelsoni at Black
Gap. Unbalanced sex ratios have been considered by

Table 7. Summary of the live-trapping of three 2.2-ha population plots (March 1958-July 1959). Total individu¬

als captured includes animals for which the sex was not determined.

Number of
Individuals Captured

Percentage
of Individuals
Captured

Number of Times
Captured

Percentage of
Total Captures

Male

Female

Total

Mean

MERRIAMI PLOT

Perognathus merriami

115

46.7

469

4.0

46.8

Chaetodipus eremicus

5.6

2.9

4.1

Chaetodipus nelsoni

0.8

1.0

0.2

Dipodomys merriami

19.5

239

5.0

23.8

Peromyscus eremicus

24.4

244

4.1

24.4

EREMICUS PLOT

Perognathus merriami

5.1

2.4

2.5

Chaetodipus eremicus

64.5

428

4.8

58.4

Chaetodipus nelsoni

Dipodomys merriami

22.5

259

8.3

35.3

Peromyscus eremicus

5.8

2.9

3.3

NELSONI PLOT

Perognathus merriami

6.2

2.6

2.7

Chaetodipus eremicus

0.9

1.0

0.2

Chaetodipus nelsoni

59.8

446

6.7

67.9

Dipodomys merriami

0.9

2.0

0.3

Peromyscus eremicus

30.4

190

5.6

28.8

Porter—Ecology of Pocket Mice in the Big Bend Region

Table 8. Nelson’s pocket mice individually marked and recaptured on the Nelsoni plot. Parenthetical numbers
indicate additional uncaptured animals that were trapped in preceding and subsequent periods and assumed to
be present on the plot but inactive during the trapping period. Superscript indicates two of the six adult males
initially captured in March 1958 that died in the trap. Subsequent trapping periods in which mice of a given
cohort survived and were recaptured can be determ ined by reading horizontally. Numbers of mice captured during
a given trapping period and the cohort in which they were originally trapped and marked can be determined by
reading vertically. Trapping period totals represent animals from all cohorts captured at least once during a
given trapping period, with parenthetical numbers showing additional animals not captured but assumed present
and inactive during that period. Each trapping period consisted of 16 nights of trapping, except the July 1959
period, which consisted of eight nights of trapping.

Marking

Cohort

Trapping Period

Age and

Sex Class

March

1958

July

1958

September

1958

December

1958

May

1959

July

1959

■EH

6 2

2 ( 1 )

1 ( 1 )

Adult S

2 ( 1 )

1 ( 1 )

July

Adult $

1958

Juvenile

1 ( 1 )

Juvenile 9

Adult S

September

1958

Juvenile S

Juvenile 9

Adult &

December

1958

Adult 9

Juvenile $

Adult S

May

Adult $

1959

Juvenile S

Juvenile 9

Adult S

July

Adult 9

1959

Juvenile f

Juvenile 9

Total trapped (all cohorts)

20 ( 1 )

16(3)

20 ( 2 )

% inactive

0 %

16%

0 %

Special Publications, Museum of Texas Tech University

Table 9. Merriam’spocket mice individually marked and recaptured on the Merriamiplot. Parenthetical numbers
indicate additional uncaptured animals that were trapped in preceding and subsequent periods and assumed to
be present on the plot but inactive during the trapping period. Superscript indicates two of the 17 adult males
initially captured in March 1958 that died in the trap. Subsequent trapping periods in which mice of a given
cohort survived and were recaptured can be determ ined by reading horizontally. Numbers of mice captured during
a given trapping period and the cohort in which they were originally trapped and marked can be determined by
reading vertically. Trapping period totals represent animals from all cohorts captured at least once during a
given trapping period, with parenthetical numbers showing additional animals not captured but assumed present
and inactive during that period. Each trapping period in 1958 consisted of 16 nights of trapping, whereas the
two 1959 trapping periods each consisted of eight nights of trapping.

Marking

Cohort

Trapping Period

/\ge ana

Sex Class

March

1958

July

1958

September

1958

December

1958

May

1959

July

1959

March

Adult S

1958

Adult 5

Adult (J

2 ( 1 )

0 ( 2 )

1 ( 1 )

July

Adult ?

0 ( 1 )

1958

Juvenile $

6 ( 2 )

0(5)

Juvenile $

2(5)

0(5)

4(1)

Adult $

1 ( 1 )

September

Adult ^

0 ( 1 )

1958

Juvenile $

1(3)

Juvenile $

0 ( 1 )

December

1958

Adult S

Adult &

May

Adult $

1959

Juvenile S

Juvenile $

Adult S

July

Adult ?

1959

Juvenile S

Juvenile $

Total trapped (all cohorts)

19 2

30 ( 8 )

3(19)

24 (3)

% inactive

0 %

21 %

86 %

11 %

0 %

Porter—Ecology of Pocket Mice in the Big Bend Region

Table 10. Chihuahuan pocket mice individually marked and recaptured on the Eremicus plot. Parenthetical
numbers indicate additional uncaptured animals that were trapped in preceding and subsequent periods and
assumed to be present on the plot but inactive during the trapping period. Superscripts indicate three of the
10 adult males originally captured in March 1958 that died in the trap, either on their initial capture . or in the
subsequent trapping period. Trapping periods in which mice of a given cohort survived and were subsequently
recaptured can be determined by reading horizontally. Numbers ofmice captured during a given trapping period
and the cohort in which they were originally trapped and marked can be determined by reading vertically. No
new or previously-marked Chihuahuan pocket mice were captured on the plot in December Therefore, there is no
December 1958 cohort, though 10 animals were presumed present but inactive. Trapping period totals represent
animals from all cohorts captured at least once during a given trapping period , with parenthetical numbers showing
additional animals not captured but assumed present and inactive during that period. The age was undetermined
for two animals captured only during the September trapping period. Each trapping period in 1958 consisted of
16 nights of trapping, whereas the two 1959 trapping periods each consisted of eight nights of trapping.

Trapping Period

Marking

Age ana

Cohort

Sex Class

March

July

September

December

May

July

1958

1959

March

Adult S

10 2

3 1

1958

Adult $

Adult S

July

Adult §

0 (1)

1958

Juvenile $

9(1)

0(4)

Juvenile $

0(3)

Adult S

Adult $

0 (1)

September

Juvenile S

1958

Juvenile §

0 (1)

S not aged

$ not aged

Adult $

May

Adult ?

1959

Juvenile $

Adult <$

July

1959

Adult J

Juvenile $

Total trapped (all cohorts)

13 2

42 1

35(1)

0 (10)

% inactive

0 %

100 %

0 %

Special Publications, Museum of Texas Tech University

Elton et al. (1931) to be associated with breeding activi¬
ties and with a greater tendency for males to wander
more than females and hence to be more susceptible to

capture than females during the breeding season. See
Porter (1962) for further discussion of sex ratios on the
population and habitat plots.

Seasonal Activity Patterns

Dixon (1959) and MacMillen (1964) noted that
cactus mice ( Perornysciis eremicus ) are much more
susceptible to trapping during winter and early spring
than during late spring, summer and fall, though Reich-
man and Van De Graaff (1973) found that trap success
for this species in the Sonoran Desert of Arizona was
slightly reduced in the winter The present investigation
revealed (Fig. 21) that cactus mice were much more
commonly collected in the winter and early spring.
Kangaroo rats were caught more frequently in the
summer of 1959 than of 1958 (Fig. 21). In the case
of cactus mice, some environmental factor affected
by climate, such as fruit and seed production or insect
populations, may be the causative factor. In field and
laboratory studies, MacMillen (1965) found that cactus
mice aestivate during summer when food or water is
scarce, and are active at high temperatures (up to 38°C)
as long as food and water are abundant. This aestivation
has been shown to prevent water loss and extend food
resources (MacMillen 1965). Reichman and Van De
Graaff (1973) also found that activity of cactus mice
was not adversely affected by high temperatures. The
greater rainfall in July 1959 than July 1958 (Fig. 5)
likely accounts for the greater activity of cactus mice
in July 1959 (Fig. 21).

On the other hand, the increased activity of kan¬
garoo rats in summer 1959 (Fig, 21) could be due to the
lower temperatures in July 1959, compared with July
1958 (Fig. 5). Reichman and Van De Graaff (1973) and
Reichman (1983) also observed that high temperatures
affected the activity of kangaroo rats more than pocket
mice, probably as a result of their larger body size.

Merriam’s and Chihuahuan pocket mice were
captured hi greater abundance and comprised a greater
percentage of individuals captured during late spring,
summer and fall than did cactus mice (Fig. 21). The fact
that pocket mice were attracted to bait in traps, whereas
cactus mice were not, during the summer months when
both pocket mice and cactus mice presumably were

active, suggests that during these months there may be
less competition between species for food than during
spring and late fall when pocket mice and cactus mice
were all susceptible to trapping.

Nelson’s pocket mice were more active during
the winter than the other two perognathine species
(Fig. 21). During 784 trap nights (16 nights) in De¬
cember 1958, 22 Nelson’s pocket mice were assumed
to occur on the Nelsoni plot. Twenty (91%) of this
number were trapped in December (Table 8). During
the December trapping period no C. eremicus were
captured on the Eremicus plot and only three P. mer-
riami (86% assumed to be inactive) were taken on the
Merriami plot (Tables 9-10). The three Merriam’s
pocket mice, trapped in December, were captured only
four times (two individuals were captured once each
and one was captured twice). These four captures
probably represented animals which became aroused
from torpor to obtain food. Pocket mice likely do not
remain in a torpid condition much longer than a week
without food (Bartholomew and Cade (1957). In the
laboratory, Bartholomew and Cade (1957) found that P.
longimembns aroused at intervals of only a few hours
to a day or more. MacMillen (1983) cites data from
Meehan (1976) indicating that Perognattms parvus
remain torpid during the winter, with torpidity of up
to eight days interrupted by arousal for less than a day.
It is likely then that most Merriam’s and Chihuahuan
pocket mice occasionally become active to obtain food.
Consequently it is possible that only a few individuals
present on the plot were active during any one night. If
the plots had been trapped continuously throughout the
winter, most of the individuals may have been captured
at one time or another.

Manning et al. (1996) compared my results (as
reported by Schmidly 1977a) with their data from the
early 1990s for C. nelsoni and C. eremicus in the Big
Bend region. They confirmed that unlike C. eremicus ,
Nelson’s pocket mouse remains active throughout the

Porter—Ecology of Pocket Mice in the Big Bend Region

Merriami Plot

1958 1958 1958 1958 1959 1959

Peroanat hus
merriami BUI

Chaetodipus _

eremicus 1:•:■: I

Chaetodipus

neisoni

Dipodomy s
merriami lili

Peromysc us
eremicus 1 Y//A

Klalonni Dint DU«

1958 1958 1958 1958 1959 1959 1958 1958 1958 1958 1959 1959

Figure 21 Relative seasonal occurrence of nocturnal rodents on the 2.2-ha population plots

winter. Manning et al. (1996) also reported similar
peaks of activity (July-September for Nelson's pocket
mouse, and June-August for the Chihuahuan pocket
mouse). They found that pocket mice were active in
March during years of above average rainfall (1990-
1992), but were inactive during March 1994, after a
dry period.

