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HARVARD UNIVERSITY

Library of the

Museum of

Comparative Zoology

UNIVERSITY OF KANSAS PUBLICATIONS
MUSEUM OF NATURAL HISTORY

Copies of publications may be obtained from the Publications
Secretary, Museum of Natural History, University of Kansas, Law-
rence, Kansas 66045

Price for this number: $6.00 postpaid

Front cover: The subspecies of the ridgenose rattlesnake
(Crotalus willardi). Clockwise, starting from the upper left, C. iv.
amahilis, C. w. meridionalis, C. w. silus, and C. w. willardi. All
photographs by Joseph T. Collins, with the cooperation of the
Dallas Zoo.

University of Kansas
Museum of Natural History

Special Publication No. 5
December 14, 1979

THE NATURAL HISTORY OF
MEXICAN RATTLESNAKES

By BARRY L. ARMSTRONG

Research Associate

and

JAMES B. MURPHY

Curator

Department of Herpetology
Dallas Zoo

621 East Clarendon Drive
Dallas, Texas 75203

University of Kansas

Lawrence

1979

University of Kansas Publications
Museum of Natural History

Editor: E. O. Wiley
Co-editor: Joseph T. Collins

Special Publication No. 5

pp. 1-88; 43 figures

2 tables

Published 14 December 1979

MUS. COMP. ZOO'

MAY 1 7 IPR?

HARVARD Copyrighted 1979

UNIVERSITY By

Museum of Natural History
University of Kansas
'~\ Lawrence, Kansas 66045

U.S.A.

Printed By

University of Kansas Printing Service

Lawrence, Kansas

ISBN: 0-89338-010-5

To Jonathan A. Campbell

for his encouragement

*?;:»:j>.^ ,_..

-V-.^. . = ^4^4

PREFACE

Beginning in November, 1966, studies on rattlesnakes (genera
Crotalus and Sistrurus) and other pit vipers were initiated at the
Dallas Zoo which included techniques for maintenance and disease
treatments, in conjunction with observations on captive and wild
populations. Maintenance techniques and disease treatments have
been published in an earlier contribution.

The results of our studies on the ecology and natural history of
Mexican rattlesnakes are contained in the present account. Since
numerous behavioral sequences were difficult to record in the field,
many rattlesnakes were maintained in the laboratory. Over one
hundred and twenty-five captive individuals, comprising over 50
taxa (including forms indigenous to the United States) were avail-
able for study.

We have attempted to show the value of a multifaceted ap-
proach to the study of a body of organisms by beginning with field
observations as a basis for understanding, followed by maintenance
in the captive state whereupon specimens can be placed upon
death in a systematic museum collection. This arrangement allows
an investigator to examine various aspects of an animal's "being"
by recording data which would be virtually impossible to record
in the field. Further, this combined approach maximized our abili-
ties as one of us is somewhat incompetent in the field and the other
is an erratic animal keeper.

The assistance and cooperation of many persons contributed to
the completion of this study. For various courtesies extended to
us, we thank Ray Ashton, James P. Bacon, Robert L. Bezy, Charles
M. Bogert, David Brown, Mary E. Dawson, J.S. Dobbs, Michael
S. Edwards, Thomas H. Fritts, James C. Gillingham, Ronald Goell-
ner, Harry W. Greene, Herbert S. Harris, Charles Hoessle, Terry
Hulsey, J. P. Jones, Thomas L. Jordan, John E. Joy, Tommy Logan,
Arthur Lopez, Danny Lopez, Edward Maruska, Hymen Marx,
Robert W. Murphy, George R. Pisani, the late Louis Pistoia, Thomas
Porter, Steve J. Prchal, Peter C. Pritchard, William F. Pybum,
George B. Rabb, Charles W. Radclifi^e, Vincent D. Roth, Thomas
Schultz, Hobart M. Smith, Barney Tomberlin, Tom Van Devender,
R. Wayne Van Devender, James Walker, Tim Walker, John W.
Wright, Richard G. Zweifel, and our many friends throughout
Mexico.

We are grateful to Walter Aufi^enberg, Charles M. Bogert,
Charles C. Carpenter, Joseph T. Collins, Roger Conant, James R.
Dixon, William E. Duellman, the late Howard K. Gloyd, Michael
Herron, Donald W. Moore, John A. Shadduck, and the late Edward

H. Taylor for reading and criticizing the manuscript and offering
many helpful suggestions. Photographic assistance was offered by
R. Terry Basey, M. Granger, Wayne Seifert, and John H. Tashjian.
The able staff of the Dallas Zoo Department of Herpetology, David
G. Barker, Raymond K. Guese, William E. Lamoreaux, and Lyndon
A. Mitchell have recorded numerous observations and have con-
tributed to the overall maintenance of the rattlesnakes discussed
through their enthusiasm and expertise. The librarian staff of
Instituto Butantan in Sao Paulo assisted us in numerous ways.
Various persons associated with the University of Texas Press,
specifically Ann Hidalgo Mauley, Philip L. Wagner, Robert Wau-
chope, and Robert C. West, allowed us to use figures from Hand-
book of Middle American Indians. Ralph R. Woodiwiss and Anton
S. Prechtel of the U.S. Department of Commerce, National Oceanic
and Atmospheric Administration, Washington, D.C., provided cli-
matic information and allowed us to use material from Climate of
Mexico by John L. Page. Senor Silvino Aguilar Anguiano, Subdi-
rector Gral. de Geografia y Meteorologia, sent climatic data and
numerous maps for our use. Members of the Dallas Zoological
Society, through the efforts of Bernard Brister, provided partial
travel expenses.

The arduous task of typing certain parts of the manuscript was
cheerfully accomplished by Kathryn Campbell, Janet Jackson,
Martha, F. Murphy, Vema S. Murphy, and Myra Smith. Deb
Bennett skillfully executed the drawing of the map of Mexico on
the inside back cover.

Finally, special recognition must be extended to Jonathan A.
Campbell, who in so many ways aided us in the preparation of
this manuscript. His unflagging enthusiasm in the field, the gener-
ous donation of specimens under his care, his thoughtful comments
and criticisms of the manuscript, his photographic abilities and
overall encouragement made the completion of this study much
more enjoyable for us. For these reasons, we have dedicated this
study to him.

Barry L. Armstrong and James B. Murphy
Dallas Zoo
Dallas, Texas
March 1979

VI

CONTENTS

PAGE

INTRODUCTION 1

GENERAL DESCRIPTION OF THE REGION 1

MATERIALS AND METHODS 3

ACCOUNTS OF SPECIES

Crotalus atrox Baird and Girard 4

Crotalus basiliscus ( Cope ) 6

Crotalus catalinensis Cliff 8

Crotalus cerastes Hallowell 9

Crotalus durissus Linne 10

Crotalus enijo ( Cope ) 16

Crotalus intermedius Troschel 18

Crotalus lepidus (Kennicott) 22

Crotalus 7nitchelli (Cope) 29

Crotalus molossus Baird and Girard 31

Crotalus polystictus (Cope) 34

Crotalus pricei Van Denburgh 38

Crotalus pusillus Klauber 43

Crotalus ruber Cope 46

Crotalus scutulatus (Kennicott) 48

Crotalus stejnegeri Dunn 50

Crotalus tigris Kennicott 50

Crotalus tortugensis Van Denburgh and Slevin 53

Crotalus transversus Taylor 53

Crotalus triseriatus (Wagler) 56

Crotalus viridis ( Rafinesque ) 61

Crotalus willardi Meek 63

Sistrurus ravus ( Cope ) 68

DISCUSSION 70

RESUMEN 77

APPENDIX: RANGE OR ALTITUDE EXTENSIONS

REPORTED IN TEXT 78

LITERATURE CITED 79

INDEX TO HERPETOLOGICAL SCIENTIFIC NAMES 86

Vll

INTRODUCTION

The view that the central plateau of Mexico was the center of
dispersal for rattlesnakes is a zoogeographical position needing no
defense (Gloyd 1940, Smith 1946). However, infomiation relating
to many species, particularly the primitive montane forms, is limited.
There are few observations dealing with the ecology and natural
history of Mexican rattlesnakes owing mainly to the inaccessibility
of many of the populations. Mexican rattlesnakes inhabit many eco-
logical niches ranging from xeric to mesic environments. One spe-
cies, Crotalus triseriatus, reaches altitudes of 4573 m (Klauber 1956).
Many mainland forms are found in the central Mexican plateau
where they tend to be restricted to arid, rocky situations. Other
species are found in lowland desert habitats, including island popu-
lations of the Gulf of California.

The purpose of this study is to: (1) analyze and discuss the
environmental components (including physiography, vegetation, and
climate) of Mexico and their effect on the distribution of rattle-
snakes, and (2) record observations of natural and captive pop-
ulations.

GENERAL DESCRIPTION OF THE REGION

Information recorded by Goldman (1951) serves to illustrate the
physical characteristics of Mexico (Fig. 1). Barbour (1973) presents
a detailed description of the central mesa, and Morafka (1977) de-
scribes the Chihuahuan Desert. In order to understand the current
geological patterns of Mexico and the geohistorical development
and paleogeography, the reader is referred to Maldonado-Koerdell
(1964) and West (1964). Soil has been treated by Stevens (1964).

The reader should consult such maps as those included in the
works of Contreras Arias (1942) and Hernandez (1923) for climatic
data. Information on weather and climate is summarized by Vivo
(1964). Shreve (1944) discussed rainfall patterns in Mexico, north
of the Tropic of Cancer. Refer to Fig. 2 for the mean annual
rainfall in Mexico.

Natural vegetation in Middle America is depicted in Fig. 3.
Detailed accounts of the vegetational relationships for various
Mexican areas are as follows: Beard (1944), Brand (1936, 1937,
1957), Duellman (1965), Eggler (1948), Gentry (1942, 1946a, 1946b),
Goldman (1916), Goldman and Moore (1946), Holdridge (1947,
1964), Leaveuworth (1946), Leopold (1950, 1972), Lesueur (1945),
Lundell (1934, 1942), MacDougal (1908), Martin (1958), Martinez
(1945), Miranda (1942, 1952-1953), Miranda and Sharp (1950), Mul-
ler (1939, 1947), Nelson (1921), Savage (1960), Sharp (1946), Shreve
(1934, 1937a, 1937b, 1939, 1942, 1944), Standley (1920-1926, 1930),
Wagner (1964).

2 SPECIAL PUBLICATION-MUSEUM OF NATURAL HISTORY

Fig. 1. Physiographic regions of Mexico. Reprinted from Handbook of
Middle American Indians. Copyright 1964 by Univ. Texas Press. Used by
permission of Robert C. West.

[ ~] UNOEH 250
P'j 2S0-1OO

500-750 MM

750 - 1000 MM

'000- 1500 UM

'500-2000 MM

I 2000-2500 Mw

I 2500-3000MM

I OvEIO 3000MM

Fig. 2. Mean annual rainfall (millimeters) for Mexico. Reproduced by
permission from the National Oceanic and Atmospheric Administration,
Washington, D.C.

THE NATURAL HISTORY OF MEXICAN RATTLESNAKES 3

TROPICAL RAINFOREST FORMATION

MONTANf FORMATION SERIES

SEASONAL FORMATION SERIES

DRY EVERGREEN FORMATION SERIES

SEASONAL SWAMP FORMATION SERIES

CALIFORNIA CHAPARRAL

STEPPE. THICKET. & SCRUB DESERT

Fig. 3. Natural vegetation of Mexico. Formations classified after Beard
and others. Reprinted from Handbook of Middle Atnerican Indians. Copy-
right 1964 by Univ. Texas Press. Used by permission of Philip L. Wagner.

MATERIALS AND METHODS

Most of the localities which are listed in the following species
accounts are based on our own observations unless otherwise spe-
cified, although voucher specimens were not collected in many
instances. Since the primary purpose of the study was not to collect
rattlesnakes for systematic collections, many of the individuals ob-
served were not collected and in some cases, parturient females
were held until they gave birth then released with the young at the
same locality. Litters of snakes were not measured and weighed
in the field. In the laboratory, weights were recorded with an Ohaus
triple-beam balance. Measurements were made on newborn snakes
in most cases, using the method described by Quinn and Jones
(1974).