Porter (1962) reported that captive P. merriami
held at ambient temperature at Big Bend became torpid
during the winter, with oral temperatures 1-2°C above
ambient as long as environmental temperatures were
above freezing. The mice apparently became active
intermittently to feed. When temperatures dropped
below freezing, P. merriami became continuously ac¬
tive. During trapping activities in December, February,

and March, Merrianrs pocket mice frequently were in
a torpid condition in the traps. The Chihuahuan and
Nelson’s pocket mice seldom were observed in torpor.
In February 1958, with temperatures ranging from
12.0°C to 26.2°C, six individuals of/! merriami and
one of C. eremicus were trapped in a lethargic condi¬
tion. Oral temperatures of the mice ranged from 1.0
to 3.2°C above ambient.

Field observations indicated that torpid Merriam’s
pocket mice were usually in traps in which the bait was
exhausted. Bartholomew and Cade (1957) found that
when their animals were removed from food for 24-
36 hours, torpor was invariably induced regardless of
the environmental temperature. According to Bartho¬
lomew and Cade (1957), aestivation in C. eremicus is

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basically the same phenomenon as hibernation except
that body temperature remains high because ambient
temperatures are high. I found no pocket mice during
this study that showed indications of aestivation.

Scheffer (1938) did not regard the dormancy
displayed by P pawns as true hibernation. According
to Reynolds and Haskell (1949) hibernation does not
appear to be equally developed in all species of pocket
mice, with P. baileyi exhibiting weaker tendencies to
hibernate than C. penicillatus. Bartholomew and Cade
(1957) believe that torpidity, together with food storage
and burrowing (as found in pocket mice), provides an
effective adaptation for a small mammal living in an
environment as severe and unpredictable as the deserts
of North America. They point out that even with all
food stores exhausted, individuals could survive a week
or more at temperatures of20-21°C. Accordingly, they
believe that a small pocket mouse could survive even

longer without food at 8°C because of the small amount
of weight loss per day at low ambient temperatures.
Bartholomew and Cade (1957) and MacMillen (1965,
1983) believe that torpidity is not as much an adaptation
to temperature conditions as it is to adverse conditions
when food is not readily available. It is assumed (Bar¬
tholomew and Cade 1957) that the ability of pocket
mice to maintain normal behavior at low temperatures
is of adaptive importance when they are active at
temperatures near freezing. An additional adaptive
advantage they did not mention is that it also places the
mice under the ground and away from predation at a
time prior to the critical spring period when, according
to Leslie and Ranson (1940), Elton (1942) and Miller
(1958) the adult population must be sufficiently large to
provide a replacement large enough for the population
to survive a high summer mortality and to establish a
new vigorous overwintering population.

Seasonal Abundance

Various methods can be used to estimate the
size of rodent populations during a particular trapping
period. The number of individuals actually captured
during a trapping period is the simplest method, but
does not include animals not captured because they are
“trap-shy” or because they are inactive as a result of
climatic conditions. Some rodents are never captured
because it becomes increasingly difficult to capture
the remaining residents as more and more animals are
captured (Davis 1956).

A better estimate can be obtained by counting not
only mice actually captured, but also those assumed to
be present because they were captured during preceding
and subsequent trapping periods (Miller 1958; Chap¬
man and Packard 1974). However, for the first few trap¬
ping periods, there is no way to determine the number
of inactive animals not trapped. This d ifficulty can be
avoided by a modification (Dixon 1958) that includes
only those animals that were recaptured in the same
period or one of the following, and omits individuals
captured only once.

Another method, a modification of the Lincoln
Index (Davis 1956) involves marking and releasing ani¬

mals for several days with a subsequent trapping of the
plot to determine the proportion of previously marked
to unmarked individuals. The following formula is used
to estimate the population size: N = Mn/m; where

N = original unknown population,

M = number marked in first trapping.,

n = number caught in second trapping, and

m = number recaptured in second trapping.

A disadvantage in comparing populations with
this method is that with confidence limits at the 0.05
level one can distinguish only large differences (Figs.
22-24) and estimates can vaiy greatly according to the
duration of the trapping period. Davis (1956) recom¬
mends a five-day period of marking and releasing fol¬
lowed by a three-day period of trapping to determine
the proportion of marked to unmarked individuals. He
believes that after three days of the second trapping pe¬
riod new mice begin to infiltrate the area. An adequate
period of trapping undoubtedly varies with the mortality
and natality rates of the population. Trapping data from

Porter—Ecology of Pocket Mice in the Big Bend Region

Modified Lincoln Index

±2 Sta e?roS| - jj” -|" ^Estimated value

3+3 5+3 8+8
Days marking + recapturing

Mice trapped plus assumed inactive

Single captures:

included

excluded

8 116

8-Day 16-Day
Trapping period

Figure 22. Seasonal population estimates for P merriami on the Merriami plot based
on modified Lincoln index and on the total mice trapped plus those assumed to be
inactive because they were trapped during preceding and subsequent trapping periods.
Modified Lincoln indexes are calculated based on 3+3,5+3 or 8+8 (days of marking and
releasing + days of recapturing) estimates. Horizontal center-lines indicate estimated
values. Modified Lincoln Index estimates could not be calculated in December due
to inactivity of the animals. The numbers of mice trapped plus assumed inactive are
calculated both with and without omitting the single captures. The Merriami plot was
trapped for only eight days during each of the 1959 trapping periods.

Figure 23 Seasonal population estimates for C. nelsoni on the Nelsoni plot.
Calculations are explained in the text and in the legend for Figure 22. The Nelsoni
plot was trapped for only eight days in July 1959.

Special Publications, Museum of Texas Tech University

Figure 24. Seasonal population estimates for C. eremicus on the Eremicus plot.
Calculations are explained in die text and in die legend for Figure 22. Modified
Lincoln Index estimates could not be calculated in December due to inactivity of
the animals. The Eremicus plot was trapped for only eight days during each of the
1959 trapping periods.

a population with a low turnover of individuals are less
subject to error as a result of long trapping periods than
from one with a high turnover. Distances between traps
are also important in determining the duration of the
trapping period. With traps spaced at greater distances
it takes longer to sample a population than with traps
set close together. Also “trap addicted mice” tend to
cause the catch to be less random; consequently, one
gets an underestimate of the population (JTayne 1949).
This method is impractical for determining population
density during periods of inactivity of the animal be¬
ing studied.

Three population estimates based on the modified
Lincoln Index of Davis (1956) were calculated for the
population plots (Figs. 22-24). One was based on an
eight-day mark and release period and an eight-day
recapture period (8+8; open diamonds in Figs, 22-24).
Another was calculated for a five-day mark and release
period, followed by a three-day recapture period (5+3;
closed rectangles in Figs. 22-24). The third estimate
was based on a three-day mark and release period and
a three-day recapture period (3+3; open rectangles in
Figs. 22-24). These estimates were compared (Figs.

22-24) with 8-day and 16-day estimates based on as¬
sumed or known populations and on known populations
less the single captures (Dixon 1958). The assumed
or known populations were based on the number of
individuals assumed to be present on the plot because
they were captured during preceding and subsequent
trapping periods.

In the 3+3 and 5+3 calculations, the first three
trapping days of the first trapping period were consid¬
ered as prebaiting days (Chitty and Kempson 1949).
Since the traps were left on the plots between trapping
periods and captures were relatively high on the first
day of each subsequent trapping period, prebaiting was
not deemed necessary for periods other than the first.

Long-term investigations of plots in marginal
habitats where two or more species of pocket mice
occur are needed to determine the correlation among
such factors as (1) long and short term phenological
changes of the habitat, (2) changes in long and short
term population levels of age and sex classes for each
species of pocket mouse residing on the area, (3)
changes in the period and intensity of reproduction,

Porter—Ecology of Pocket Mice in the Big Bend Region

(4) food preferences of each species, and (5) nest and
burrow requirements for each species. A study of this
nature would answer many of the questions regarding
the effects of these variants on the population levels of
the species of pocket mice occupying the plots.

Abundance of Merriam's Pocket Mouse .—
Population estimates were lowest in March 1958 and
highest in July 1958 (Fig. 22). The small number of
pocket mice recorded in March probably was due both
to mortality and to lessened activity because of cold
weather (Figs. 3-5; 21). The number of inactive mice
could not be ascertained because March was the first
trapping period. During the 16 trapping days in March
and early April only 2 females and 17 males were
captured (Table 9). This apparent lack of activity of
females reduced the catch by almost half.

Only three animals were captured in December
(16 trapping days), but at least 19 others were assumed
to occur there because they were captured in preceding
and subsequent trapping periods. Tn September, 30
animals were captured but 38 were assumed to occur
on the plot (Table 9). Based on calculations for an
8 +8-day trapping period these represent significant
changes from the July total of 49 individuals (Fig.
22). The decrease in numbers in September may be
attributable to lessened activity of the mice because of
unusually high rainfall (9.8 cm) during the trapping
period (Fig. 5A).

Abundance of Nelson's Pocket Mouse. —In March
the number of C. nelsoni captured on the Nelsoni plot
was low (9 animals for 16 days of trapping; Table 8).
Although 21 individuals were known to occur on the
plot in July, the known population was low compared

with that of P. merriami (49) and C. eremicus (42) on
their respective plots for the same period (Tables 8-10).
Only a slight decrease in the known population of Nel¬
son’s pocket mice occurred in September.

In contrast to the other two species. Nelson’s
pocket mouse was active during December at which
time more individuals were captured than in July 1958
(Fig. 23 and Table 8). Ten new animals (all but one
were adults) appeared on the plot in December from
an unknown source. The addition of 12 juveniles to
the population in May 1959 (16 days of trapping) ac¬
counted for most of the increase in numbers during that
period (Table 8 and Fig. 23). The number of animals
trapped during the eight-day trapping period in July
1959 was the same as that in May (Fig. 23).

Abundance of the Chihuahuan Pocket Mouse .—
On the Eremicus plot Chihuahuan pocket mice reached
a peak of activity (numbers captured) in July 1958 (Fig.
24). The number of individuals captured on the plot
decreased in September, and in December no pocket
mice were trapped on the plot, although two individuals
of C. eremicus were captured on the Merriami plot. At
least 10 inactive individuals were assumed to be present
on the Eremicus plot during December.

In May the active populations (numbers captured)
appeared to be nearly back to the level reached in July
1958 (based on eight trapping days; Table 11), but in
July 1959 the population was significantly smaller (Fig.
24). The cause of this decline is not known but it may
have been the result of a marked decrease in the number
of juveniles added to the population as compared with
1958 (Table 11).

Table 11. Numbers ofpocket mice captured on the population plots during the first eight days of trapping.
PTA = previously trapped adults; NTA = newly trapped adults; NTJ = newly trapped juveniles.