Snakes that were removed from wild populations have been
deposited (or will be upon their death) in the following vertebrate
collections: University of Colorado (UCM), Dallas Museum of
Natural History (DMNH), Herbert S. Harris/ Robert S. Simmons
Private Collection (HSH-RSS), University of Kansas Museum of
Natural History (KU), and University of Texas at Arlington (UTA).
Each museum number refers to an individual specimen.

4 SPECIAL PUBLICATION-MUSEUM OF NATURAL HISTORY

ACCOUNTS OF SPECIES

Crotalus atrox Baird and Girard

Crotalus atrox is one of the largest and most aggressive of the
Mexican rattlesnakes (Klauber 1972: 448). Although it has been
confused with C. scutulatus (Klauber 1972: 541), C. atrox is gen-
erally well known throughout its range. These rattlesnakes are
found in a wide variety of habitats at varying elevations. Snakes
from southern Oaxaca (Gloyd 1940: 206; Klauber 1952: 102) and
central Veracruz (Klauber 1952: 103) present problems in C. atrox
distribution.

Specimens obtained by us are from the following localities:
BAJA CALIFORNIA: Progreso; SONORA: 4.8 km S Sonoyta,
Caborca, Esqueda; CHIHUAHUA: 16-32 km S Chihuahua (4 spe-
cimens), near El Sueco; NUEVO LE6N: 16-48 km NE Sabinas
Hidalgo (10 specimens).

In Mexico, C. atrox inhabits deserts, plains, grasslands, and foot-
hills from near sea level at La Posa, Sonora (Taylor 1936: 497) to
at least 2440 m near Alvarez, San Luis Potosi (Klauber 1972: 527).
Individuals are generally found in arid country and are not neces-
sarily associated with rocks. Mesquite, dry wash banks, creosote
bushes, and burrows provide likely retreats. Taylor (1936: 497)
found this snake common along the seashore where shrubs and
sandy beaches met. We found C. atrox especially plentiful in mes-
quite grasslands north of Sabinas Hidalgo in Nuevo Leon. Several
snakes have been observed in this area along roads at night during
the hot summer months. Hardy and McDiamiid (1969: 213) con-
sidered this species to be rather uncommon in tlie lowlands of ex-
treme northern Sinaloa, and R. T. Basey (pers. comm.) stated that
C. hasiliscus was much more common than C. atrox in the nearby
Alamos area of southern Sonora. In northern Sonora, near Esqueda,
these snakes are not uncommon in sandy, creosote habitats.

Throughout its range, tliis rattlesnake is mostly nocturnal, at
least during the summer montlis. Most snakes obtained by us have
been collected while crossing roads during the early evening. Some
individuals have been seen in the early morning or on cloudy days,
but they are generally not active and are usually coiled in the
mouth of a burrow or under the cover of available brush.

We have observed ritualized combat between males in captivity.
Wiley (1929), Hoessle (1963) and Petzold (1963) have discussed
reproductive biology and mating behavior in this species. We ob-
served eight C. atrox courtship sequences in captivity and a gen-
eral pattern was apparent. The enclosure in which observations
were made measured 80 X 40 X 40 cm high, and the snakes mea-

THE NATURAL HISTORY OF MEXICAN RATTLESNAKES 5

sured ca. 1.2 m total length. The courtship behavior detailed below
was typical for C. atrox observations made by us.

0900h The female was placed with the male.

0901 The male sensed her presence and began rapid tongue flicking
( 1 per sec).

0906 The female responded defensively for a few minutes, but began
to react to male.

0911 Male raised anterior third of body 5-6 cm above the surface. Spas-
modic jerking of male was apparent throughout entire period (1
every 2 sec ) and increased to 1 per sec.

0917 Male started rapid tongue flicking on closest part of body and be-
gan to investigate entire lateral and dorsal body surface of female.
Jerking of male became more intense ( 2 per 3 sec ) . Male held head
at 45° angle with mental scale touching dorsum of female. Head of
male pressed against dorsum of female, and side-to-side motions
(approx. 2.5 cm) were undertaken in clusters (4-5) at irregular
intervals.

0927 By moving in an anterior plane, male began to drape large radius
coils over female.

0931 Sequence lasted until most of male's body was in contact \vith
female. Male attempted to insert hemipenis but no external eversion
of hemipenis was in evidence. Both snakes lifted the tails at the
location of the cloacae and occasionally flicked the tails with a
side-to-side motion. The substrate was not touched by the tails.
Male constantly moved tail in an attempt to align cloacae. When
cloacae were juxtaposed, male tried to slide tail under tail of female.

0951 Cloacae were aligned and male inserted. During the latter part of
courtship activity, female began twitching with same frequency as
male. Jerking and twitching stopped when male inserted. Courtship
lasted 20 min and no movement other than steady pulsation of the
body near male's tail was in evidence after penetration. Both sexes
would occasionally drag each other slowly by the attached hemi-
penis; this behavior perhaps was initiated by external stimuli.

Actual copulation lasted from 2-8 hours. Breeding dates were
recorded from 21 January to 26 January 1974.

Variation of pre-coital behavior was observed on 28 February
1975. During the courtship period, the male was loosely looped
over the female and undulated slowly. In addition, the male tried
to position his tail under the cloaca of the female. In contrast to
the behavior observed previously, the female raised her tail and
repeatedly opened her cloaca. When the cloacae were juxtaposed,
the male inserted his hemipenis but no external extrusion of the
hemipenis was noticed. The male exhibited pulsating throbs (once
every 3 seconds) intermittently and the snakes were in copulation
nearly 7 hours. The female remained passive throughout the re-
mainder of the sequence. When the snakes were placed in a larger
unit (1.0 X 0.8 X 1.2 m high), the female assumed a vertical anterior
trunk posture as the male directed courtship behavior to her. This
may be a female rejection posture. Of nine captive litters, two
were born in June, one in July, three in August, two in September
and one in October. Sexual maturity was reached in 30-36 months.

6 SPECIAL PUBLICATION-MUSEUM OF NATURAL HISTORY

Litter size varied from six viable young (4 infertile egg masses) to
25 viable young.

Vegetational characteristics associated with the habitat of C.
atrox in Mexico include the Seasonal Formation Series, the Steppe,
Thicket and Scrub Desert as defined by Wagner (1964). Gloyd
(1937) and Lowe (1964) described the faunal areas inhabited by
this species in Arizona.

Crotalus hasiliscus (Cope)

Crotalus basiliscus basiliscus. Crotalus h. hasiliscus is one of
the largest and gentlest of the Mexican Crotalus. Gloyd (1940: 161),
Bogert and Oliver (1945: 394), and Klauber (1952: 87) have com-
mented on its status in relationship to C. molossus. As far as is
known, the distribution of these two species usually does not over-
lap, but Hardy and McDiarmid (1969: 216) gave reasons for be-
lieving that the two species live sympatrically in northern Sinaloa.
Further investigation is necessary in this area. Klauber (1972) indi-
cates an overlap in the ranges of C. h. oaxacus and C. molossus
nigrescens in Oaxaca. We have found C. h. basiliscus to be a com-
mon snake (30 specimens) of Mexico's west coast with individuals
being observed in tropical thorn forests and, to a lesser degree, in
tropical deciduous forests.

Many records of C. h. hasiliscus are available, no doubt owing
to the fact that this rattlesnake is very common. Examples of this
snake are known from extreme southern Sonora, south through
Sinaloa, Nayarit, Jalisco, Colima, and western Michoacan where its
range apparently terminates at the Rio Balsas. It would not be
surprising, however, to find this rattlesnake on the coastal plain of
Guerrero where topographical similarities to its known habitat are
apparent. Specimens obtained by us are from the following lo-
calities: SINALOA: 4 km S Santa Lucia, 5 km W Concordia;
NAYARIT: 4.8 and 8 km E San Bias; JALISCO: 8 km N Ciudad
Guzman, Tamazula; MICHOACAN: Dos Aguas.

Crotalus h. hasiliscus is generally a lowland resident inhabiting
the previously mentioned tropical thorn or tropical deciduous for-
ests. Klauber (1972: 528) reported that a specimen from Apatzingan,
Michoacan, was collected at 975 m elevation in the arid tropical
scrub forest. He felt this to be the upper limits of the altitudinal
range of this form, but he was no doubt unfamiliar with the speci-
men from 19.2 km NE Santa Lucia in Sinaloa (KU 78966) which
was collected at 1940 m in a humid pine-oak forest. We can extend
the altitudinal limits with a recently collected specimen (UCM
51313) from near Dos Aguas, Michoacan. This snake was found in
July 1974, well within the confines of the Sierra de Coalcoman at
an elevation of 2225 m. The rattlesnake was located within 3 m
of a stream in a canyon bottom in the humid pine-oak forest. Local

THE NATURAL HISTORY OF MEXICAN RATTLESNAKES 7

residents recognized the individual, but reported that C b. hasiliscus
was not nearly so common as the abundant C. pusillus or the fairly
rare C. durissus cuhninatus. Despite the fact that occasional snakes
may reach elevations above 2000 m, we consider such occurrences
rare, for C. h. hasiliscus is apparently a lowland foim. Most of our
specimens have come from below 1000 m in various tropical forests
(see Hardy and McDiarmid 1969: 51-58). Another snake (UTA
R-6120) was found on the south slope of Cerro Baralosa in the
Sierra de Coalcoman. On 21 July 1976, an individual snake (UTA
R-6071) was found coiled in shade among rocks at 915 m at Plomo-
sas, Sinaloa, in a transition pine-oak and tropical deciduous forest
(Fig. 4). This snake was collected in the same rock slide as an
example of Crotalus stejnegeri. C. b. basiliscus appears to be most
common, or at least most active, during the summer rainy season,
and most individuals are located crossing roads. Hardy and Mc-
Diarmid (1969: 214) reported that these rattlesnakes had also been
found in the dry season in Sinaloa, although this is the period of
least activity.

Vegetative components associated with this rattlesnake are the
Arid Tropical Scrub Forest (Duellman 1965) and the Tropical Thorn
Forest and Tropical Deciduous Forest as defined by Leopold

Fig. 4. Habitat of Crotalus basiliscus basiliscus, C. lepidus maculosus and
C. stejnegeri near Plomosas, Sinaloa, Mexico, September 1976. Crotalus I.
maculosus occurs in the pine-oak forest covering tlie top of the bkiffs. Crotalus
stejnegeri and C b. basiliscus are found in the tropical deciduous forest on
the lower hillsides and the transition zone at the base of the bluffs. (Photo-
graph by Jonathan A. Campbell.)

8 SPECIAL PUBLICATION-MUSEUM OF NATURAL HISTORY

(1950). According to Goldman (1951), mesquites, acacias, wild figs,
Spanish cedar, palo mulato (probably Bursera grandifolia), wild
guava (Psidium guajava), Costilla elastica, silk-cotton trees and
palms grow in the area inhabited by this rattlesnake.

Crotalus basiliscus oaxacus. A Oaxacan subspecies, C. h. oaxa-
cus, separated geographically from C. h. basiliscus by the entire
state of Guerrero, is a rather neglected form from the interior high-
lands of central Oaxaca. Described in 1948 by Gloyd from two
specimens, this subspecies has remained rare in systematic collec-
tions. Much of the vegetation in the area once inhabited by C. h.
oaxacus is now either destroyed or near destruction. This rattlesnake
was apparently quite common in the Valley of Oaxaca, but is now
rarely encountered there, most individuals now being taken from
the mountains surrounding Oaxaca. These mountains exhibit typical
pine-oak habitat where rock outcrops are occasionally found, and
these areas are superficially similar to the habitat of C. molossus.
They seem to prefer the more heavily forested areas and, according
to the local people, are often seen among rocks where C. intermedius
gloydi is found. The first specimen obtained by us from El Tejocote
(also known as Tejocotes) was located on a steep hillside within the
pine-oak forest at 2438 m. We had previously collected in this area
rather extensively for three years, and this was the first example
of C. b. oaxacus we had encountered. A second specimen was se-
cured along the Rio Colorado near El Tejocote during the first
week in February 1976 at an elevation of 1982 m (J. A. Campbell,
pers. comm.). Another was collected on 24 March 1976 in a pine-
oak forest at an elevation of 2285 m (UTA R-6820). An individual
(UTA R-6060) was found killed on the road on 31 July 1976 at 1030
h in an oak forest (1932 m) at 35.5 km NW Telixtlahuaca.

C. b. oaxacus inhabits the Montane Formation Series and Mon-
tane Thicket as defined by Wagner (1964) and Pine-Oak Forest
(Duellman 1965).

Crotalus catalinensis Cliff

Crotalus catalinensis is found only on Santa Catalina Island,
Raja California del Norte, and is related (ecologically) most closely
to C. ruber lucasensis. The island is rocky and barren with sparse
brush and cacti (Klauber 1972). The temperatures of the coastal
islands are similar to the adjacent mainland (Nelson 1921).

One of the male snakes maintained by us, born during September
1975, exhibited head-bobbing and tongue-flicking courtship be-
havior toward another individual (KU 173096). Another pair copu-
lated on 15 Januar>' 1978. This rattlesnake is found in the Seasonal
Formation Series and Cactus Scrub as defined by Wagner (1964).

THE NATURAL HISTORY OF MEXICAN RATTLESNAKES 9

Crotalus cerastes Hallowell

The two subspecies of this rattlesnake that occur in Mexico,
C c. laterorepens and C. c. cercobornbus will be referred to as C.
cerastes, since their habitats, habits, and behavior are essentially the
same. Approximately 300 individuals have been observed by us on
the eastern drainage of the Sierra Juarez and Sierra de San Pedro
Martir as far south as Puertecitos, a fishing village on the Gulf of
California in Baja California del Norte. In Sonora, C. cerastes is
located north and west of the Nogales-Hermosillo-Guaymas high-
way (Route 15), with the heaviest concentration of snakes being
found in the Desierto de Altar, an arid sandy desert in the extreme
northwest part of the state.

Crotalus cerastes generally prefers the sandy areas of its desert
environment, although it is not restricted to that particular habitat.
Dammann (1961) reported that C. cerastes populations may be af-
fected by the amount of vegetation present. In Arizona, the popula-
tion density of this rattlesnake tended to decrease as the vegetation
increased. Too much vegetation may limit the unique locomotor
abilities of this species. These rattlesnakes can be found on hard
stony terrain, such as the Cottonwood Springs area of Joshua Tree
National Monument. These particular individuals are not as com-
mon as those found in sandy situations, but they appear to be
slightly larger in body girth. This may be due in part to a different
diet as mammals rather than lizards are more plentiful in these
areas. This terrain also has a more luxuriant growth of desert vege-
tation. Generally, C. cerastes will commonly be found on sandy
alluvial fans, sand dunes, sandy washes, and the fringes of desert
dry lakes. The greatest concentrations of these rattlesnakes are
usually found in areas where wind-blown sand forms small mounds
at the bases of creosote bushes (see Brown 1971). Miller and Steb-
bins (1964) found C. cerastes resting under bushes. The areas around
San Felipe, Baja California, Yuma, Arizona, and Mexico Route 2
between Mexicali and Sonoyta are good examples of this type of
habitat. In these situations, C. cerastes is easily located, usually at
the mouth of a burrow, or beneath bushes where shade is available.
One snake from north of Puerto Peiiasco, Sonora, was found in a
creosote bush about 30 cm above the sandy soil. This individual
appeared to be basking rather than hunting, as the specimen was
not alert and did not seem to sense our presence. In these areas
the sand shifts constantly and C. cerastes is often observed coiled
near a bush, partially concealed by sand. Brown (1971) suggested
that the unusual cratering behavior serves primarily for thermo-
regulation and secondarily for concealment during the day, whereas
at night the reverse is true. Individuals have also been found
within and under the wreckage of dilapidated buildings where as

10 SPECIAL PUBLICATION-MUSEUM OF NATURAL HISTORY

many as five specimens have been located in one search. This
rattlesnake is mostly nocturnal but may be seen during the day in
early spring and late fall. Activity cycles seem to be related pri-
marily to thermal considerations (Brown 1971). Moore (1976) sug-
gested that due to circulatory adjustment, tightly coiled, inactive
sidewinders (and C. mitchelli pyrrhus) were able to conserve heat
more effectively than uncoiled snakes. Often, ten or more C. cerastes
may be encountered by driving roads at night in areas where they
are common, with as many as 30 occasionally being seen. Peak ac-
tivity appears to be late May and early June, particularly in the
hours just after sundown. Sidewinders will often coil on asphalt
roads where automobiles cause heavy mortality. Brown (1971) felt
that the snakes used roads as a source for irradiated heat.

Sidewinders are rather pugnacious and will strike readily. They
will often turn and bite when restrained, a characteristic usually
associated with montane forms (C. ivillardi, C. polystictus). They
prefer a diet of lizards (Uma, Dipsosaurus, Cnemidophorus, Uta,
Crotaphytus) in captivity, and captive sidewinders will often refuse
mice, but readily consume an iguanid lizard. We observed a captive
adult female C. c. cercohombus feeding on three of her newborn
young.

The vegetational regimes inhabited by this species include the
Seasonal Fonnation Series and Cactus Scrub as defined by Wagner
(1964). Lowe (1964) described the habitat of this snake in Arizona.

Crotalus durissus Linnaeus

The Neotropical rattlesnakes, C. durissus, are among the largest
and sometimes most aggressive rattlesnakes indigenous to Mexico.
With the exception of an isolated subspecies, C. d. totonacus found
in Tamaulipas, Queretaro, Veracruz, and southern San Luis Potosi,
snakes of the C. durissus complex are inhabitants of the country
south and east of the Mexican Plateau, including the entire Yucatan
peninsula. Despite the fact that the common name given C. durissus
suggests a jungle inhabitant, these rattlesnakes rarely will be found
in such moist places, much preferring savannahs or partially wooded
hillsides.

Crotalus durissus durissus. Crotalus d. durissus is a resident of
the Mexican states of Tabasco, Chiapas, central Veracruz, and
southeastern Oaxaca (Klauber 1972). Klauber (1952: 65) cited differ-
ences in snakes from near Jalapa and Orizaba, Veracruz, and sug-
gested that with examination of more specimens this population
may merit subspecific recognition. Crotalus d. durissus has been
recorded by us from the following locaHties: CHIAPAS: 12.8 to
18 km NE Tapanatepec; OAXACA: 19.3 km W Tehuantepec.

This rattlesnake prefers dry savannah habitats with frequent
rock outcrops. Hartweg and Oliver (1940) characterized the habitat

THE NATURAL HISTORY OF MEXICAN RATTLESNAKES 11

of this snake in the vicinity of Tehuantepec as plains. Klauber
(1972: 531) suggested that the habitat of C. d. durissus was some-
what hke that of the arid southwestern United States. Specimens
obtained by us from extreme southeastern Oaxaca and southwest-
em Chiapas were found to be very common in rocky, grassy situa-
tions where forests had been partially cleared. In late June, three
weeks after the rainy season had begun, individuals were found
at night on the road between Tapanatepec and Rizo de Oro. Most
were observed on evenings when rains were in the form of light
drizzle. Crotalus d. durissus ranges from near sea level to at least
1585 m near Comitan, Chiapas (Klauber 1972) and is found in the
Tropical Rain Forest as defined by Wagner (1964). According to
Goldman (1951), the wild gourd tree, various cacti, Cassia, Acacia
farnesiana, Prosopis julifiora, two species of Jatropha, Annona, two
or more species of Ficus, Ipomoea, two species of Pithecollobium,
fan palms and the guasima (Guazuma uhnifolia) are found in the
area inhabited by tliis snake.

On 8 December 1976, W. E. Lamoreaux (pers. comm.) introduced
an adult male C. d. cumanensis into an enclosure (100 X 80 X 75 cm
high) containing an adult female C. d. durissus. The male initiated
head-bobbing movements with rapid tongue-flicking over the fe-
male's dorsum. The female remained passive and no intromission
was observed. The next day, the pair was in copulo at 1300 hours.
The coital position of the pair was unusual for although the snakes
were firmly joined by the hemipenis, the tails were pointing in
directly opposite planes. The male's body pulsated near the vent
and the tail was curled upward. During intromission, the male
opened his mouth intermittently and began a series of vertical head
and neck jerks with occasional tongue flicking. Parturition occurred
on 10 June 1977, and produced one dead and nine viable neonates.

Crotalus durissus culminatus. Crotalus d. culminatus, a Pacific
Coast race, ranges from near sea level near La Placita, Michoacan,
and Copala, Guerrero (Wayne Seifert, pers. comm.), to 1982 m near
Morelia, Michoacan. These rattlesnakes may reach elevations near
2285 m in the Sierra de Coalcoman where we have seen tanned
skins. Local residents have identified live individuals, but this
identification should remain tentative until more reliable data are
secured. This subspecies occurs from near the Colima border at
La Placita, Michoacan, northeast to Morelia, east through southern
Morelos and western Puebla to extreme southwestern Oaxaca, in-
cluding the entire state of Guerrero. Specimens have been recorded
by us from: MICHOACAN: near La Placita, 7 km W Morelia
(J. A. Campbell, pers. comm.), Morelia; MORELOS: 5 km W
Jojutla.

Crotalus d. culminatus is an inhabitant of rough, rocky, generally
arid habitats (Fig. 5). Aldiough Klauber (1972: 530) thought that

12 SPECIAL PUBLICATION-MUSEUM OF NATURAL HISTORY

Fig. 5. Habitat of Crotalus durissus culminatus, near Jojutla, Morelos,
Mexico. Open tropical deciduous thorn forest characterized by limestone
outcrops. (Photograph by M. Granger.)

this rattlesnake avoided lowland situations, snakes from Copala,
Guerrero, and La Placita, Michoacan, have been recorded from
near sea level. This lowland habitat can be described as arid
tropical scrub forest, characterized by frequent rock outcroppings.
Davis and Smith (1953: 141) found C. d. culminatus common in
lowland mountains south of Tepaltzingo, Morelos, on the northern
slopes of the Rio Balsas basin. This area is also interlaced with
extensive limestone outcroppings. Duellman (1961: 121) commented
that 18 specimens from El Sabino, Michoacan, were found near the
upper limits of the arid scrub forest (1050 m) on the lower slopes
of the Cordillera Volcanica. A specimen from near Morelia,
Michoacan, the nortliern limit of the range of C. d. culminatus,
was taken on the edge of a lava How at 1982 m in a mesquite grass-
land habitat with isolated stands of scrubby oaks. Throughout
much of its range, this subspecies is generally nocturnal, and seems
to reach a peak period of activity during the summer rainy season,
as do most Mexican Crotalus. A captive pair was observed copu-
lating on 4 February 1978.

Crotalus d. culminatus inhabits the Arid Tropical Scrub Forest
(Duellman 1965) and the Temperate Pine-Oak Forest as defined by
Leopold (1950).

Crotalus durissus totonacus. The northeastern subspecies, C. d.
totonacus, is probably the least studied form of C. durissus. Since
its description in 1940 (Gloyd and Kauffeld), few specimens have

THE NATURAL HISTORY OF MEXICAN RATTLESNAKES 13

reached collections, and only recently, with the construction of
new roads, has C. d. totonacus been observed with some frequency
by field investigators. Records obtained by us are from TAMAULI-
PAS: 48 km N Soto la Marina, 16 km N Aldama. Four subadult
individuals were found killed on the road in Tamaulipas: El Carrizo
at Kilometer Post 26 on 2 March 1977 (KU 174825), 20.8 km N Soto
la Marina on 29 April 1977 (KU 174826), 38.4 km N Soto la Marina
on 29 April 1977 (KU 174827), 33.6 km N Soto la Marina (KU
174828). Topographical similarities suggest that C. d. totonacus
may be found in the state of Hidalgo.