Merriami Plot Nelsoni Plot Eremicus Plot

PTA

NTA

NTJ

PTA

NTA

NTJ

PTA

NTA

NTJ

March 1958

July 1958

September 1958

December 1958

May 1959

July 1959

Special Publications, Museum of Texas Tech University

Breeding Habits

Arnold (1942), Reynolds and Haskell (1949),
and Scheffer (1923, 1938) discussed breeding in
pocket mice. Scheffer (1938) believed gestation to be
somewhere in the range of 21 -28 days for P. parvus.
Eisenberg (1963) and Eisenberg and Isaac (1963)
reported gestation of 23-26 days for live species of
pocket mice.

Determination of stage of reproduction in pocket
mice as indicated by such external criteria as swollen
vulva, perforate vagina, presence or absence of vaginal
plugs, lactation, visible pregnancy, and scrotal testes
were found to be inadequate, difficulties also noted by
Chapman and Packard (1974). An attempt was made
first to determine reproductive condition during the
trap and recapture program of the population plots,
but with little success. Later, animals taken from the
habitat plots were examined for external indications of
pregnancy and other reproductive criteria, but little if
any correlation was found between these external crite¬
ria and the true reproductive condition of the animals.
Handling of animals frequently caused testes to descend
into the scrotal sacs. Hence, the following discussions
are based on dissection and internal examination of
animals taken from the habitat plots and the occur¬
rence of juveniles on both the habitat and the 2,2-ha
population plots. The presence or absence of embryos,
the presence of fully formed spermatozoa in the testes
and the average monthly weights of the testes were the
criteria by which reproductive condition of the mice
was determined. The presence of spermatozoa usually
is considered indicative of fecundity even though the
physiological condition of the spermatozoa cannot be
established (Jameson 1950). It has been established
that the testes increase in size as the mating season
progresses, and they regress in size at its close (Jameson
1950). Unless otherwise noted, the data that follow
were based on the examination of animals taken from
the habitat plots during 1959.

Breeding Season of Merriam’s Pocket Mouse

An increase in the reproductive activity of male
Merrianrs pocket mice from February to March 1959
was indicated by a rise in the average weight of the
testes and a conspicuous increase in the percentage

of animals having spermatozoa in the epididymides
(Fig. 25A; Table 12). The testes acquired their greatest
average weight in April, declined during the May-June
period, and again increased in July. A sharp decrease
in the breeding activity of males during August is
suggested by the low weight of a testis from a single
individual, and this is confirmed by small testis sizes
in mice collected in August by Yancey et al. (2006) in
the Harte Ranch area in the northern part of BBNR An
analysis of variance revealed no significant differences
between the monthly means (Porter 1962). Data of
Genoways et al, (1979) from the Guadalupe Moun¬
tains provide further evidence of a July peak m male
reproductive condition followed by a gradual decline
through October.

The first pregnancy was recorded in March and
the first extensive captures of pregnant females were
in April (Fig. 25A). The highest percentage of preg¬
nant females was recorded in May. The incidence of
pregnancy decreased sharply between May and June;
during July and August no pregnant females were taken
(Fig. 25A), Genoways et al. (1979) collected a female
with two placental scars in the Guadalupe Mountains in
June, and nonpregnant females in August and October.
Yancey (1997) and Yancey et al. (2006) collected preg¬
nant females from the Big Bend Ranch area in March,
June, July, August, and October.

Although a few juvenile mice appeared in April,
the first large influx of juveniles was in May (Fig. 25A)
with a peak in June. In spite of the reduction in pregnant
females after May 1959, the percentage of juveniles
had not declined much by the first week of August.
Consequently, one might infer that a second peak of
sexual activity occurred during 1959. The increased
weight of the testes in July supports this. There was a
decrease in the percentage of young mice on the Mer-
riami plot in September 1958 compared with July 1958
(Fig. 26). A very young individual was trapped on the
Merriami plot September 24, and juveniles which had
not yet entered the post juvenile molt were captured 7
October 1958. These young individuals probably were
offspring of late breeding females rather than the result
of a second peak of reproductive activity. The testes
increased in weight noticeably about a month before

Porter—Ecology of Pocket Mice in the Big Bend Region

Figure 25. Correlation of the reproductive activities of three species of pocket mice on the
habitat plots during 1959 A range that goes off scale is represented by an arrow with the high
or low value shown. Confidence intervals were not calculated for P. merriami.

Special Publications, Museum of Texas Tech University

Table 12. Summary of the reproductive condition (in percent) of three species ofpocket mice trapped on the
habitat plots.

Juvenile Females

Adults with Spermatozoa
in Epididymis

Placental

Scars

Juvenile Males
with Spermatozoa -
(testes or epididymis)

1958

1959

Pregnant

February

March

February

March

P. merriami

100

C. nelsoni

80*

100

C. eremicus

100

*100% contained spermatozoa in their testes.

100 -

80H

<1)

I 60-

1 ^

I 40-

a 20H

Perognathus merriami H§|
Chaetodipus netsoni
Chaetodipus eremicus 1 |

March

1958

July

1958

September

1958

Figure 26. Seasonal percentage of animals in juvenile pelage in the pocket mouse populations
on the 2.2-ha population plots, based on 8-day trapping periods. The black dot indicates
percentage of juvenile C. nelsoni based on a 16-day trapping period in July 1958.

a marked number of gravid females was trapped. An
influx of juveniles occurred about one month later.
Peaks for each phase were also about one month apart
(Fig. 25A).

Breeding Season of Nelson’s Pocket Mouse

Spermatozoa were present in the epididymides or
testes of all adult male Nelson’s pocket mice trapped
in February (Table 12). The testes showed a continual
increase in average weight, beginning in February,
and reached a maximum during the May-June period.

In July the average weight declined to about the same
value observed in February . During August the aver¬
age weight of the testes sampled declined to a value
significantly less than that observed during the previous
months (Fig. 25B). Yancey et al. (2006) found similar
results for this species in BBNP.

The percentage of pregnant females reached a
peak in March, declined in April, reached a secondary
peak in May, declined to zero in June and increased
slightly in July (Fig. 25B). Yancey et al. (2006) col¬
lected pregnant females during June and August in the

Porter—Ecology of Pocket Mice in the Big Bend Region

Harte Ranch area of BBNP. Baker (1956) reported
pregnancies from late March through July in Coa-
huila.

Juvenile Nelson’s pocket mice first appeared in
April, one month after the first pregnancy was observed.
The largest percentage of juveniles was taken in June,
after which their numbers declined conspicuously. An
increase in the percentage of juveniles on the Nelsoni
plot from 19% in July to 31% in September (Table 8)
suggests a late July or August peak of pregnancy dur¬
ing 1958. This is also suggested by the fact that 78%
of the individuals of this species trapped on the habitat
plots late in August 1958 were juveniles. Very young
animals were trapped on the Nelsoni plot on 26 Sep¬
tember 1958, and juveniles which had not yet molted
were recorded on October 7. During the December
trapping period, one juvenile male was marked and
released on the Nelsoni plot, An adult female taken
18 November 1957 on a habitat plot had enlarged
mammary glands and showed signs of having nursed
young. Thus it is probable that a few individuals are
bom as late as October or November, at least during
warmer years. There was little correlation between
the May-June peak of male breeding activity and the
pregnancy peaks (Fig. 25B). However, the pregnancy
peaks preceded by nearly a month the peak of occur¬
rence of young. Yancey et al. (2006) found juveniles
in BBNP in July and August.

Breeding Season of the Chihuahuan Pocket
Mouse

Of the nine adult male Chihuahuan pocket mice
taken in February 1959, 67% had spermatozoa in the
epididymides (Table 12). By March all individuals ex¬
amined had spermatozoa. Unlike the other two species,
males of C. eremicus experienced two annual peaks in
the size of testes. The average weight of the testes in¬
creased significantly between February and March (Fig.
25C), indicating a spring peak in the breeding activity
of males, and decreased in April and May. In June the
testes reached their greatest average weight, indicating
a second peak in sexual activity. Subsequently the tes¬
tes decreased in size and by July their average weight
was significantly less than that for the peak months of
March and June. Manning et al. (1996) and Yancey et
al. (2006) also reported two peaks in testes size in this
species from Big Bend, with the first peak occurring

in March, but, the second peak not until August. The
later date for the second peak could be due to different
climatic conditions at the time of the study. In three
of the four years of Manning et al.’s (1996) study, the
Big Bend region experienced above average rainfall.
My study of reproductive data was also conducted
during a time of high rainfall (Fig. 6), particularly in
late spring and early summer (Fig. 5). The long-term
average annual precipitation in the Big Bend area for
the period 1954-1960 (Fig. 6; Porter 1962) was 28.9
cm, as compared with 27.5 for the period 1961-1990
(Yancey 1997), suggesting that the baseline for what
constitutes a year of average rainfall did not change
more than 1,5 cm during the >30 years that intervened
between the two studies. In New Mexico, Whitford
(1976) found females in reproductive condition from
February to September, with a peak in May, followed
by occurrence of juveniles in June.

For Chihuahuan pocket mice, the first pregnancy
and highest percentage of pregnancies were recorded in
April (Fig. 25C). A second peak of pregnancy occurred
in June and a third in August. The low incidence of
pregnancy in May probably was attributable to the fact
that the mice were collected near the Rio Grande where
the reproductive periods probably occur earlier. Conse¬
quently a higher percentage of the catch was juveniles,
and probably most of the adult females had already
produced at least one litter. There was no indication of
a decline in number of pregnancies in August. Yancey
et al. (2006) reported pregnancies as late as September
and November. In Mexico, Baker (1956) reported a
pregnancy as early as February.

The first juveniles appeared in the traps during the
latter part of May (Fig. 25C), A second peak of abun¬
dance was recorded in August. A definite decline in
the percentage of young was recorded for the Eremicus
plot between July and September 1958 (Table 10). Two
very young individuals and several young animals in
juvenile pelage were trapped on that plot in late Septem¬
ber. Nearly a month elapsed between the first marked
increase in the average weight of the testes (and also
the first peak of pregnancy) and the first appearance
of juveniles (Fig. 25C). Although some reproductive
activity occurs throughout the months when this species
is active, the data suggest two peaks of reproduction,
one in spring and the second in late summer.