Like most races of C. durissus, C. d. totonacus is typically a
lowland inhabitant, although specimens have been reported from
1680 m in the lower cloud forest of the Sierra de Guatemala (Martin
1958). Throughout most of its range, the areas inhabited by this
snake are either tropical thorn or tropical deciduous forests marked
by distinct wet-dry seasons. Two specimens (both juveniles) from
east-central Tamaulipas, collected by John E. Joy (pers. comm.) on
7 and 8 August 1975, were found active shortly after dark during
rain showers (Fig. 6). A juvenile (UTA R-6707) found killed on the
road at Kilometer Post 17 near La Marina Viejo on 2 March 1977
contained rodent hair. Crotalus d. totonacus appears to prefer areas
around any type of watercourse. Local residents assured us that
this is where most of these rattlesnakes were seen, but Dixon et al.
(1972) found this subspecies in eastern Queretaro at 1585 m in a

'^

Fig. 6. Subadult Crotalus durissus totonacus. Specimen from 48 km N
Soto la Marina, Tamaulipas, Mexico. (Photograph by Jonathan A. Campbell.)

14 SPECIAL PUBLICATION-MUSEUM OF NATURAL HISTORY

pine-oak forest near no standing water. P. Pritchard (pers. comm.)
found a large individual on the bank of a small dam. These rattle-
snakes are common, but as is the case of most forms of C. durissus,
the peak period of activity seems to be limited to the rainy season.
Growth rates of captive young are the most rapid of any rattlesnake
taxa maintained by us.

C. d. totonacus inhabits the Tropical Thorn and Tropical De-
ciduous Forests, some elements of the temperate deciduous forest
as defined by Leopold (1950), and the Seasonal Formation Series
and Steppe, Thicket and Scrub Desert as defined by Wagner (1964).
The area supports mesquite, huisache, ebony and other shrubs.
Along the streams, bald cypresses (Taxodium) and cottonwoods
are common (Goldman 1951).

L. A. Mitchell and J. E. Joy (pers. comm.) discovered a male
C. d. totonacus measuring 1.7 m total length, in combat with an
adult ocelot, Felis pardalis, during February 1977. The animals
were below a thick canopy of thorn scrub in early afternoon and
the ground beneath the canopy supported little vegetation. A dis-
tance of 1.3 m separated the combatants, and the snake was in a
defensive posture typical of C. durisnis, and rattling vigorously.
The ocelot remained immobile with the right foreleg raised. When
the snake was extended fully with its mouth open after a strike,
the ocelot raked with its claws within the snake's mouth, then
withdrew the paw before the snake could close its mouth. Later
examination of the oral cavitv and throat of the snake revealed a
number of lacerations in the soft tissue and masseter muscles which
were inflicted by the ocelot, but the interstitial skin and scales ex-
terior to the lacerations were not affected. Lacerations were found
on tlie dorsal aspect of the head and neck, but not near the punc-
tures within the snake's mouth or throat. The snake's lower jaw
was out of alignment and the mandibular bone was visible when
the mouth was open. A large female C. d. totonacus was located

15 minutes later, within 12 m of the male. The male rattlesnake
was treated for its injuries, but succumbed about two weeks later
(KU 174824). The snake regurgitated a large number of ascarids
during the two weeks prior to its death.

Crotalus durissus tzabcan. The Yucatan race of this species,
C. d. tzabcan, unlike C. d. durissus and C. d. cuJminatus, is strictly
a lowland inhabitant from the Yucatan peninsula, and ranges from
eastern Tabasco to include all of Campeche, Quintana Roo, and
Yucatan. Neill and Allen (1959, 1960) reported on this rattlesnake
in British Honduras.

Crotalus d. tzabcan is quite common throughout the Y'ucatan
peninsula. Duellman (1965: 611) described its habitat as scrub
forest with numerous limestone outcrops. Four specimens obtained
by us were found between Kantunil and Chichen Itza, Yucatan,

THE NATURAL HISTORY OF MEXICAN RATTLESNAKES 15

after 1600 hours (UTA R-5659-60), following a two-hour rainshower.

A male, taken on 18 September 1974 between Kantunil and
Chichen Itza, Yucatan, and a female obtained on 29 August 1969
at Uxmal, Yucatan, were placed together on 11 October 1974. The
following observations were recorded by Terry Hulsey (pers.
comm.). The female exhibited nervousness and refused to coil near
the male. On 24 October she was placed in isolation and again
introduced on 7 November. The male shed on 29 December and
the female was coiled next to the male for the first time since they
were introduced. Courtship behavior was seen on 30 December.
The male began rapid head-bobbing and tongue-flicking. He ap-
proached the female and tapped her dorsum repeatedly in tlie
vicinity of the 13th and 15th scale rows. The male's activity lasted
ca. 30 seconds and he remained immobile for ca. 30 seconds. At the
end of each period of activity, the male's tail encircled the female
in the vicinity of the vent and he jerked violently. Generally, the
head-bobbing motions were directed anteriorly. Pre-coital activity
was continued after the male discontinued the cloacal searching
motion. The snakes did not copulate. On 2 January 1975 the cage
was sprayed with water at 0830 hours and the male began twitching
shortly thereafter. The snakes were found in copulo at 1500 hours
and the male slowly moved his tail in a laterally directed twitching
motion. The female was weighed periodically after copulation and
the weight increase was as follows: 24 February 1975 (2.1 kg); 20
April (2.6 kg); 12 May (2.7 kg); 25 July (3.0 kg). Twenty-one young
(two dead, KU 158547-8) were born on 31 August and were immedi-
ately weighed and measured. The range of variation is as follows:
total length 290-350 mm, mean 316; weight 18.4-26.8 g, mean 23.4.
Other parturition dates are 31 August 1975 (UTA R-6732, 6802), 2
August 1976 (UTA R-6733) and 3 August (UTA R-6734-35). Figure
7 represents one of the young at 35 months.

Crotalus d. izahcan is found in the Tropical Evergreen and
Tropical Rain Forest as defined by Leopold (1950). The area sup-
ports Enterolobium cyclocarpum, two or more species of Ficus,
Spanish cedar, logwood, two species of silk-cotton, the chico zapote
(Achras zapota), palo mulato (Bursera), two or more species of
Cassia, giant nettle (Urera caracasana) and Agave (Goldman 1951).

The subspecies of C. diirissus are very unpredictable in tem-
perament. At times they are aggressive and stand their ground,
whereas at other times they are placid. Our specimens have ex-
hibited both of these behavioral extremes. Most C. durissus are at
least partially nocturnal, and all seem to reach peak activity during
the summer rainy season. Young of this species have been collected
from late June through late August.

16 SPECIAL PUBLICATION-MUSEUM OF NATURAL HISTORY

*?

m^'^'i.

Fig. 7. Crotalus durissus tzabcan. See text. (Photograph by Jonathan A.
Campbell. )

Crotalus enijo (Cope)

Crotalus enyo, though widespread throughout Baja CaHfornia,
remains biologically neglected in literature on rattlesnakes (Klauber
1931). It is characterized by an unusually small head and rather
large rattles, also traits of C. tigris, a species not found in Baja
California. Races of C. enyo are moderately sized rattlesnakes, and
throughout much of the range, are lowland or mountain desert
dwellers. Van Denburgh and Slevin (1921) found this rattlesnake
around human habitations.

Crotalus enyo enyo. The subspecies C. e. enyo is a rather com-
mon rattlesnake in the Cape region of Baja California del Sur (Fig.
8), Specimens are known from El Marmol, Baja California del
Norte, south throughout the peninsula to the southernmost point.
Specimens obtained by us were collected at San Antonio (64 km S
La Paz) and 14.4 km N Buena Vista (B. Tomberlin, pers. comm.).

Crotalus e. enyo is an inhabitant of arid, rugged deserts and
desert mountains throughout its range. Klauber (1972: 531) sug-
gested that snakes found from La Paz to San Jose del Cabo were
from an arid region containing a profusion of cacti, xerophytic
shrubs, and rocks. In central Baja California, this rattlesnake lives
in association with large boulders, the giant cardon cactus (Cereus
pringlei), the boojum (Idria), and the long-lived elephant tree. B.
Tomberlin (pers. comm.) commented that, in the area south of La
Paz, this subspecies lives sympatrically with C. ruber lucasensis

THE NATURAL HISTORY OF MEXICAN RATTLESNAKES 17

1

\m

■St'

=^- >-

Fig. 8. Crotalus enyo enyo. Specimen from 14.4 km N Buena Vista, Baja
California del Sur, Mexico. (Photograph by John H. Tashjian. )

and, to a lesser degree, with the speckled rattlesnake, C. m. mitchelli.
He also stated that specimens of C. e. enyo are found in that area
from arid flat desert plains to rocky desert mountains.

Crotalus e. enyo is nocturnal for a good portion of the year, and
seems to reach a peak activity period in the early fall when southern
Baja California receives a fair amount of its annual rainfall. At
this time it is commonly found crossing roads at night.

Captive reproduction in C. e. enyo has been reported (Tryon
and Radcliffe 1977). On 15 January 1975 courtship activity among
snakes maintained by us was observed between 1530-1700 hours.
The female had just shed and the male rubbed his mental area on
the female's dorsum with convulsive forward jerks in the manner
of C. willardi silus (described later in this paper). During this pe-
riod the male tried vigorously to thrust his tail beneath the cloacal
region of the female, and in some instances, the male's tail com-
pletely encircled the female's cloaca. The male would slide forward
1-2 cm by extending his loosely draped coils and bracing his body
at the cloacal region of the female. Low intensity twitches accom-
panied this behavior. Copulation was not observed.

Crotalus e. enyo is an inhabitant of the Seasonal Formation
Series as defined by Wagner (1964).

Crotalus enyo furvus. The Rosario rattlesnake, C. e. furvus, is
a northern form of this species from the San Quintin Plain of the
west coast of Baja California del Norte (Lowe and Norris 1954).
Until recently, with the completion of a paved road through Baja

18 SPECIAL PUBLICATION-MUSEUM OF NATURAL HISTORY

California, C e. furvus was known from few specimens, no doubt
due to the previously poor road conditions which prevented access
to the optimal habitat of this snake.

The San Quintin Plain is characteristic of coastal, open, low-
growing scrub habitats in Baja California del Norte. Klauber (1972:
531) described the area as much like western San Diego County,
California, but somewhat more rocky and with a lighter brush
cover. Lowe and Norris (1954: 57) characterized the area as con-
taining a markedly more mesic climate and biota tiian the harsh
environments both southward and in the interior of the peninsula.
The average annual rainfall is about 12.7 cm (Beal 1948), and coastal
fog produces some additional moisture in the area. One specimen,
obtained by us near El Socorro, appeared to come from a sandy
hill where vegetation was sparse, and there was a profusion of
coastal ground cover. Many burrows were noted. A specimen col-
lected by R. T. Basey in October 1972 (ambient temperature 21°C)
came from a light-colored mud wash where vegetation was ex-
tremely sparse (pers. comm.). This specimen was a young adult
female which later gave birth to seven young on 26 August 1974.
Total length of the young was from 206-222 mm, mean 213. Dennis
Bostic (pers. comm.) related that his seven specimens of C. e. furvus
from near Punta Camalu were collected where two alluvial fans
met. Eight additional snakes were taken by one of us (BLA) be-
tween 10-15 October 1977 in the vicinity of El Rosario.

Crotalus e. furvus appears to be at least partially nocturnal in
that two individuals recorded by R. T. Basey and T. Porter (pers.
comm.) were collected on a road at night. The type specimen of
this race was taken at noon in the mouth of a small rodent burrow.
The Seasonal Formation Series as defined by Wagner (1964) is
typical of areas inhabited by C. e. furvus.

Crotalus intermedius Troschel

The smallhead rattlesnakes, C. intermedius, of southern Mexico
remain biologically among the least understood species of rattle-
snakes. Little published material on these snakes is available, and
few specimens are available in museum collections. Most examples
of this rattlesnake are from southeastern Mexico, but Duellman's
(1961: 121) record of an individual from Cerro Tancitaro, Michoacan,
is indeed unusual. When more field work is done in areas between
known localities, the taxonomic status and geographic variation
within C. intermedius should become more apparent.