Special Publications, Museum of Texas Tech University

Discussion of the Extent of Breeding Season in
Pocket Mice

The main reproductive period was in spring
and, to a lesser extent, summer. There was a slight
suggestion of both spring and late summer peaks of
reproduction in all three species. These peaks were
more pronounced in C. eremicus than in the other
two species, especially at lower elevations where the
weather warmed up earlier in the spring and stayed
warm longer in the fall. Similarly, Reynolds and
Haskell (1949) reported that in southeastern Arizona the
breeding activity of C. penicillatus was highest in late
spring, decreased during the drought period of June and
early July, but increased again in August. They found
that periods of greatest sexual activity were concurrent
with seasons of new vegetative growth of spring and
summer. A similarly divided breeding season, observed
by Reynolds (1960), in Dipodomys merriami in the
same general area of Arizona likewise corresponded
closely with the periods of new vegetative growth of
spring and late summer. As pointed out by Reynolds
(1960), who cited Bodenheimer and Sulman (1946),
there is some evidence that nutrients contained in fresh
vegetation have a stimulating effect on the breeding
activity of rodents. Holdenried and Morland (1956)
found that 77 pregnancies in Perognathus flavus were
distributed as follows: January—0; February—2;
March—0; April—11; May—10; June—29; July—8;
August—1; September—10; October—6; November
and December—0. Hence there were also two repro¬
ductive periods in P.flavus. The double peak may be
a widespread phenomenon among heteromyid rodents
for the southwestern deserts where spring and fall
periods of precipitation are the rule. A more detailed
study of the reproductive activity of Perognathus and
Chaetodipus in the Big Bend area is needed to verily
the presence or absence of a divided period of sexual
activity. In all three species, reproduction appeared to
begin abruptly in the spring and to taper off gradually
in the fall.

Breeding activities of C. nelsoni began about one
month earlier in the spring of both 1958 and 1959 than
did those of the other two species. Likewise, Nelson’s
pocket mice bred later in the fall than did the other
species. The longer breeding period of C. nelsoni is
probably correlated with the species’ winter activity

pattern. Manning et al. (1996) and Yancey et al. (2006)
collected pregnant females of C. nelsoni during June
and August in the Big Bend region, and they found juve¬
niles throughout the summer and as late as November.
For C. eremicus , they reported pregnant females nearly
every month from March though November.

The onset and duration of reproduction appear
to be associated with phenological conditions of the
habitat. Hence, these phenomena vary from year to
year with those conditions rather than with calendar
dates (Brown and Harney 1993). The reproductive
season in 1959 began nearly one month earlier than
it did in 1958. A greater percentage of adult male P.
merriami and C. eremicus contained spermatozoa in
the epididymides during February 1959 compared with
February 1958 (Table 12). The difference between the
two years is less evident in male C. nelsoni probably
because its breeding begins earlier. The later breed¬
ing season in 1958 is further pointed up by the later
emergence of female pocket mice compared with 1959.
This fluctuation was also noted by Dixon (1958) who
found that the March 1957 population values for the
Black Gap area (see Fig. 2) were more similar to those
for April 1958 than to those for March of that year. He
also recorded a greater trap response for C. nelsoni in
March 1957 than at comparable calendar dates a year
later. Further, he recorded juveniles earlier in 1957
than in 1958. During January-March 1957 the average
temperatures at Panther Junction were conspicuously
above nomial whereas a year later they were well below
normal (Fig. 5)

Analysis of Numbers and Uterine Distribution of
Embryos and Placental Scars

The difference in the average number of embryos
or placental scars recorded for each of the three species
of pocket mice (Table 13) was small. Nelson’s pocket
mouse had a slightly smaller average number of em¬
bryos per female than did the other two species, and
C. eremicus had a somewhat higher average number of
placental scars than either of the others. Baccus (1971)
reported similar mean numbers of embryos in P. mer¬
riami (3.7 embryos, ranging in number from 3 to 5) and
C. eremicus (3.5 embryos, ranging in number from 2 to
5) in BBNP, but found slightly larger litters (4 embryos
in each of two pregnant females) in C. nelsoni. Yancey

Porter—Ecology of Pocket Mice in the Big Bend Region

Table 13. Number of embryos and placental scars of three species ofpocket mice.

Number of Embryos Number of Placental Scars

Sample

Size

Range

Mean

Sample

Size

Range

Mean

P. merriami

1-4

3.6

2-21

5.9

C. nelsoni

2-4

3.2

2-10

5.1

C. eremicus

2-6

3.6

2-19

7.4

et al. (2006) reported a mean of 4 embryos (range 3-5)
in C. nelsoni and 3.6 (range 2-6) in P. merriami in the
Harte Ranch area of BBNP.

The average of 3.6 (range 2-6) embryos per fe¬
male recorded for C. eremicus in this study is similar to
the 3-4 embryos reported by Baker and Greer (1962),
the 3.5 embryos (range 2-5) by Baccus (1971), and
3 embryos by Genoways et al, (1979), but slightly
smaller than the 4.4 (range 2-8) reported by Yancey
et al. (2006). The number of embryos and placental
scars I observed in each uterine horn did not differ
significantly from a ratio of 1:1 (see Porter 1962). C.
eremicus had an average of 2.0 embryos in the right
horn and 1.6 in the left.

Incidence of Breeding in Juveniles

There was considerable difference among the
three species in their ability to reproduce before acquir¬
ing adult pelage. None of the juvenile female Mer-
riam’s pocket mice examined was pregnant or had pla¬
cental scars. Conversely, 30% of the young males had
spermatozoa either in the testes or in the epididymides
(Table 12). Males of the other species were less sexu¬
ally precocious. Only 7% of the juvenile males of C.
eremicus had spermatozoa in their gonads. Females
of C. nelsoni and C. eremicus , however, were more
precocious than those of P. merriami (Table 12). There
was a much higher evidence of reproductive activity in
juvenile males of P. merriami than in juvenile males of
the other species which was difficult to explain.

Longevity

The data were not suitable to construct life tables
because animals were inactive during the winter and
the trapping periods were too infrequent. However,
information was compiled on longevity for the three
species of pocket mice. Mortality estimates based
on trapping data would represent the upper limit of
mortality, since the data cannot distinguish between
individuals that died and those that dispersed from the
plot (Brown and Harney 1993).

Merriam’s Pocket Mouse

The annual turnover in the population of P. mer¬
riami on the Merriami plot from July 1958 to July
1959 was somewhat greater (84%) than that reported
by Dixon (1958) at Black Gap (75%). The annual
probability of living was 0.16 (16% of the population

would probably live one year) on the Merriami plot.
Of the seven adult males captured on the Merriami plot
in July 1958,29% (2 animals) survived until July 1959
(Table 9). A slightly lower percentage (16%) of the
juveniles survived a full year (males 14% and females
19%) than of the adults (18%).

Considering the small size of MerrianYs pocket
mouse, its longevity is much higher than normally
would be expected. Dixon (1958) recorded a maxi¬
mum life span of 33 and 22 months, respectively, for
two mice at Black Gap. He recorded 25% survival of
24 mice which were marked 12 months or more be¬
fore the last trapping period. Of these, the ones living
the longest were immature when marked; a male was
recorded for 89 weeks and a female for 96. On the
Merriami plot, one animal was first captured as an adult

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(at least a year old) in March 1958 and last recorded in
September 1958. Two males, first captured as adults
in July 1958, were taken again during the last trapping
period (July 1959) and were suspected of being at least
15 months old and possibly older. Of 38 young mice,
first marked in July 1958,6 were still alive in July 1959.
They were also approximately 15 months of age when
last recorded.

Nelson’s Pocket Mouse

Dixon (1958) reported an annual turnover of
nearly 75% in the population of C. nelsoni at Black Gap
which is somewhat lower than I recorded for this spe¬
cies on the Nelsoni plot (86%). The annual probability
of living was 0.14 for C. nelsoni on the Nelsoni plot.
The percentage of adult males (13%) that survived a
year was similar to that for adult females (11 %) whose
“life span” was at least one year. A greater percentage
of juveniles (25%) than of adults (14%) survived from
July 1958 to July 1959. At Black Gap, Dixon (1958)
found C. nelsoni to be unusually long-lived. He re¬
corded two individuals which survived 30 months, one
24 months and two others about 20 months.

Data from the Nelsoni plot were similar. An adult
female, first taken in March 1958 and last trapped in
May 1959, was at least 20 months old (Table 8), Of 14
adults taken in July 1958, two survived to July 1959,
at which time they were at least 15 months old. One
of four juveniles marked in July 1958 was taken a year
later when it was at least 13 months old (Table 8).

Chihuahuan Pocket Mouse

The annual turnover in the population of this
species (95%) was greater than that of the other two
species. The probability of living at least one year was
correspondingly low (0.05). An 81% turnover was
recorded from July 1958 to May 1959. None of the
annuals marked as adults in July 1958 survived to July
1959, although one was recorded in May (Table 10).
Only 7 of the 33 juveniles marked in July 1958 were
recorded in May 1959, and only 2 were recaptured in
July. One of the oldest known individuals which was
an adult in March 1958, was recaptured in September.
This animal was at least 12 months of age. A young

male and a young female, first captured in July 1958,
were known to have survived more than a year.

Discussion and Comparisons of Longevity

A remarkably high percentage of the individuals
of P. merriami and C. nelsoni marked in July 1958 was
recaptured in July 1959. That P. merriami showed the
smallest turnover in population is particularly inter¬
esting because of its small size. Chihuahuan pocket
mice were not as long lived as MerrianTs and Nelson’s
pocket mice. The high survival rate of pocket mice in
desert scrub areas might be attributed to food storage
and hibernation (Dixon 1958) and better cover. Mild
winters in the southern deserts also play an important
role in survival of these rodents. The differences be¬
tween the survival of pocket mice at Black Gap and in
BBNP may be associated with more severe winters and
longer trapping periods in the latter area.

Except for C. eremicus , the longevity of pocket
mice was greater than usually encountered among small
rodents (Blair 1953). In California, Fitch (1948) found
that only 5% of the Dipodomys heermanni population
survived one year or more. Burt (1940) reported that
only 4% of 1,382 Peromyscus leucopus survived a full
year in Michigan. Out of 559 individuals of the same
species trapped by Snyder (1956) in Michigan only one
individual survived a year. Snyder also showed that
15.8% of the mice marked while only a few weeks old
lived 53 weeks under one set of environmental condi¬
tions, but only 1.8% survived 53 weeks under more
severe conditions. In England, Evans (1942) reported
that in marked populations only 1% of the Myodes
glareolus and 1% of the Apodemus sylvaticus were
trapped a year after the original capture. But in 1941,
Manville (1949) recaptured 34% of the P. mqniculatus
he had marked in 1940, and 14% of the mice marked
in 1941 were retaken a year later.

Figure 27 shows the percentage of July 1958
captures on the 2.2-ha population plots that survived to
subsequent trapping periods. The low survival rate of
C. eremicus was probably related to the substantial loss
in numbers between July and September 1958. The rate
of decline in the percentage of surviving C. eremicus
parallels that for P. merriami for the December and
May trapping periods. Although a larger percentage of

Porter—Ecology of Pocket Mice in the Big Bend Region

C. nelsom survived between July and September 1958
as compared with P. merriami , the rate of decline or
C. nelsom between May and July 1959 was slightly

greater than for P. merriami. Most of the mortality or
loss to the populations of all three species took place
between July and December 1958.