Crotalus intermedius intermedius. Crotalus i. intermedius has
been recorded from eastern Hidalgo, west central Veracruz, and
northeastern Puebla (Klauber 1972: 39-40). Pianka and Smith
(1959) found C. i. intermedius 16-24 km W Japala, Veracruz. A
single specimen (KU 155530; Fig. 9) from near Cacaloapan, Puebla,

THE NATURAL HISTORY OF MEXICAN RATTLESNAKES 19

captured in the summer of 1974 by one of us (BLA), extends the
range of this form some 160 km (airHne) to the south. This area is
at an elevation of 2195 m and is dominated by high desert vegeta-
tion where cacti, agave, and yucca are prevalent. A rattlesnake of
this race obtained by us was taken at 0995 hours when sunlight
was diffused and humidity was high. It was basking in partial shade
under a large agave on a southern facing hill. Thunderstorms oc-
curred the previous evening, but the porous limestone soil surround-
ing the collecting site showed little evidence of moisture. The
vegetational habitat preference of C. i. inter me dius includes the
Tropical Evergreen Forest as defined by Leopold (1950).

Crotalus intermedius gloydi. A Oaxacan subspecies, C. inter-
medius gloydi (Fig. 10), is a montane race from the mountains sur-
rounding the city of Oaxaca. To our knowledge it has not been
taken from the desert areas of that region. In that respect, it
differs greatly from the nominate northern subspecies. Three snakes
obtained by us have come from the humid pine-oak forests above
2440 m (Fig. 11) and another specimen (KU 155529) from north of
Ixtlan de Juarez at 3020 m. This latter individual was found on a
west-facing slope the day following a violent hailstorm which left
about 5 cm of hail on the ground. The day it was captured, how-
ever, was hot (27 °C) and sunny. The snake was found in the shade
of an oak that had begun to sprout new foliage after being burned
in a fire. J. R. Dixon (pers. comm.) found a specimen of C. i. gloydi

Fig. 9. Crotalus intermedius intermedius. Specimen from Cacaloapan,
Puebla, Mexico. (Photograph by John H. Tashjian. )

20 SPECIAL PUBLICATION-MUSEUM OF NATURAL HISTORY

crawling up a vertical stone wall at night in a rainstorm along a
highway cut 57.2 km N Oaxaca at 2286 m on 6 June 1956. Other
specimens of C. i. glotjdi collected by us came from near El Tejocote
(UTA R-4915, 5627, 5637, 7163), which is northwest of Oaxaca. This
area has been extensively logged, and only scattered areas support
rattlesnake populations. Six additional specimens were collected
in July 1975 from near Cerro Machin (Sierra de Juarez), Suchixtepec
(Sierra de Miahuatlan) and El Tejocote (Sierra de Cuatro Venados).
Other snakes from the vicinity of El Tejocote are as follows: (UTA
R-4915) collected on 20 August 1975, (UTA R-5627) during August
1975 from a pine-oak, madrofio forest at 2440 m, (UTA R-5637) dur-
ing August 1975 in a pine-oak forest at 2285 m, (UTA R-5791-2)
on 17 June 1976 in a pine-oak forest at 2285 m, (UTA R-6062-63)
on 31 July 1976 in a pine-oak forest at 2285 m, (UTA R-6122) during
July 1976 in a pine-oak forest at 2285 m, (UTA R-6229) during
August 1976 in a pine-oak forest at 2285 m and (UTA R-6356) dur-
ing June 1975. We have observed ritualized combat between male
C. i. gloijdi shortly after capture.

The Montane Formation Series and Montane Thicket as defined
by Wagner (1964) is characteristic of the habitat of C. i. gloydi,
and Duellman (1965) listed it as a resident of Pine-Oak Forest.
Blue-flowered Solanura, Ceanothus coeruleus, wild cherry, and two
or more species of oaks and pines are found in the area (Goldman
1951).

Crotalus intermedius omiltemanus. A Guerreran subspecies,
C. /. omiltemanus (Figs. 12, 13) is probably the most thoroughly
studied race of the smallhead rattlesnakes. Most examples of this
snake have been taken in the humid pine-oak forests {Pinus herrerai,
P. pseudostrobus, Quercus sp.) wliich surround Omilteme in the
Sierra Madre del Sur (Davis and Dixon 1957, 1959). Nearly all
specimens (35) obtained by us were collected within pine-oak forest
(UTA R-4707-10, 6232-33, 6245, 6821); only one individual, collected
on 25 May 1975 from 3.2 km W Omilteme, was taken in an area
of secondary growth in the adjacent hardwood cloud forest at
2653 m by J. A. Campbell (UTA R-5626). This latter snake may have
reached this area via a power line cut. These rattlesnakes also were
found only during the major rainy season which usually began in
May. This area may receive precipitation every month, but from
January to May, it is rather dry. C. i. omiltemanus is apparently
a rock-dweller, and none of the specimens obtained by us was
found any distance from a rocky retreat, with the exception of the
cloud forest specimen. In most instances they were found basking
on rocks, especially after a rain. One snake (UTA R-2813) was found
dead on a grassy hillside on 19 May 1973; cause of death was not
determined. We collected snakes from 2075-2592 m, and doubt
that these rattlesnakes are found near Chilpancingo which has an

THE NATURAL HISTORY OF MEXICAN RATTLESNAKES 21

Fig. 10. Crotalus intermeditis glotjdi. Specimen from 31.3 km N Ixtlan
de Juarez, Oaxaca, Mexico. (Photograph by John H. Tashjian.)

Fig. 11. Habitat of Crotalus intermedins gloydi, pine-oak forest 31.1 km N
Ixtlan de Juarez, Oaxaca, Mexico, July 1975. Area had been partially cut
and burned the previous year, characterized by epiphytic plants. (Photograph
by Jonathan A. Campbell.)

22 SPECIAL PUBLICATION-MUSEUM OF NATURAL HISTORY

elevation of 1372 m (Klauber 1972: 533). Although C. i. omiltemanus
has been definitely recorded only from the vicinity of Omilteme,
it was to be expected that with additional field collecting other
specimens would be secured throughout the upper reaches of the
Sierra Madre del Sur. Supportive evidence of this statement is in-
dicated by the collection of a specimen (UTA R-5638) on 28 May
1975 from 1.6 km SW Filo de Caballo by J. A. Campbell. Another
adult snake, observed approximately 1 km N Puerto del Gallo (some
60 km WSW Omilteme), escaped before being captured. The ele-
vation at this site was 2950 m, and an example of Bothrops barhouri
was discovered nearby. Melanistic specimens of C. i. omiltemanus
were collected in San Vincente, Guerrero (UTA R-4538-9). Five
newborn young of this race were found 1.6 km S Omilteme in pine-
oak forest at 2286 m on 28 May 1975. Range of variation for length
and weight is as follows: total length 194-212 mm, mean 205;
snout-vent 182-195 mm, mean 190; weight 4.9-5.4 g, mean 5.2.
Ritualized combat between adult males was observed by us among
four recently captured snakes.

Campbell (1977) examined the stomachs of 18 C. i. omiltemanus
from Omilteme, and revealed that these snakes fed almost exclu-
sively on lizards (genus Sceloporus) whereas the stomachs of 15
examples of Bothrops undulatus from the same area contained
mostly rodent remains.

Crotalus lepidus (Kennicott)

The rock rattlesnakes, C. lepidus, are typical examples of the
various small montane, rock dwelling rattlesnakes inhabiting Mexico.
All four subspecies, lepidus, klauberi, maculosus, and morulus, are
generally timid, yet rather irritable and curious at times. Through-
out much of the range of C. lepidus in Mexico, only limited numbers
of specimens have been recorded, probably because the terrain
inhabited by these snakes is generally steep, rugged, and inacces-
sible. More specimens are needed from the southern limits of the
ranges to clarify the relationship between C. lepidus and C. tri-
seriatus (Gloyd 1940: 81).

Crotalus lepidus lepidus. The nominate race, Crotalus I. lepidus,
is found in pine-oak forests tlirough much of its range, though desert
populations are not uncommon. This snake ranges from southeast-
ern New Mexico and Trans-Pecos Texas, south through Coahuila,
west-central Nuevo Leon, eastern Zacatecas, and northwestern San
Luis Potosi (Klauber 1972: 62).

Gloyd and Smith (1942: 235) reported a specimen of C. /. lepidus
from the Sierra del Carmen at 1830 m on an open, southwestern
facing slope where junipers were present. Taylor (1952) reported
a snake from Cerro Penon Blanco, San Luis Potosi at 2928 m. The
single specimen of this rattlesnake, obtained by us from La As-

THE NATURAL HISTORY OF MEXICAN RATTLESNAKES 23

Fig. 12. Crotalus intermedins omiltemanus. Melanistic specimen from
San Vicente, Guerrero, Mexico. (Photograph by John H. Tashjian. )

i-^ "I-^ §5

Fig. 13. Crotalus intermedius omiltemanus. Specimen from near Omilteme,
Guerrero, Mexico. (Photograph by John H. Tashjian.)

24 SPECIAL PUBLICATION-MUSEUM OF NATURAL HISTORY

cension, Nuevo Leon, was found under similar conditions at 2075
m where junipers, pines, and agaves were prevalent. The specimen,
a gravid female weighing 44 g and measuring 435 mm total length
(400 mm S-V), appeared to be assignable to C. /. morulus according
to Klauber (1972: 130) by its having 28 crossbands. However, three
of the six young born to this female on 5 June 1974 had fewer than

25 crossbands, and three had 25 or more (one male had 31 cross-
bands). Range of variation for length and weight for this brood
is as follows: total length 165-190 mm, mean 177; S-V 150-173 mm,
mean 162; weight 5.0-5.7 g, mean 5.2.

Milstead, Mecham, and McClintock (1950) found C. I. lepidus
in a persimmon, shin-oak association, and the Scrub Desert as de-
fined by Wagner (1964) is its characteristic habitat. Minton (1959)
described the habitat of this race in the Trans-Pecos area.

Crotalus lepidus klauberi. Crotalus I. klauberi is normally a
resident of moderate elevations in forested mountains throughout
most of its range. Snakes from extreme west Texas in the Franklin
Mountains are an exception since this mountain range is very arid
with little vegetation. Specimens obtained by us are from: CHI-
HUAHUA: 16 km S Chihuahua, Arroyo Mestefio and Canon del
Alamo in the Sierra del Nido, near Villa Matamoros; DURANGO:
near Villa Ocampo.

Most of the data on C. I. klauberi obtained by us have been
from examples collected in the Chiricahua and Huachuca Mountains
of southeastern Arizona. With the exception of the snakes from
the Sierra del Nido, Chihuahua, specimens obtained by us from
Mexico were collected on roads, and all came from generally the
same type of terrain, primarily desert canyons with rock outcrops
or ledges, and sparse vegetation. C. /. klauberi was collected at
elevations from 915 m south of Chihuahua to 1372 m in the vicinity
of Villa Ocampo, Durango, where grasses and oaks form a savannah
environment. In the Chiricahua, Huachuca, and Sierra del Nido
Mountains, C. I. klauberi is often located along rocky stream beds,
and during the summer rainy season is observed in open pine-oak
forests. These rattlesnakes are mostly diurnal, at least in forested
mountains, and are most easily observed on warm, humid summer
days when they reveal their presence by rattling. Ritualized combat
between males of this subspecies has been observed (Carpenter et al.
1976). A female specimen, collected at Herb Martyr Dam in the
Chiricahua Mountains, Arizona, gave birth to three young on 28
August 1968. Four young were born on 30 August 1975 to another
female specimen collected 4.8 km S Madera, Chihuahua, and range
of variation for length and weight is as follows: total length 181-
196 mm, mean 191; S-V 164-177 mm, mean 172; weight 6.3-7.0 g,
mean 6.6. A captive pair of C. I. kluuberi was observed to breed
on 21 February 1977. R. K. Guese (pers. comm.) observed a captive

THE NATURAL HISTORY OF MEXICAN RATTLESNAKES 25

pair of this race from the Sierra del Nido, Chihuahua, engaged in
courtship activity on three occasions between 14-28 September 1977.
The male directed rapid head-bobs (3-5/5 sec) on the dorsum of
the female. Tongue-flicking occurred at the same speed. The
snakes were disturbed after 5 minutes and discontinued the be-
havior. Another pair of these snakes was observed courting on 11
October 1977, and the male exhibited the same head-bobbing and
tongue-flicking behavior (D. G. Barker, pers. comm.).