Pelage Changes

Hansen (1954) discussed molt patterns among
genera of ground squirrels and found three basic types.
The “diffuse type” of Urocitellus occurred once a year
and was characterized by the lack of a molt line. The
second type, found in Callospermophilus and Otosper-
mophilns , was also a single annual molt, but with a
distinct molt line and with hair replacement on the
head and shoulders before the molt line appeared. The
third type, found in Ammospermophilus , Jctidomys, and
Xerospermophihis, had two hair replacements annually
yielding winter and summer pelages. The molt patterns
in pocket mice resembled the second type described by
Hansen but with certain noteworthy differences among
the species.

Progression of Molt

Although different in details, the general pattern
of molt in the three species of pocket mice was simi¬
lar in that it progressed from the head posteriorly and
terminated at the ankle (Fig. 28). The venter usually
did not have an evident molt line and the rate of molt
was slower there than on the dorsum. Consequently,
by the time the dorsal molt line reached the rump the
ventral molt was still in the region of the belly. As a
result, the final stages of molt in all three species was
characterized by a molt line extending from the rump
laterally and anteriorly across the thighs and sides (Fig.
28D 4). The molt then progressed posteriorly on the

Special Publications, Museum of Texas Tech University

Drawings by R. □. Porter

New mature pelage

Growing hair
Molt l»ne
Old pelage

Figure 28. Sequence of molt of three species of pocket mice. Numbers indicate successive
stages of molt. A. Perogncithus merriami. B Chaetodipus eremicus. C. Chaetodipus
nelsoni. D. Final stages of molt for all three species.

Porter—Ecology of Pocket Mice in the Big Bend Region

belly and down the hind legs. The last stage of molt was
indicated by molt lines near the ankles (Fig. 28D 5).

The dorsal molt lines originated in different areas
on each of the three species (Fig. 28A-C). In Nelson’s
pocket mouse, it started on the nose and progressed
posteriorly (Fig. 28C). In Merriam’s pocket mouse, a
molt line usually did not appear until the head region
was covered completely with new mature hair (Fig. 28A
1). Only one individual of this species was observed
to have a molt line on the head. Most C. eremicus
observed during this investigation had two dorsal
molt lines, one generally originating on the nose as in
C. nelsoni and the other on the neck as in P. merriami
(Fig. 28B 1). The molt line originating on the nose
progressed posteriorly until it disappeared in the new
hair of the neck. It usually reached the posterior part
of the head about the same time the second molt line
reached the rump (Fig. 28B 3). There were no observed
differences in the sequence of molt between juveniles
and adults of the same species. Speth (1969) reported
that the Great Basin pocket mouse, P. parvus, begins
its molt behind the ears, and with a second area of molt
originating later on the nose.

Size of Juveniles at Time of Molt

Juveniles of all three species molted directly
into adult pelage. There was considerable variation in
the size of individual animals when the molt line first
appeared. Juveniles of P. merriami and C. nelsoni in
molt were intermediate in size between juveniles not in
molt and adults, as measured by weight and body length
(Fig. 29). In these two species, juveniles completed the
postjuvenile molt before they have acquired their ma¬
ture weight and body length. On the other hand, young
Chihuahuan pocket mice observed in molt were nearly
as large as adults (Fig. 29). This probably accounts for
the higher percentage of pregnant juveniles recorded
for C. eremicus than for the other two species. Juvenile
C. eremicus not in molt were significantly smaller than
molting juveniles and adults (Fig. 29).

Duration and Season of Molt

My observations of a single annual molt in adult
pocket mice are in general agreement with those of
Osgood (1900). Kangaroo rats also follow this pattern

(Grinnell 1922). On the other hand. Great Basin pocket
mice ( P. parvus ) from Utah molt semiannually (Speth
1969), with the first molt beginning in January or Feb¬
ruary and concluding in March or April. The summer
molt in that species begins in June and is completed
by August or September. 1 examined a dozen or more
mice per month of each species beginning in February,
and found no evidence of a late winter molt in any of
the Big Bend pocket mice (Fig. 30).

For most juveniles and adults the duration of
molt was one month or less. Some juveniles which
were about half then mature weight and sparsely haired
ventrally completed the molt within a period of 30-45
days. A juvenile female Merriam’s pocket mouse,
for example, which was sparsely haired beneath and
showed no signs of molt when first captured on 25
July 1958, had completed her molt by 6 September.
A young male C. eremicus which had no indications
of molt on 5 July 1958 had completely molted by 29
July. It was first observed in molt on 13 July when it
was in a stage similar to Fig. 28B 3. Most individuals
captured in juvenile pelage in July 1958 had completed
the postjuvenile molt when recaptured in September.
Two or three juveniles of P, merriami and of C. nelsoni
captured during the last week in May 1959 had not
yet completed the postjuvenile molt when recaptured
9 July 1959. The duration of molt is similar to that
seen in P. parvus, which generally takes 30-35 days
to complete its molt, with a range between 11 and 90
days (Speth 1969).

For adult pocket mice the annual molt takes place
lfom May through October in Big Bend (Fig. 30). More
recent studies by Genoways et al. (1979), Manning et
al. (1996), and Yancey et al (2006) indicated similar
periods of annual molting, with molting specimens
of Nelson’s pocket mouse collected June-August,
Chihuahuan pocket mice molting in June through
October, and Merriam’s pocket mice collected in molt
during June-August. In Washington, Scheffer (1938)
found that by the end of August 65% of the males of
P. parvus were in molt, but only an occasional female.
He found that females reached their peak of molt 3-4
weeks later. There did not appear to be any difference
between sexes as regards the peak of molt in the three
species occurring in the Big Bend area.

Special Publications, Museum of Texas Tech University

Figure 29. Size of juvenile pocket mice with and without molt compared with
adults. Horizontal lines indicate mean values. Vertical lines indicate ranges.
Numbers indicate sample size. For both weight and length, adults and molting
juveniles of C. eremicus show no significant differences in weight. All other
pelage classes show significant differences within species. A. Body weight.
Height of symbol indicates 95% confidence intervals. B. Body length.

Porter—Ecology of Pocket Mice in the Big Bend Region

Figure 30. Seasonal incidence of molt in three species of pocket mice. Numbers in rectangles indicate
the number of animals examined for molt, with P. merriami on the top, C. eremicus in the middle,
and C. nelsoni on the bottom. None of the mice collected in February, March, April, or December
were in molt. No specimens were examined in January, October, or November.

Spatial Organization

Home Range

The home range of an animal is that area which
it habitually traverses during its normal daily activi¬
ties (Burt 1943; Blair 1953). This definition excludes
occasional sallies outside the area for exploration or
establishment of a new range. A variety of methods
have been employed to determine home range in wild
populations. The most accurate method is direct ob¬
servation, but with nocturnal rodents this is impracti¬
cal. Consequently, the size of home range usually is
estimated by recording the places of capture of marked
mice in live traps arranged in grid. The most distant
traps in which the animal is taken determine the limits
of the trap-revealed range. Flayne (1949) describes a
home range as revealed by trapping as “an area over
which the animal enters traps with greater or lesser
frequency according to the location of the traps,”

Doubt exists in the minds of some regarding the
validity of this method, since Hayne (1950) and Stickel

(1954) have found that there is a positive relationship
between the ranges revealed by trapping and the dis¬
tance used between traps. Despite this apparent fault,
Blair (1953 ), among others, contends that peripheral
points of capture are a fairly reliable estimate of the
home range. Some methods allow the data of a large
number of animals to be considered, whereas other
procedures deal with only those individuals that enter
the traps a large number of times (Brown 1956). Esti¬
mates of home range derived from calculating the area
enclosed within peripheral points of capture require a
large number of captures. Blair (1942) found that for
Peromyscw maniculatus 10 or more captures were
necessary to reveal the maximum home range. God¬
frey (1954) found that 16-19 records from her Geiger
counter were needed before she was able to obtain
maximum values for Microtus agrestis. Conversely,
the extent of movement may be ascertained by presenta¬
tion of the data as distances between points of capture
rather than as areas occupied. With this method, only
the distances between two or more captures are needed.

Special Publications, Museum of Texas Tech University

Measurements from most of the captures may thus be
used in analyzing the data.

In this report the mean recapture distances were
compared on a seasonal (trapping period) basis for the
age and sex classes of each species (Figs, 31-34). Trap-
revealed home ranges were compared for adult males,
adult females and juveniles of each species captured six
or more times (Fig. 31). A mean range was calculated
for each of these groups and for every trapping period
using the exclusive boundary strip method (Stickel
1954). The calculated home ranges were then averaged
together irrespective of the trapping period from which
they came. In his studies, Hayne (1949) estimated a
“center of activity” which is the geographic center of
all points of capture. Blair (1951) believes that a bet¬
ter term for such a point is “average point of capture.”
The center of activity was estimated for each pocket
mouse captured during each of two or more trapping
periods. Distances were calculated between centers of

activity from one period of capture to the next and from
the period of first capture to each subsequent period for
the purpose of measuring the amount of shift in relative
position of individual home ranges during the course
of the investigation. Data on home range and shifts in
center of activity are presented in Figs. 31-35 and in
Tables 39-53 of Porter (1962).

According to Blair (1953), young animals that
have recently ventured from their nests apparently
range over smaller areas than do adults. As the juve¬
niles approach sexual maturity, some of them may dis¬
perse, apparently to set up residence elsewhere (Brown
1956). Howard (1949) found that some young Peromy-
scus maniculatus moved as much as 1 km when they
became sexually mature, whereas other juveniles set
up residence in the area of their birth. Reynolds (1960)
reported that young Dipodomys merriami ranged over
a significantly smaller area than did adults.

94.5

121.9
1104.2

64.0

66.8 66.8

50 —]

Adults:

Significant

differences'

D VS. CEF**
GH vs. IJ**
AK vs. CG**
H vs. BDL**

March July September May

1958 1958 1958 1959

Figure 31 Home range of sex and age classes of
individuals of three species of pocket mice captured six
or more times. Horizontal bars indicate mean values.
Vertical lines indicate the range. Vertical bars indicate
95% confidence limits comparing adult males, Sample
sizes are indicated with numbers. Adult males of P.
merriami differ significantly in their home range from
both juvenile P. merriami and adult male C. eremicus
(see Porter 1962).

Figure 32. Daily recapture distances in Merriam’s pocket
mouse. Horizontal lines indicate mean values. Vertical
lines indicate the range with maximum recapture distances
indicated for values that go off scale. Vertical bars
indicate 95% confidence limits comparing adult classes
represented with large sample size (sample size indicated
next to each symbol). Significant differences (all at the
0.01 level of probability) based on an analysis of variance
(Porter 1962) are listed.

Porter—Ecology of Pocket Mice in the Big Bend Region

Brown (1956) suggests that once individuals
of certain rodent species have established their home
ranges they seldom move from these positions, except
for occasional exploratory trips from which they return.
Males of many species of rodents range more widely
than do females presumably because of the female’s
greater attachment to the nest and young (Blair 1953).
Blair (1943) found this to be the case with the Chihua-
huan pocket mouse, as did Dixon (1959) with adult
male C. nelsoni. Females of a few species range more
widely than do males (Blair 1953). York (1949), for
example, estimated a much larger average home range
among adult female Merriam’s pocket mice than he did
among adult males. His sample size, however, was too
small to indicate statistical significance.