The Montane Formation Series, Steppe Thicket and Scrub Des-
ert as defined by Wagner (1964) are habitats in which C. I. klauberi
typically resides.

Jacob and Altenbach (1977) described sexual dichromatism in
body coloration in C /. klauberi from New Mexico, Sonora and
Chihuahua. Van Devender and Lowe (1977) reported similar sexual
dichromatism in this race from Chihuahua. We have seen twelve
snakes from Arroyo Mestefio (7 males, 5 females) which exhibit
sexual dichromatism. A female gave birth to five young on 18
August 1977. Two male young were only slightly mottled with
black speckling and black bands, whereas the two female young
were grey, mottled with black speckling and had dark grey bands.
The fifth neonate was defective, exhibiting a fusion of the ventral
surface in the lower cervical area and the anterior trunk region
(UTA R-6946).

Crotalus lepidus maculosus. C. /. maculosus (Fig. 14) is a re-
cently described race of rock rattlesnake (Tanner, et at., 1972)
whose geographic distribution remains virtually unknown because
much of its habitat is inaccessible. It is a resident of the Pacific
versant of the Sierra Madre Occidental, and apparently intergrades
with C. /. klauberi on the high Mexican Plateau in the vicinity of
El Salto, Durango.

Where they are most abundant Crotalus I. maculosus and an-
other subspecies C. I. morulus apparently prefer similar habitat
conditions. Both races seem to prefer humid, pine-oak forests with
well-defined rainy seasons (Fig. 15). We have unsuccessfully tried
to obtain both subspecies in the late spring. However, after the
summer rains begin, the two subspecies apparently surface and
are quite common. This is unlike many populations of C. I. lepidus
and C. I. klauberi, whose combined distribution divides the ranges
of C. I. maculosus and C. I. inorulus, and have adapted in some
areas to desert or chaparral conditions. These populations are active
above ground throughout the year if the weather is sufficiently
warm (24° C). Almost all specimens of C. /. maculosus obtained by
us were secured under cloudy or partly cloudy conditions, and were
particularly common just before and after showers during the rainy
season. On 10 July 1973, a female was observed on a southeastern
facing rock outcrop 4.8 km W El Alazan, Sinaloa, with eleven young

26 SPECIAL PUBLICATION-MUSEUM OF NATURAL HISTORY

L^^^^"^

•**• ~»r-.

'0'i'.

■.^j^

Fig. 14. Crotalus lepidus maculosus. Specimen from Los Bancos, Durango,
Mexico. (Photograph by John H. Tashjian.)

coiled or draped about her. This race exhibits behavior typical of
all subspecies of C. lepidus in that it is alert and quick to seek
shelter when discovered. Crotalus I. maculosus possesses an un-
usually small rattle, and it is barely audible even in larger specimens.
The accepted technique historically employed to discover C. lepidus
in its native habitat (by hearing them rattle) is virtually useless
when trying to collect this subspecies.

Most of the snakes observed by us, which total forty sightings
(KU 155532-534, UTA R-5847), were normally located by exploring
southern facing slopes where an abundance of grass and rocks occur
in breaks in the pine-oak forest. This subspecies was not found in
rock slides as were many C. /. lepidus and C. /. klauheri populations.
However, C. /. maculosus is usually found near some kind of rock
cover. Lizards of the genus Sceloporus appear to be the main food
item of this rattlesnake.

Other rattlesnakes found in association with C. I. maculosus are
C p. pricei, C. molossus nigrescens, and possibly C. willardi me-
ridionalis in the eastern part of the range. Some local residents
gave positive identification of C. to. meridionalis from photographs
which we provided.

Crotalus I. maculosus inhabits the Pine-Oak Forest and Pine-
Oak Woodland as defined by Leopold (1950).

Crotalus lepidus morulus. Crotalus I. morulus (Figs. 16, 17) is
the least known of the subspecies of C. lepidus. This race was de-

THE NATURAL HISTORY OF MEXICAN RATTLESNAKES 27

Fig. 15. Habitat of Crotalus lepidus maculosus, C. pricei pricei and C.
molossus nigrescens. Los Bancos, Durango, Mexico. Steep hillsides covered
with pine-oak forest. These snakes were most frequently encountered in
open areas of rock outcroppings. (Photograph by R. Terry Basey. )

scribed by Klauber (1952: 52) from specimens obtained from P. S.
Martin, who collected in the Gomez Farias area of Tamaulipas.
Most of the known specimens have been collected in the Gomez
Farias area, and an additional specimen is known from Chiche (Mar-
tin 1958: 78), which is northwest of Cuidad Victoria, Tamaulipas.

Recently, more snakes of this race have been secured in the
Sierra de San Francisco (H. S. Harris, pers. comm.), and we have
located an apparently isolated population (19 specimens) in the
Sierra Madre Oriental some 24-32 air kilometers northwest of
Galeana near the Nuevo Leon-Coahuila border (KU 159360). No
doubt, more C. /. morulus will be collected which will fill the pres-
ent gaps between the few presently known localities.

Most of the rattlesnakes collected by P. S. Martin were located
on the humid, eastern slopes of the Sierra de Guatemala at altitudes
ranging from 1190-1890 m. These snakes were usually found along
the abundant rocky mountain trails in the Gomez Farias area. Pines,
oaks, and firs constitute the major vegetational formation of this
area, although there is an abundance of agave on the steep rocky
sides of the Sierra. Martin (1958) stated that C. /. morulus was
present in the upper part of the Cloud Forest, but its presence was
not confinned in the Lower Cloud Forest. This area is extremely
rugged, and more extensive collecting is needed to delineate the
extent of the range of C. /. morulus.

28 SPECIAL PUBLICATION-MUSEUM OF NATURAL HISTORY

Fig. 16. Crotalus lepidus morulus. Specimen from near San Antonio de
las Alazanas, Coahuila, Mexico. (Photograph by John H. Tashjian. )

m

Fig. 17. Crotalus lepidus morulus. Juvenile specimen bom to female col-
lected near San Antonio de las Alazanas, Coahuila, Mexico. (Photograph by
John H. Tashjian.)

THE NATURAL HISTORY OF MEXICAN RATTLESNAKES 29

In late August 1973, Danial Lopez and one of us (BLA) collected
16 specimens of C. I. morulus 24-32 km northwest of Galeana; the
specimens ranged from mature adults to newborn young. All were
identified as C. /. morulus in having at least 28 primary dorsal
blotches with the exception of an adult female which had 19
blotches and no mottling. This female gave birth, several days after
capture, to four young which had the normal complement of pri-
mary blotches, suggesting that the female parent exhibited an aber-
rant pattern, not uncommon in the C. lepidus complex. During
summer 1974 an additional individual (UTA R-6123) was taken
from the same locality in a pine-scrub oak situation on a steep
rocky hillside.

Unlike the specimens collected by Martin which were associated
with humid pine-oak forests at the elevations previously mentioned,
all of the specimens obtained by us were located at elevations be-
tween 2380-2592 m amongst limestone bedrock which had a south-
ern exposure. A few scattered pines and oaks were present, but
most of the vegetation consisted of various species of agaves and
low growing shrubs, which provided excellent cover for the snakes.
These snakes appeared to be extremely common during the only
four hours we spent collecting (due to inclement weather) and
would often make their presence known by rattling long before we
were within their view. They would immediately seek shelter
among the rocks and agaves, and several escaped us in this habitat.
Captured snakes defended themselves vigorously and often bit them-
selves or the tongs that were used to secure them.

This subspecies has been reported from the Humid Pine-Oak
Forest to Pine-Oak Forest (Martin 1958).

Crotalus mitchelli (Cope)

The speckled rattlesnake, C. mitclwlli, is an inhabitant of the
American Southwest, Baja California, islands in the Gulf of Cali-
fornia, and one island, Santa Margarita, off the Pacific coast of Baja
California del Sur (Klauber 1972). It is a highly variable rattlesnake,
both in size and color, and to a lesser degree in pattern. Color and
pattern seem to be correlated with the general color of the rocky
terrain which any given population inhabits.

Crotalus mitchelli mitchelli. The San Lucan subspecies, C. m.
mitchelli, ranges from the southern border of Baja California del
Norte, south throughout the peninsula. Island populations are lo-
cated on Cerralvo, Espirito Santo, San Jose, Cannen, and Santa
Margarita Islands (Klauber 1972). The range does not include
Santa Cruz Island (Soule and Sloan 1966). This rattlesnake is com-
mon throughout its range, but is most easily observed in the Cape
region. Klauber (1936) recorded specimens from Cape San Lucas,
San Jose del Cabo, Miraflores, Todos Santos, La Paz, Mulege,

30 SPECIAL PUBLICATION-MUSEUM OF NATURAL HISTORY

Santa Rosalia, and San Ignacio. Specimens obtained by us are
from 8 km N San Jose del Cabo and 16.1 km N Buena Vista
(KU 173097).

Crotalus m. mitchelli is a dweller of the rocky portion of Baja
California. Rarely will it be found any distance from rocky re-
treats, and it seems to prefer rocky canyons and rock outcrops
(B. Tomberlin, pers. comm.). This rattlesnake has, however, been
found on desert flats where desert shrubs and burrows provide
shelter. Crotalus e. enyo and C ruber lucasensis are often found
sympatrically with C. m. mitchelli, although the first two are not
as prevalent in rocky situations. Klauber (1972: 536) found that
C. m. mitchelli was a rock dweller, but also indicated it was found
in brushy areas. Most of these rattlesnakes are nocturnal, since
temperatures in southern Baja limit diurnal activity. Activity
reaches peak periods during the late summer rainy season. These
rattlesnakes are most often observed at night as they are crossing
roads,

A captive pair of C. m. mitchelli was observed in copulo on
13 October 1975 at 0800 hours. The right hemipenis of the male
was inserted, and a prominent bulge was evident which extended
ten scale rows anterior to the vent of the female. The snakes
separated at 1500 hours. On 29 June 1976 the female gave birth to
one neonate and one infertile egg mass. The newborn snake
measured 275 mm in total length, S-V 263 mm, weight 17.5 g
(UTA R-6939).

The Seasonal Formation Series and Cactus Scrub and Savannah
as defined by Wagner (1964) are typical vegetative regimes in-
habited by C. m. mitchelli. These areas support mesquite, creosote
bushes (Larrea), cacti and other arid vegetation (Goldman 1951).

Crotalus mitchelli pyrrhus. A southwestern subspecies, C. m,
pyrrhus, is a rock-dwelling rattlesnake that ranges in Mexico from
the California border to the southern portions of Baja California
del Norte. This rattlesnake is common on the desert portions of
the Sierra Juarez and Sierra de San Pedro Martir. It is also found
on the western slopes in chaparral situations, but not commonly.
Two rattlesnakes obtained by us from south of Puertecitos were
found in the vicinity of abandoned sulphur mines in extremely
rocky mountains. They were found just after sundown in early
spring, a period of the year that seems to be a peak activity time
for C. m. pyrrhus. We have observed many individuals of this
subspecies north of the U.S. border in southern San Diego County,
California, during early spring. Fewer specimens were found dur-
ing summer and fall. To take advantage of favorable ambient tem-
peratures, this rattlesnake shifts from a diurnal mode of activity
in the spring and fall to nocturnal activity in the summer (Moore
1976).

THE NATURAL HISTORY OF MEXICAN RATTLESNAKES 31

Crotalus m. pyrrhus inhabits the California Chaparral, Seasonal
Formation Series, Cactus Scrub, and Montane Thicket as defined by
Wagner (1964).

Crotalus mitchelli muertensis. A dwarf subspecies, C. m. muer-
tensis, is an insular form of C. mitchelli from El Muerto Island of
the San Luis group in the Gulf of California. El Muerto is the
second largest island in the group, is found ca. 6.4 km southeast
of Huerfanito Island (the northernmost island in the San Luis
group), and is an extremely arid, rocky island supporting sparse
vegetation. During the summer, temperatures on the island reach
high extremes and limit reptilian activity.