Among other rodents, such as kangaroo rats
(Dipodomys ), there does not appear to be a sex differ¬
ence in size of home range (Blair 1953). Blair (1943)
investigated Dipodomys merriami and D. ordii in New

Mexico; Fitch (1948) studied D. heermanni in Cali¬
fornia, and Reynolds (1960) reported on Dipodomys
merriami in Arizona. None of these studies showed sex
differences in the size of home range. Conversely, York
(1949) found that male Dipodomys merriami ranged
more widely than did females in western Texas. But his
results were based on a small number of samples.

Home Range of Merriam Is Pocket Mouse .—
There were no significant differences in the mean
recapture distances between the adult males and adult
females for the month of May 1959. During July 1958,
however, the mean recapture distance of adult males
was significantly greater than it was for adult females
and for juveniles of both sexes (Fig. 32). In September,
juvenile females moved significantly greater distances
on the average than did adults (Fig. 32). Juvenile
males moved significantly greater distances than did
adult females.

60H

50—

E —

! 4cH

3 30—I

*3 ^0 I

10—i

84,8

Tt7.7

26 |

73.5

E G

120.7

I J

9 <*

venues:

Significant

differences:

A vs. B*
K vs, L“
F VS, E“
F vs, 0“

II N
I I M
K L

60—

50—

q 40

) 30

- 20 —

10H

AB C E

Adults:

Juveniles:

9 ? 2 Y

Significant

differences:

D vs CEF**
G vs. A*

6 vs. H*

G vs. r
G vs. J*

G vs. K”

March July September December May

1958 1958 1958 1958 1959

Figure 33. Daily recapture distances in Nelson’s pocket
mouse. Horizontal lines indicate mean values. Vertical
lines indicate the range with maximum recapture distances
indicated for values that go off scale. Vertical bars indicate
95% confidence limits for selected classes. Sample sizes
are indicated next to each sy mbol. Significant differences
at the 0.05 (*) or 0.01 (**) levels of probability based on
an analysis of variance (Porter 1962) are listed.

March July September May

1958 1958 1958 1959

Figure 34. Daily recapture distances in the Chihuahuan
pocket mouse. Horizontal lines indicate mean values.
Vertical lines indicate the range with maximum recapture
distances indicated for values that go off scale. Sample
sizes are indicated next to each symbol. Significant
differences at the 0.05 (*) or 0.01 (**) levels of probability
based on an analysis of variance (Porter 1962) are
listed.

Special Publications, Museum of Texas Tech University

Figure 35 Shift in center of activity from one period
of capture to the next in three species of pocket mice.
Horizontal lines indicate mean values. Vertical lines
indicate the range with maximum recapture distances
indicated for values that go off scale. Vertical bars indicate
95% confidence limits for adult males. Sample sizes
are indicated next to each symbol. Analysis of variance
(Porter 1962) indicates that adult male Chihuahuan pocket
mice differ significantly from other classes of the same
species and from adult males of P. merriami.

During September 1958, adult males of P mer¬
riami moved significantly shorter distances between
captures on the average than they did during trapping
periods in March and July 1958, and May 1959. The
mean recapture distance of adult females was signifi¬
cantly greatest in May 1959 and least in September.
Young females moved insignificantly greater distances
in September than they did in July (Fig. 32).

Adult males and females of this species were not
compared because there was an insufficient number of
recaptures of adult females. The home range of adult
males, however, averaged more than 0.3 ha and that
of juveniles nearly 0.2 ha (Fig. 31). The difference
between the estimated home range of juveniles and
that of adult males was significant at the 0.05 level

(Fig. 31). York (1949) found that the average home
range of three male pocket mice captured 10 times or
more was 0.60 ha, and that of two females was 1.58
ha, which is much larger than was recorded during the
present study. In New Mexico, Blair (1943) estimated
the average home range of males (C. eremicus) as 1.10
ha; of females, 0.44 ha.

Home Range of Nelson s Pocket Mouse. —On the
average, adult males moved significantly greater dis¬
tances between captures than did adult females during
March 1958 and May 1959 (Fig. 33). There was not a
sufficient number of juveniles captured in March and
July 1958 to determine the extent of their movements
for these months. During September, however, the
mean daily movements of juvenile females were sig¬
nificantly greater than those of adult males and females.
During May 1959, adult males moved significantly
greater distances than did juvenile females. Dixon
(1959) recorded a mean recapture distance for males of
32.6 ±9.8 m during June and July and shorter distances
for adult females. For the two aforementioned periods,
adult females at Black Gap averaged 22.6 ±4 m.

The mean distances moved by adult males did not
differ significantly among any of the trapping periods
(Fig. 33). Dixon (1959), on the other hand, found a
considerable difference between the mean recapture
distance of adult males and adult females recorded from
February through May and those recorded for June and
July. He also found that females moved less in June
and July than they did during the period from February
through May Conversely, I found during this study that
females moved significantly greater distances in July
than they did in March and May (Fig. 33).

Movements of juvenile females were more ex¬
tensive during September than for May and December.
Differences between the mean recapture distances of
juvenile females during September and those for May
were highly significant (Fig. 33). The home range
of adult males encompassed over 0.30 ha (Fig. 31),
whereas adult females ranged over somewhat smaller
areas. Although juveniles apparently used less area than
did adults, the difference was not significant.

Home Range of the Chihuahuan Pocket Mouse .—
The mean daily recapture distances of adult males were

Porter—Ecology of Pocket Mice in the Big Bend Region

significantly larger than those of adult females for the
July and September trapping periods (Fig. 34). Juve¬
niles of both sexes ranged more widely in September
than did adult females (Fig. 34).

During July 1958, the ranges of adult males were
significantly larger than in any other trapping period
(Fig. 34); they were significantly smaller in September
than in March and July 1958, but the difference between
September 1958 and May 1959 was not significant.
The mean daily recapture distances of adult females in
September 1958 were significantly less than in March
1958 and May 1959, but not July 1958 (Fig. 34).

There were no significant differences in extent
of home ranges among adult males, adult females and
juveniles (Fig. 31). All of them encompassed approxi¬
mately 0.25 ha. In southern New Mexico, Blair (1943)
estimated the average home range of male C. eremicus
as 1. 10±0.19 ha and that of females as 0.44 ±0.06 ha.
The maximum area in hectares for males was 2.24;
that of females, 0.58. These represent considerably
larger areas than were recorded for C. eremicus during
the present study (Fig. 31). Conversely, August et al.
(1979) reported a mean home range of only 0.047 ha in
11 specimens of C. eremicus in Guadalupe Mountains
National Park.

Discussion of Home Range and Movements .—
According to Blair (1953), within closely related
groups, a species living in a sparsely vegetated habitat
tends to range more widely than those occurring in
an area covered with dense vegetation The reverse
appeared to be true of C. eremicus and C. nelsoni,
however, both in area of the home range and in the
mean recapture distance. Nelson’s pocket mouse, for
example, which was trapped in relatively dense ground
cover, ranged significantly (Porter 1962) more widely
during July and September 1958 than did the Chi-
huahuan pocket mouse which was trapped in a sparse
ground cover (Figs. 31-33). But this difference may be
a function of population density, since the population
of C. nelsoni was relatively low during those months
whereas that of C. eremicus was relatively high.

As mentioned previously, my estimates of home
ranges of adult C. eremicus and P. merriami were much
smaller than those reported by Blair (1943) for C. er¬
emicus in southern New Mexico and by York (1949)

for P. merriami in western Texas. As observed by
Hayne (1950) and Blair (1953), it is difficult to compare
the estimates of home range of various investigators,
because of the lack of uniformity of treatment among
different studies. Many factors may cause a disparity of
results, among them (1) distance between trapping sites
(Hayne 1950; Shekel 1954); (2) size of plots (Shekel
1960); (3) number of captures used in calculating home
range size; (4) density of cover; (5) seasonal density
of population (Stickel 1960); (6) breeding condition
of the animals (Stickel 1960); and (7) availability of
food (Stickel 1960). The first three factors undoubtedly
contributed most to the disparity between my estimates
and those of Blair (1943) and York (1949).

Extent of range as determined by area presum¬
ably occupied by individual mice did not always agree
with the results of the mean recapture distance data.
For example, judged by mean recapture distances,
Merrianrs pocket mice moved significantly smaller
distances than the other two species. But when home
ranges were compared, the home range of C. eremicus
was significantly smaller (Fig. 31) than that of either
of the other two species; that of P. merriami was not
significantly smaller than that of C. nelsoni.

In this study more reliability was placed on
mean recapture distance than on a real extent of the
home range for the following reasons: (1) There is
a much better chance for uniformity of interpretation
when ranges are expressed in terms of mean recapture
distances. (2) The method involving mean recapture
distances requires fewer captures in order to compare
the data on the basis of season, age and sex.

Shift in Center of Activity

The center of activity was calculated for each
mouse captured during two or more trapping periods.
The distance of the shift in center of activity was
determined in two ways: (1) The change in center of
activity from one period of capture to the next and (2)
the change from the first period of capture to each sub¬
sequent period. Figure 34 shows the change in center
of activity from one period to the next. The changes
from the first period of capture are given and analyzed
in Porter (1962). The two methods yielded essentially
the same results.

Special Publications, Museum of Texas Tech University

The mean shift in the center of activity of each
species was comparable to the mean recapture distance
of that species for any one period. No one age or sex
group of P. merriami and C. nelsoni showed a sig¬
nificantly greater shift in center of activity than other
groups of the same species (Fig. 35). The reason is
not understood, but it suggests that the population of
C. eremicus was not as stable as it was for the other
two species. The data were also analyzed to determine
whether an interval of one or more trapping periods
between periods of capture increased the average shift
of the center of activity. This was accomplished by
comparing the distance between centers of activity of
those animals having an interval of one or more trap¬
ping periods between captures with the distance for
those having no interval between captures. There was
no significant difference between the two (see analysis
of variance in Porter 1962).

Miller (1958) observed three typical patterns of
movement in his recapture data on Apodemus: (1) home
range, (2) home range + dispersal, and (3) dispersal.
All three patterns were observed in pocket mice during
this study, plus movements which might be interpreted
as exploratory trips (Plate 17, P. merriami , B-17; Plate
18, C. nelsoni , B-6, D-8; Plate 19, C. ere miens. A-1).
Typical home ranges are shown for individuals of each
species (Plate 17, P. merriami , A-4, B-6, C-13; Plate
18, C. nelsoni, A-2, B-5, C-5; Plate 19 C. eremicus,
A-3, B-6, C-5). For many species of mice dispersal is
most frequent as the juveniles approach maturity (Blair
1953). Dispersal was probably more common among
juveniles than is indicated by the recapture data, but it
was not recorded because individuals leaving the plots
were not recaptured.

Several mice displayed movement similar to that
Miller (1958) described as dispersal plus home range
(Plate 17, P. merriami , A-l, A-2, A-18; Plate 18, C.
nelsoni. A-16, D-9; Plate 19, C. eremicus , D-17) except
that the extended movements in my examples were be¬
tween the penultimate and the final captures. Miller’s
were between the first and second captures and he
assumed that the animals remained in their new home
ranges. These examples could represent exploratory
movements as well as shifts in home range and dispersal
because there were no captures at either end of the long
dispersal movement. Two or more captures at either

end of the long dispersal movements would be needed
to determine any shift in home range.

Dispersal

True shifts in home range of a magnitude to be
called dispersal rarely were observed during the study,
and they occurred only from one trapping period to the
next (Plate 17, P. merriami , C-13, D-13, E-13; Plate
18, C. nelsoni , B-4, C-4, F-4; Plate 19 C. eremicus,
B-4, C-4, D-4, E-4). Miller (1958) believes that these
patterns of apparently aimless wandering represent a
continued series of dispersal movements which are best
explained as a search by unmated males for breeding
females. This seems a plausible explanation for the
wanderings of the above mentioned adults during pe¬
riods of reproduction The peregrinations of juvenile
females possibly may be explained as the movements
of a young animal in search of a home range.

Territoriality

Territoriality is the defense of area, usually
against others of the same species and sex (Blair 1953).
Burt (1943) considers territory as that part of the home
range which is defended by fighting or by aggressive
gestures against other individuals of the same species.
Crowcroft (1955) demonstrated territoriality, as well as
a social order, in a population of wild house mice ( Mus
musculus). He found that fighting tended to disperse
the population into spatially distinct breeding areas,
territorial in function, occupied by one male and one or
two females. Conversely, Young et al. (1950) and Scott
(1956), who also studied wild-captured populations of
this species in the laboratory, found territoriality weekly
developed if present at all.

Although direct observation of defense of terri¬
tory is the best method of demonstrating territoriality,
trapping records can give indirect evidence of territorial
behavior. For example, if the home ranges of individual
animals of one or the other sex or of both sexes are
mutually exclusive, territoriality apparently exists.

Although not of general occurrence, several spe¬
cies of small nocturnal rodents are thought to show
territorial behavior as determined by occurrence of
mutually exclusive home ranges. Because there was

Porter—Ecology of Pocket Mice in the Big Bend Region

Recaplunes far each trapping peri curt
■ AdulE mates
* Adult teivialGS
d Juvenile males
D Juvenile lemetes

Center of activity for each trapping period

• March 1956
V July 195&

+ S^pLembtfi 1956

★ May 1959
A July 1959

50 m

Plate 17. Maps of the seasonal movements, home ranges (A-D), and shifts in
home range (E) from one period of capture to the next of P. merriami on the
Merriami plot, expressed as points of capture. Points of capture or centers
of activity for each individual are connected by lines indicating the sequence
of capture. Broken lines in part E indicate the change in center of activity
between when the animal was a juvenile and when it became an adult. Colors
are used when necessary to differentiate individuals with overlapping home
ranges or to identify certain animals captured during more than one period.
The same color may be used for more than one individual. Numbers identify
selected individuals and match the color of the symbols.

Special Publications, Museum of Texas Tech University

A—March 1958 50 m B—J uly 19&6

Plate 18. Maps of the seasonal movements, home ranges (A-E), and shifts
in home range (F) from one period of capture to the next of C. nelsoni on the
Nelsoni plot, expressed as points of capture. Points of capture or centers of
activity for each individual are connected by lines indicating the sequence
of capture. Broken lines in part F indicate the change in center of activity
between when the animal was a juvenile and when it became an adult.
In parts A-E, males are indicated by squares; females by circles. Closed
squares and circles are adults; open squares and circles are juveniles. For
the key to shifts in center of activity, see Plate 19. Colors are used when
necessary to differentiate individuals with overlapping home ranges or to
identify certain animals captured during more than one period. The same
color may be used for more than one individual. Numbers identify selected
individuals and match the color of the symbols.

Porter—Ecology of Pocket Mice in the Big Bend Region

1958

E—Shift in center of activity

Recaptures lor each trapping period
■ Adult males

• Adult females
□ Juvenile mates
O Juvenile females

Center uf activity for each Lrappiuy pedcd

• March 19SE5

▼July iasa

+ September 195fl
X December 1958

• May 1959
A July 1959

50 m

Plate 19. Maps of the seasonal movements, home ranges (A-D), and shifts in
home range (E) from one period of capture to the next of C. eremicus on the
Eremicus plot, expressed as points of capture. Points of capture or centers
of activity for each individual are connected by lines indicating the sequence
of capture. Broken lines in part E indicate the change in center of activity
between when the animal was a juvenile and when it became an adult. Colors
are used when necessary to differentiate individuals with overlapping home
ranges or to identify certain animals captured during more than one period.
The same color may be used for more than one individual. Numbers identify
selected individuals and match the color of the symbols.

Special Publications, Museum of Texas Tech University

no broad overlap of ranges among female Peromyscus
leucopus , Burt (1940) deduced that they possessed ter¬
ritorial behavior. Blair (1951) found similar evidence
of territoriality in Peromyscus polionotus. Blair (1940)
found evidence of territoriality in females of Microtus
pennsylvanicus based on recapture records. Getz (1961)
found that territorial behavior appeared better devel¬
oped in females than in males and that the size of the
defended area of M. pennsylvanicus probably was not
more than 7 m in diameter.

Blair (1953) pointed out that, although there is
considerable antagonism among individuals of the same
species of heteromyids, there is little evidence of any
defense of territory. He attributed this antagonism to
the habit of food storage and the protection of food
caches. Blair (1943) observed complementary home
ranges in females of C. eremicus in April and May, but
not in March. Dixon (1959) observed that females
of C. nelsoni had complementary home ranges from
February through July. York (1949) found that the
ranges of females of P merriami overlapped during
June and July. Data from the present study indicated
that adult females of all three species of pocket mice
showed a greater tendency toward territoriality than
adult males.

Home ranges of adult female Merrianvs pocket
mice were mutually exclusive during every trapping pe¬
riod except May 1959, at which time there was a slight
overlapping. It is possible that the mutually exclusive
ranges were a result of the paucity of adult females in
the population. The ranges of adult males overlapped
somewhat except during periods when the density
of the adult population was low (Plate 17). Juvenile
females had ranges which were exclusive of those of
adult females and other juvenile females. The ranges
of juvenile males appeared to be exclusive of those of
other juvenile males, but not of adult males.

Ranges of adult female C. nelsoni were entirely
exclusive of each other during March and September
and slightly overlapped during July and December
1958, and May 1959 when the density of the popula¬
tion was high (Plate 18). Ranges of adult males were
complementary during periods when population density
was low (July and September) with the exception of
March. The overlapping observed in March is probably

due to the greater movement of adult males as a result
of increase breeding activities during that period (Plate
18A). During December and May, when population
densities were high, the ranges of adult males over¬
lapped to a greater extent than they did when popula¬
tions were low. Juvenile females had ranges during
May which were generally exclusive of those of adult
females and other juveniles (Plate 18D).

Only during July 1958, when there was a low
density of adults, were the ranges of adult female C.
eremicus entirely exclusive of each other (Plate 19B).
During the remaining trapping periods, when there were
more adults than juveniles, the ranges of adult females
overlapped slightly. The seasonal relationship of the
ranges of adult males with that of other adult males
was similar to that of adult females with other adult
females. During July when populations of juvenile C.
eremicus were at a peak, the ranges of juvenile males
were not complementary. The same may be said of
juvenile females. During September, however, when
there were considerably more adults than juveniles
(also more individuals with established home ranges)
most of the juvenile males had spatially distinct home
ranges, whereas some of the juvenile females did not.

There was a tendency for adult females of all three
species of pocket mice to display territorial behavior.
This w'as indicated by the fact that their ranges were
spatially distinct w hen populations were low and nearly
distinct w^hen populations were high. Males showed a
similar tendency, but to a much lesser extent. During
periods of high population density and during the peak
of reproduction when males ranged most widely, the
ranges of a larger number of adult males overlapped
than did those of females, Two factors needing further
investigation, which appeared to change the exclusive¬
ness of the ranges of adult males, were (1) reproductive
condition of the mice and (2) density of the population.
As suggested by Getz (1961), if territoriality were not
functioning, one would expect a greater percentage of
completely overlapping home ranges during periods
of high densities of population than during periods of
low f densities. The previously mentioned data suggest
this to be applicable to the population of adult males.
Even the females apparently show some indication
of territoriality, since completely overlapping ranges
were few.

Porter—Ecology of Pocket Mice in the Big Bend Region

Territorial tendencies of juvenile female C.
nelsoni appear to be more pronounced than those of
juveniles of the other two species. This is implied by
the fact that most young females of Nelson’s pocket
mouse had ranges which did not overlap even during
the periods of greatest density of population (May
1959 trapping period), In addition, j uvenile and adult
females of Nelson's pocket mouse responded in the
population as separate units because the ranges of
juveniles overlapped those of adults. The exclusive¬
ness of the ranges of juvenile males and females of the

other two species were correlated with the density of
the population (Plates 17-19). Thus it appears that ter¬
ritorial behavior functions at an earlier age in Nelson’s
pocket mouse than it does in the other two species.

Although evidence indicates the presence of ter¬
ritorial behavior in these three species of pocket mice,
this hypothesis needs to be tested more extensively with
a behavioral study, to determine the extent to which
individual ranges are defended.

Acknowledgments

The Big Bend region was severely overgrazed
prior to 1945 when administration of the area was as¬
sumed by the National Park Service. In 1955, the Texas
State Game and Fish Commission contracted with the
Texas Agricultural Experiment Station (Project 965) to
study the biological changes taking place in BBNP and
the Black Gap Wildlife Management Area. I collected
the material upon which this report is based while work¬
ing on this project. As chair of my doctoral committee.
Dr. William B. Davis suggested this problem and su¬
pervised my research. I thank him for his many helpful
suggestions and advice. Dr. O. Charles Wallmo and Dr.
Keith L Dixon, both of whom conducted research in the
Big Bend area on the same ecological project concur¬
rently with this investigation, were particularly helpful
with many of the problems and techniques encountered
during the course of the field work. The National Park
Service staff graciously extended to me and my family
facilities of the national park, where we resided during
the field work. 1 am particularly indebted to Mr. George
W. Miller, then superintendent of BBNP, for the use
of a Park Service vehicle during the study, and also
to Mr. Harold J. Broderick, Park Naturalist, for use of
the museum preparation room and especially for mak¬
ing the herbarium available.. Mr. Broderick and Mr.
Miller issued permits for collecting animals and plants
in the Park. My thanks go to Dr. Barton H. Warnock,
Sul Ross State College, whose identification of plants
proved was an invaluable service. The Bureau of Public
Roads provided laboratory facilities and soil sieves. Dr.