Sixteen specimens obtained by us (KU 155535, 174830) were
taken from the western and southwestern portions of the island on
30 May 1969 between 1000-2000 hours; most were released after
capture. The majority of these specimens were found shortly after
sunset, since daytime temperature reached approximately 38°C.
Several specimens were found within 3 m of the water, foraging
in beach debris. Other specimens were located on rocks, and some
were found under sparse bushes. A single specimen was recorded
at 183 m elevation. Two lizards, Uta stanshuriana and Streptosaurus
mearnsi, as well as a species of Peromyscus, occur on the El Muerto
Island in large numbers, and probably make up a sizable portion of
the diet of C m. muertensis.

These dwarf rattlesnakes would rattle long before we were
within 6 m of them, indicating a high degree of alertness. Several
individuals escaped under massive rockpiles before we were able
to observe them. Those captured resisted vigorously, and all
thrashed violently. Although all were observed before sundown,
it is reasonable to assume that C. m. muertensis is mostly nocturnal
because of excessively high daytime temperatures.

Two of the snakes captured by us bred in captivity on 23 March
1977. At the time of collection, the male measured 355 mm, and
the female 280 mm, in total length. At the time of breeding, the
male measured 680 mm, and the female 610 mm, in total length.
Two live and two dead young (UTA R-7218-19), and three infertile
masses, were born on 13 September 1977. Total length of the four
young, measured and weighed eight days after birth, was 143-179
mm (mean 167) and weight was 3.3-7.0 g (mean 4.9).

Crotalus m. m^uertensis inhabits the Seasonal Formation Series
and Cactus Scrub as defined by Wagner (1964).

Crotalus molossus Baird and Girard

The blacktail rattlesnakes, C. molossus, of the American south-
west and the central plateau of Mexico, are well known throughout
their range despite the fact that they were at times confused with
C. hasiliscus (GloydT940: 161; Klauber 1952: 87). Large collections

32 SPECIAL PUBLICATION-MUSEUM OF NATURAL HISTORY

of the subspecies C. m. molossus and Cm. nigrescens have been
made in past years, and only C. m. estebanensis, from San Esteban
Island in the Gulf of California, remains biologically unknown.
All three subspecies of C. molossus attain moderate size, and speci-
mens of C. m. molossus and C. m. nigrescens are among the most
handsome rattlesnakes we have had the opportunity to observe.
This rattlesnake appears to be adaptively successful; the range and
variety of habitats within its area of distribution are unsurpassed
by most other Mexican rattlesnake species.

Crotalus molossus molossus. Crotalus m. molossus lives in di-
verse habitats from the Edwards Plateau in west-central Texas
through Arizona to the Grand Canyon. In Mexico, C. m. molossus
has been reported from the states of Sonora, Chihuahua and
Coahuila; it intergrades with C. m. nigrescens in the southern por-
tion of its range. See Klauber (1952: 91) for a discussion of inter-
gradation in this species. Specimens obtained by us have been
recorded from: SONORA: 4.8 km S Sonoyta; CHIHUAHUA: the
Sierra del Nido complex, 16 km S Chihuahua. We have observed
approximately 30 individuals.

Although somewhat unspecialized ecologically (Dammann 1961),
C m. molossus is generally considered a montane species, pre-
ferring rocky retreats within dominant pine-oak forests. Typical
habitat niches for this rattlesnake from mountain ranges such as
the Chiricahuas and Huachucas in southern Arizona, and the Sierra
de Ajos and Sierra del Nido of northern Mexico, reflects this prefer-
ence. This snake is often observ^ed at high elevations (up to 2592 m)
in the Chiricahua Mountains of Arizona. However, it appears to
be more common below 2135 m on southern or southeastern facing
slopes of the Chiricahuas where oak, madroiio, agave, and various
grasses are the dominant vegetation. Pough (1966) observed many
in the talus rockslides where C. lepidus klauberi is quite common
and our observations corroborate those made by him. Rocky stream
beds are also inhabited by C. m. molossus, especially during the
summer rainy season. C. m. molossus is not restricted to forested
mountains, however, as Klauber (1972: 537) cited specimens from
the valleys and plains of Cochise County in Arizona which average
ca. 1220-1525 m elevation. We have observed this subspecies in
desert situations north of Scottsdale, Organ Pipe National Monu-
ment, and near Gates Pass, south of Tucson, Arizona. Taylor (1936:
497-498) found C. m. molossus to be common in rocky habitats
northwest of Guaymas at La Posa, which is near sea level. Perhaps
the most unexpected place where this snake has been found is the
sand dunes south of Cuidad Juarez in Chihuahua, where a speci-
men was taken by T. Walker (pers. comm.).

Most C. m. molossus are gentle in disposition and generally seek
escape when approached. They rarely rattle, although those found

THE NATURAL HISTORY OF MEXICAN RATTLESNAKES

33

in rockslides will at times reveal their presence by rattling, a char-
acteristic they share with C. lepidus and C. pricei. Allen (1933)
found C. m. molossus in trees ca. 2 m above the ground.

C. m. molossus inhabits the Seasonal Formation Series, Montane
Formation Series and Scrub Desert vegetational classifications as
defined by Wagner (1964). See Gloyd (1937) and Lowe (1964) for
a description of the habitat of this race in Arizona.

Crotalus molossus nigrescens. The Mexican blacktail rattle-
snake, C. m. nigrescens (Fig. 18), is generally a resident of temperate
pine-oak forests of the central plateau of Mexico, and Gloyd (1940:
164) listed localities for every state within the plateau. We have
observed twenty snakes from the following localities: DURANGO:
17 km N Las Nieves (UTA R-5630), 16 km W Durango, Los Bancos,
8.3 km E Coyotes (UTA R-5700), La Ciudad; JALISCO: W Za-
coalco; MICHOACAN: 4.8 km S Carapan, Contepec, 7 km W
Morelia, Morelia (UTA R-5112-4), Tacicuaro; MORELOS: the
lava beds off the toll road between Mexico City and Cuernavaca;
VERACRUZ: the lava beds near Perote.

Crotalus m. nigrescens does not occupy as many varied habitats
as C m. molossus, but is not restricted to a specific environment.
Individuals were observed by us near Las Nieves, Durango, in rock
outcrops at 1220 m within an oak-grass savannah. They were com-
mon there, and the local residents were well aware of their presence.
The other previously listed locales in Durango were typically pine-
oak situations where broken rock and various grasses provided

'**

Fig. 18. Crotalus molossus nigrescens. Specimen from near Morelia,
Michoacan, Mexico. Juvenile. (Photograph by John H. Tashjian. )

34 SPECIAL PUBLICATION-MUSEUM OF NATURAL HISTORY

excellent cover and a varied food supply (Fig. 15). Most of the
rattlesnakes observed by us from these areas in Durango were found
basking in the late morning during the July-August rainy season.
Those from Carapan, Michoacan, Perote, Veracruz, and north of
Cuernavaca, Morelos, were all located in lava bed areas. Davis
and Smith (1953: 141) thought that the occurrence of C. rn. nigres-
cens in Morelos was doubtful based on geographic grounds. At
Carapan in Michoacan they were extremely common (10 observed).
The specimens from Tacicuaro and Contepec, Michoacan, were in
mesquite grassland, a habitat in which Klauber (1972: 538) had
previously speculated that this subspecies might occur. J. R. Dixon
(pers. comm.) found a specimen in sweet-gum, oak forest on the
east-facing slope of Sierra Madre, 11.2 km ENE Pinal de Amoles,
Queretaro, at 1981 m elevation.

Dunkle and Smith (1937) found a female C. m. nigrescens with
16 young in a canyon west of La Colorado, Zacatecas. Copulation
was observed by us between a captive pair on two occasions (28
May 1973 and 2 March 1974); no pulsations or other movements
were noticed. No young were born from these observed unions,
but on 9 June 1975 five young were born in captivity, and were
weighed and measured. Range of variation is as follows: total
lengtli 291-316 mm (mean 304); S-V 267-290 mm (mean 284); weight
25.4-27.9 g (mean 26.6).

Two C. m. nigrescens were observed copulating on 1 February
1978 at 0920 hours. The diameter of the female's cloaca was 15 mm
and was distended due to the male's hemipenis. A noticeable bulge
in the female's body extended 35 mm anterior to her cloaca. The
shoulder spines of the hemipenis were visible and the organ was
dark purple in coloration. Coitus lasted 105 minutes and detu-
mescence of the hemipenis occupied 100 seconds. Intermittent
head-bobbing and tongue-flicking sequences by both snakes oc-
curred during coitus.

An open mouth defensive posture was observed in three indi-
viduals. The mouths were held open for over five minutes when
the snakes were provoked.

The subspecies C m. nigrescens inhabits tlie Temperate Pine-
Oak Forest as defined by Leopold (1950) and Mesquite Grassland
(Klauber 1972).

Crotalus polystictus (Cope)

Previous accounts of Crotalus polystictus indicated a preference
for marshy situations and this snake was generally referred to as
the "aquatic rattlesnake." This presumed preference for aquatic
situations was reinforced by specimens, captured in 1919 by Paul
D. R. Ruthling, which were secured in the tules of Lake Chapala,
Jalisco (Klauber 1956). However, this was probably an unusual

THE NATURAL HISTORY OF MEXICAN RATTLESNAKES

35

situation since Ruthling pointed out that this area was, at the time
of collection, a flooded swampland. In this ephemeral environment,
it is easy to understand why so many specimens (20-25) were ob-
served by Ruthling, and also why the species was thought to prefer
an aquatic habitat. It is unlikely for a snake with a warning mech-
anism such as a rattle to live in aquatic situations where the rattle
would be rendered useless.

In recent years we have captured or observed over 100 speci-
mens of C. pohjstictus in the vicinity of the Nevado de Colima in
southern Jalisco (Fig. 21) and northern Michoacan. Smith and
Higareda (1965) commented on specimens from this area. In Jalisco,
this snake has been found on plateaus which occur in breaks in
the pine-oak forest at elevations of 2075-2317 m. Often these areas
are interlaced with gently flowing streams. This rattlesnake can
be found during most months of the year; we have records from
December, February, and May through September. Snakes ob-
tained by us are from the following localities: JALISCO: near
Chapala (UTA R-4000), Rancho San Francisco (UTA R-4499, 4906-8,
4916, 5666-7, 6043, 6250, 6704, 6822-23, 6927; Figs. 19, 20); MICHO-
ACAN: Tacicuaro (KU 155540-541), 8 km W Morelia, 18 km W
Jiquilpan, 5 km N Cato de las Esperanzas.

Most C pohjstictus are found in rocky situations with an abun-
dance of tall grass called zacaton. None of these rattlesnakes have
as yet been discovered in pine-oak forest, but they may occur in
that habitat. In Michoacan, near Tacicuaro, C. polystictus has been

'%".;'i:

Fig. 19. Crotalus polystictus. Specimen from Rancho San Francisco,
Jalisco, Mexico. (Photograph by John H. Tashjian.)

36 SPECIAL PUBLICATION-MUSEUM OF NATURAL HISTORY

fc'*^..

>• * 1

Fig. 20. Crotalus polystictus. Juvenile specimen bom to female collected at
Rancho San Francisco, Jalisco, Mexico. (Photograph by John H. Tashjian.)

Fig. 21. Habitat of Crotalus polystictus. Rancho San Francisco, Jalisco,
Mexico. Snakes were commonly found among boulders or in gopher burrows
in grassy meadows with surrounding pine forest on hillsides. (Photograph by
M. Granger.)

THE NATURAL HISTORY OF MEXICAN RATTLESNAKES 37

collected in mesquite grassland (Duellman 1965: 655). This area
has rocky outcrops where the species is easily located. We have
also collected this species in plains-grassland areas, although it does
not appear to be abundant in this environment.