Robert H. Black, Dean of the College of Agriculture
and Home Economics, New Mexico State University
provided facilities of the Agronomy Department for
the analysis of soil samples. Dr. R. C. Williams of
the Department of Agronomy at New Mexico State
University deserves particular credit for the use of his
laboratory and for giving instruction on techniques of
soil analysis. Mr. D. V. Allison and Mr. O. L. Kay, Soil
Conservation Service, assisted this work by allowing
Mr. August Turner to make two special trips into the
study area. The suggestions of Dr. Robert C. Pendleton
regarding analysis of the material in this report is much
appreciated. The following have been of considerable
help: At Texas A&M: Dr. Morris E. Bloodworth and
Dr. Harvey Oakes, U. S. Department of Agriculture,
helped with early planning of the soil analysis. At the
University of Utah: Dr. Charles Wolfe, Department
of Genetics, advised me regarding the most practical
statistical applications for the data. To my wife Lois
G. Porter, T am most grateful for the many hours she
spent setting traps, assisting me with the measuring
and recording of the vegetation, weighing rock and
soil samples, and finally typing the manuscript. Dr.
Keith L. Dixon, Utah State University; Dr. Robert C.
Pendeleton, Stephen D. Durrant and William H. Behle
of the University of Utah, reviewed parts of this manu¬
script and gave editorial advice. Drs. William B. Davis,
Robert A. Darrow, Sewell H. Hopkins, John J. Sperry
and O. Charles Wallmo, all members of my advisory
committee, critically examined the manuscript.

Special Publications, Museum of Texas Tech University

Afterword

Studies of the mammalian fauna of Texas have a
history extending well back into the nineteenth century
(Schmidly 2002). The Latin names of two species
prominent in this study (Merrianrs pocket mouse and
Merriam’s kangaroo rat) are patronyms honoring C.
Hart Merriam, father of the U. S. Bureau of Biological
Survey, and founding president of the American Society
of Mammalogists. Merriam himself authored the name
of C. nelsoni , another of the pocket mice examined in
this study. Recognizing the importance of the diverse
habitats of Texas, Merriam commissioned an extensive
biological survey of the state that spanned the years
1889-1906 (Schmidly 2002), and Vernon Bailey was
appointed to lead the field work in Texas. The result
of this assignment was a classic work (Bailey 1905)
on the reptiles and mammals of Texas. Ba iley (1905)
reported on the range, habitat, behavior, and geographic
variation of pocket mice, and described field methods
appropriate to each of the species.

Since Bailey, other prominent researchers have
taken up the cause of Texas mammalogy. Upon earn¬
ing his Ph.D. in 1937 under Joseph Grinnell, William
B. “Doc” Davis accepted a position at Texas A&M,
where he established a notable program in mammal¬
ogy (Layne and Hoffmann 1994). Davis authored or
coauthored five editions of The Mammals of Texas,
earning the title of “father of mammalogy” in Texas
(Schmidly 2002). Thirteen of Porter’s photographs
of rodents and carnivores were included in the second
edition (Davis 1960), and the third (Davis 1966) and
subsequent editions also incotporated information from
this study. It was Davis who proposed the study and
directed Porter’s (1962) graduate committee.

Early mammalogists recognized the Big Bend
as an important and fascinating ecosystem. Among
his many localities, Bailey (1905) collected a few
sites in the Big Bend area, and found both species
of Chaetodipus. He did not report P. merriami from
the Big Bend region, though it was collected in other
areas of the state. When writing of Merriam’s pocket
mouse, Bailey’s (1905) affection for “the little fellows”
is obvious in the warm style characteristic of scientific
writing in that period. Though his scientific writing

style was more detached, my father often spoke of P.
merriami in similar terms.

Borell and Biyant (1942) mention a few sporadic
mammal collecting trips by other field biologists in the
early twentieth century. However, the work of Borell
and Bryant (1942) represents the first extensive mam¬
malian study focusing specifically on the Big Bend
region, and was done in anticipation of the creation of
BBNP They made five collecting trips, some lasting
for several weeks, during 1936-1937, collecting 51
specimens of C. eremicus, 29 of C nelsoni , and four
ofP. merriami . They incorrectly regarded P. merriami
as rare in the area, probably because of the use of snap
traps, rather than of the hand-collecting method de¬
scribed by Bailey (1905) or of live traps as was done
in this study. Borell and Bryant (1942) reported on
intraspecific variation and habitat of the specimens
they collected. Tasmitt (1954) reported habitat data on
pocket mice collected from the Black Gap area (Fig.
2), near the national park.

This study (Porter 1962) filled many gaps noted
by Davis (1960) in our knowledge of these species of
pocket mice. Subsequent authors of species accounts
(Schmidly 1977a, 2004; Best 1994; Best and Skupski
1994) have relied heavily on Porter (1962). The study
is unique in focusing intently on pocket mice in the Big
Bend area, and in its scope and duration. Few faculty
or graduate students have the means or opportunity
to live and work full-time in the field for two years.
With federal support, Bailey and his field crew worked
2,185 man-days in the field (Schmidly 2002), but spent
relatively little time in any one area. Porter’s work is
particularly noteworthy for addressing nearly every
aspect of the natural history of these organisms and
their environment. Although previous workers had
identified habitat preferences of pocket mice, none had
attempted to quantify the habitat and community to the
extent done in this study.

In the fifty years since this investigation was
completed, there have been few studies comparable.
Among the most extensive subsequent studies were
Baccus’ (1971) dissertation on the effects of vegetation

Porter—Ecology of Pocket Mice in the Big Bend Region

on small rodents in BBNP and Chapman and Packard’s
(1974) yearlong survey of the ecology of Merriam’s
pocket mouse in southern Texas. Baccus (1971) studied
vegetational recovery from overgrazing in BBNP and
its effects on the distribution of small rodents including
pocket mice. Baccus (1971) also reported chiggers col¬
lected from mammalian hosts. He extensively sampled
a variety of habitats throughout the park during the
period 1969-1971. Chapman and Packard’s (1974)
mark-and-recapture study encompassed 5,922 trap
nights and provides important data on abundance, activ¬
ity, reproduction, movements, home range, and molting
that can be compared with the similar data reported
by Porter (1962) in this study. Chapman and Packard
(1974) also reported on diet and burrow architecture.
Their study differs in the habitat and community where
the animals were found and thus can be used to evalu¬
ate geographic, community, and habitat differences in
the species. Ruthven et al. (2003) studied seasonal
abundance of R merriami in southern Texas.

Several important studies have been performed
in the Big Bend or surrounding regions, but were more
broadly focused on mammals or were narrowly focused
on one or a few specific questions. Judd (1967) reported
on habitat, burrows, and cheek pouch contents of pocket
mice in Big Bend. Schmidly (1977b) surveyed the
distribution of mammals in the Chihuahuan Desert.

Boeer and Schmidly (1977) surveyed mammals along
the Rio Grande in BBNP. Wilkins and Schmidly (1979)
studied morphology and distribution of pocket mice
in the Trans-Pecos of Texas. Franklin D. Yancey, II
and his coauthors (Manning et al. 1996; Yancey 1997;
Yancey and Jones 2000; Yancey et al. 2006) reported on
distribution, habitat, reproduction, molting, and natu¬
ral histoiy of pocket mice as part of a comprehensive
study of mammals in the Big Bend area. Loomis and
Crossley (1963), Baccus (1971), Loomis et al. (1972),
Loomis and Wrenn (1972,1973), Whitaker and Easterla
(1975), and Wrenn et al. (1976) reported mammalian
parasites from BBNP. The results of all of these studies
have been cited in the text as appropriate.

Richard Porter’s study follows in the tradition
of the many natural history investigations (Schmidly
2002 ) that preceded it. This work added extensively
to our knowledge of these species and their ecology
and natural history. However, it is to be hoped that
future mammalogists, ecologists, and naturalists will
continue to expand this work across the dimensions of
time, space, and phylogeny.

Calvin A. Porter

Department of Biology
Xavier University of Louisiana

Special Publications, Museum of Texas Tech University

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Address of author:

Richard D. Porter

Deceased

Address of editor :

Calvin A. Porter

Department of Biological Sciences
Xavier University of Louisiana
New Orleans, LA 70125
cporter@xula. edu

102

Special Publications, Museum of Texas Tech University

Appendix I.

Percentage composition of plant species on the population plots based on numbers of individual plants.

Merriami Plot

Nelsoni Plot

Eremicus Plot

OVERSTORY

Ephedra

1.0

5.0

0.5

Dasylirion leiophyllum

1.0

13.0

Yucca torreyi

3.0

Acacia constricta

0.5

5.0

Dalea formosa

16.0

Prosopis glandulosa

1.0

0.5

6.0

Larrea tridentata

6.0

64.0

Guajacum angustifolium

1.5

Croton dioicus

2.0

Rhus microphylla

1.0

Janusia gracilis

1.5

Ziziphus sp.

1.0

Fouquieria splendens

3.0

5.0

Opuntia leptocaulis

24.0

0.5

9.0

Forestiera

1.0

Menodora

2.0

0.5

Tiquilia greggii

10.0

Carlowrightia linearifolia

3.0

Leucophyllum

0.5

16.0

Ruellia parryi

1.0

Flourensia cernua

42.0

0.5

1.0

Parthenium incanum

1.0

0.5

Porophyllmn scoparium

2.0

Trixis californica

4.0

Viguiera stenoloba

6.0

3.0

Unidentified

12.0

9.0

8.0

UNDERSTORY

Aristida

2.0

0.5

3.0

Bouteloua breviseta

44.0

0.5

Bouteloua eriopoda

1.0

Bouteloua trifida

1.0

Dasyoch/oa pulchella

29.0

5.0

15.0

Agave lechuguilla

20.0

37.0

8.0

Echinocereus stramineus

0.5

7.0

Mammillaria

0.5

Porter—Ecology of Pocket Mice in the Big Bend Region

103

Appendix I. (cont.)

Merriami Plot

Nelsoni Plot

Eremicus Plot

Opuntia engelmannii

8.0

0.5

8.0

Opuntia aureispina

1.0

Opuntia schottii

2.0

46.0

Senna bauhinioides

6.0

Krameria

23.0

1.0

Jatropha dioica

1.0

Unidentified

1.0

0.5

5.0

Characteristics of the important species of plant cover on the population plots. Relative dominance is calculated according to Cottam and Curtis
(1956).

104

Special Publications, Museum of Texas Tech University

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Opuntia engelmannii 43,568 7,297 11,013 73.0 1.8 53.1 50.5 35.6 25.7

Opuntia schottii - 1,700 1,300 ~ 2.1 31.7 0.0 11.2 10.7

Porter—Ecology of Pocket Mice in the Big Bend Region

105

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Rock-free Sandy Fine Gravelly Fine Gravelly Fine Gravelly Coarse Gravelly Coarse Stony Coarse Stony Rough-broken
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106

Special Publications, Museum of Texas Tech University

Porter—Ecology of Pocket Mice in the Big Bend Region

107

Echinocereus stramineus 1 0 5 <0.5

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Full text of "Movements, populations, and habitat preferences of three species of pocket mice (Perognathinae) in the Big Bend Region of Texas"

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