Crotalus polystictus is found rather commonly in late spring
when summer rains have not yet been sufficiently heavy to con-
tribute to growth of grass cover. During this period, they become
somewhat nocturnal, active individuals having been recorded as
late as 2200 hours. In late spring, C. polystictus displays a rather
mild disposition and is usually quite inoffensive. Some specimens
have been observed attempting to hide their heads beneath a coil
of their body, behavior similar to that exhibited by Lichanura
trivirgata roseofusca. During the summer, this species becomes
much more aggressive and will generally not retreat when ap-
proached. We observed an individual which, upon discovery,
opened its mouth in a threatening pose similar to the behavior ex-
hibited by Agkistrodon piscivorus. Lampropeltis triangulum ar-
cifera, a harmless colubrid snake, is common in the vicinity of the
Nevado de Colima, and may be an important predator upon these
rattlesnakes.

The known range of C. polystictus is decreasing due to habitat
destruction. The plateados inhabited by these rattlesnakes are be-
ing altered for agricultural use because they are more easily cleared
than adjacent pine-oak forests.

The preferred habitat of C. polystictus is Temperate Pine-Oak
Forest as defined by Leopold (1950), and Mesquite Grassland
(Duellman 1965).

Cuesta Terron (1930-31) reported on a brood of 14 newborn
young whose average total length was 12 cm, a figure considerably
smaller than the mean total length of broods measured by us. It is
possible that Cuesta Terron may have misidentified his snakes (see
Fig. 6, p. 54), although we have not seen his specimens.

Data are available to us for six broods born to females of this
species, as follows: Female A, weighing 111 g and measuring 608 m
total length (575 mm S-V), gave birth to ten young on 20 June
1974: total length 200-223 mm, mean 211; S-V 184-208 mm, mean
195; weight 6.3-10.9 g, mean 7.2. Female B, weighing 166 g and
measuring 660 mm total length (620 mm S-V), gave birth to twelve
young on 13 June 1974: total length 211-287 mm, mean 222; S-V
198-271 mm, mean 208; weight 8.5-11.5 g, mean 9.9. Female C
gave birth to seven young on 26 June 1975: total length 198-232
mm, mean 216; S-V 185-211 mm, mean 200; weight 9.9-10.5 g,
mean 10.1. Female D gave birth to seven neonates on 30 June
1975: total length 182-205 mm, mean 197; S-V 167-185 mm, mean
179; weight 9.9-11.1 g, mean 10.6. Female E gave birth to five
stillborn young on 30 June 1975: total length 155-203 mm, mean

38 SPECIAL PUBLICATION-MUSEUM OF NATURAL HISTORY

177; weight 2.4-5.5 g, mean 4.0. Female F gave birth to seven
young on 25 June 1975: total length 227-239 mm, mean 232; S-V
201-219 mm, mean 212; weight 9.5-10.9 g, mean 10.2. The range
of variation of the six broods is as follows: total length 155-287
mm, mean 214; S-V 167-271 mm, mean 201; weight 2.4-11.5 g,
mean 8.9. Two of the snakes born to Female A were maintained in
captivity and exhibited courting behavior on 31 January 1978.

Crotalus pricei Van Denburgh

Crotalus pricei pricei. Crotalus p. pricei (Fig. 22) is the more
commonly known subspecies of this snake, large collections having
been made in Santa Rita, Huachuca, Chiricahua, and Graham
Mountains of southeastern Arizona. Fewer individuals of this race
have been recorded from Mexico, yet they are rather abundant in
Mexico in pine-oak forests at elevations ranging from 2135 to at
least 2745 m. We have observed these rattlesnakes from the fol-
lowing localities in Mexico: SONORA: Sierra de Ajos (UTA
R-6931-34); CHIHUAHUA: the Sierra del Nido complex; DU-
RANGO: Las Adjuntas, near Coyotes, 14 km ENE El Salto (UTA
R-2021), Llano Grande (KU 158561), Los Bancos (UTA R-6251).
Klauber (1972: 45) suggested that C. p. pricei may also occur in
eastern Sinaloa and northern Nayarit, but to our knowledge, no
specimens have been secured in either of these states.

Crotalus p. pricei is a resident of high rocky pine-covered slopes
(Fig. 23). In the United States, it seems to prefer talus rock slides

Fig. 22. Crotalus pricei pricei. Specimen from near Los Bancos, Durango,
Mexico. (Photograph by John H. Tashjian.)

THE NATURAL HISTORY OF MEXICAN RATTLESNAKES

39

in the previously mentioned mountain ranges. This rattlesnake will
often divulge its presence by rattling when approached. However,
like many of the montane rattlesnakes, C. p. pricei is curious and
usually retreats only a short distance. Although generally asso-
ciated with rocks, this rattlesnake may be found also in grassy
mountain valleys where prey, such as lizards and small rodents, is
abundant. One adult snake (UTA R-2021) contained a juvenile
lizard (Sceloporus pohisetti). We have frequently observed this
subspecies on warm, humid days when diffused sunlight is preva-
lent. Individuals were collected near rocky retreats from March to
November, even when a light snow cover was present. No doubt
these retreats are the warmest micro-habitats available to this rattle-
snake in mountains. On 17 March 1965, two C. p. pricei were found
on a south-facing talus slope at Barfoot, sometimes known as Buena
Vista Peak, Chiricahua Mountains, Arizona at 1220 hours. Snow
surrounded the talus slope, and the ambient temperature was 11 °C.

In Mexico, C. p. pricei is probably the most common rattlesnake
at higher elevations within its range, where it is found sympatrically
with C. tvillardi, C. lepidus, and C. molossus. We have collected
C. p. pricei from May through September, sometimes under the
most adverse weather conditions. In June 1973 we secured four
individuals on a south-facing slope near Los Bancos, Durango (Fig.

Fig. 23. Habitat of Crotalus pricei pricei and C. tvillardi meridionalis.
Sparse pine-oak forest near Llano Grande, Durango, Mexico. Crotalus p.
pricei was generally discovered in rocky areas while C w. meridionalis was
collected in thick grass or underbrush, mainly manzanita and scrub oak, on
hillsides. (Photograph by Lyndon A. Mitchell.)

40 SPECIAL PUBLICATION-MUSEUM OF NATURAL HISTORY

15), two hours after a driving hail storm. The hail melted quickly
and all four snakes were active, apparently foraging for food.
These four snakes exhibited an unusual array of body patterns.
One individual had the typical coloration of a slate-gray back-
ground with brown dorsal spots. The second example exhibited a
blue-gray background with rusty colored spots. The third specimen
was overall salmon-pink with tan spotting, and the fourth snake
was a completely patternless light tan. These four specimens were
captured within a 45 m radius.

A female specimen of C. p. pricei obtained at Los Bancos,
Durango, gave birth to three young on 24-28 July 1973. Another
female, taken near Onion Saddle, Chiricahua Mountains, Arizona,
gave birth to four young on 20 May 1967. Three young C. p. pricei
were found on 11 July 1973 near Los Bancos, Durango, in a rock
crevice; they had not shed. A captive mating produced four young
on 9 July 1971 (J. A. Campbell, pers. comm.). Three of the four
young were born alive and weighed 2.7, 3.9, 2.4 g; mean 3.0. Four
female specimens from Llano Grande, Durango, gave birth in the
laboratory on 10, 14, 27, and 29 July 1977, respectively. Data on
these four broods is as follows: Female A produced eight young
(2 stillborn, less than 50 mm total length), total length 157-167 mm,
mean 164; weight 4.0-4.2 g, mean 4.1; Female B gave birth to nine
viable young (four died within Itwo days, UTA R-6935-8), total
length 152-173 mm, mean 162; weight 3.5-4.0 g, mean 3.6; Female
C gave birth to six viable young, total length 159-169 mm, mean
162; weight 5.1-5.6; Female D produced six young, total length
160-185 mm, mean 168; weight 3.6-3.9 g, mean 3.7. The range of
variation and mean for all litters was as follows: total length 152-
185 mm, mean 164; weight 3.5-5.6 g, mean 4.1. All but one of the
females were collected while basking in the early morning on
rocks (L. A. Mitchell and D. G. Barker, pers. comm.). Kauffeld
(1943a, 1943b) reported that an example of C. p. pricei collected at
Barfoot, Buena Vista Peak, Chiricahua Mountains, Cochise County,
Arizona, gave birth to six young on 19 August. He cited another
record of a brood of this race bom on 3 August 1941. Keasey (1969)
recorded a birth of eight young on 23 September 1953. Six young
were bom on 19 July 1971 to a female secured at Los Leones,
Chihuahua (Van Devender and Lowe 1977).

C. p. pricei inhabits the Temperate Pine-Oak association as de-
fined by Leopold (1950). See Gloyd (1937) and Lowe (1964) for a
description of the habitat in Arizona.

Crotalus pricei miquihuanus. Although much literature is avail-
able about C. p. pricei, little is known about C. p. miquihuanus
(Figs. 24, 25), no doubt due to the inability of collectors to get
into the limited areas inhabited by it. Specimens of this rattlesnake
have been known since 1898 when Nelson and Goldman succeeded

THE NATURAL HISTORY OF MEXICAN RATTLESNAKES 41

in capturing an adult male near Miquihuana, Taumaulipas. How-
ever, it was not placed as a subspecies of C. pricei until the holo-
type, a subadult male, was secured near Galeana, Nuevo Leon, in
1938, and described by Gloyd (1940: 102). Since that time, occa-
sional specimens have been found but were unfortunately lost be-
fore being placed in systematic collections.

Recent information, still quite limited, presents a somewhat
clearer picture of the apparent range of C. p. miquihuanus. In
1961 an adult snake of this race was collected in the Sierra de Los
Amargos, Coahuila, extending the known range some thirty miles
north from the type locality in Nuevo Leon (Axtell and Sabath
1963). This will probably become the range limit of this rattle-
snake to the north, since there is a distinct topographical and faunal
break at this site wherein a much more arid and desert-type habitat
replaces the pine-oak forest of the Sierra Madre Oriental. J. R.
Dixon (pers. comm.) secured three specimens of this race from
14.8-17.2 km E San Antonio de las Alazanas between 1981-2804 m
elevation during May and August 1972. All were taken from a
pinon-pine, agave slope. South from Cerro Potosi, the specimen
obtained by Nelson and Goldman in 1898 is the only known record
for this subspecies. This is not to suggest, however, that the area
surrounding Miquihuana, Tamaulipas, is the southernmost limit of
the range of C. p. miquihuanus. Due to topographical, faunal, and
to some degree, geological similarity, it is possible that this sub-

.^^BR^^&

■ . — —

^^ ■ f'r^^.-^-

n

■mp"^

Fig. 24. Crotalus pricei miquihuanus. Specimen from near San Antonio
de las Alazanas, Coahuila, Mexico. (Photograph by John H. Tashjian.)

42 SPECIAL PUBLICATION-MUSEUM OF NATURAL HISTORY

Fig. 25. Crotalns pricei miquihuanus. Juvenile specimen bom to female
collected from Cerro Potosi, Nuevo Leon, Mexico. (Photograph by John H.
Tashjian. )

species will eventually be found as far south as the San Luis
Potosi/ southwestern Tamaulipas border, among the high forested
mountain slopes.

During our travels throughout Mexico from 1967 to 1974, we
secured four adult C. p. miquihuanus. Two were discovered above
3050 m near the type locality, in mid-July 1974, in a limestone
bedrock, agave, scrub oak habitat at 1100 hours. They were both
sunning on the limestone rock and seemed placid and indifferent
to being captured, unlike C. p. pricei which reacts defensively.
The summit of Cerro Potosi has been heavily deforested, and a
luxuriant growth of low-growing vegetation, such as agave and
scrub oak, has replaced the original pine forest. There are still
many pines in the area, but Cerro Potosi is no longer strictly a
pine forest. With this change in vegetation, the population of
C p. miquihuanus appears to have increased; possibly the low-
growing shrubbery offers more protection to both the rattlesnake
and its main food prey, lizards of the genus Sceloporus.

A female C. p. miquihuanus, collected on the east slope of
Cerro Potosi, Nuevo Leon, at 3203 m, gave birth to five young
(KU 155542, 157869-871) on 19 August 1974. The range of varia-
tion is as follows: total length 130-143 mm, mean 135; S-V 117-130
mm, mean 121; weight 2.6 g, mean 2.6. Another female (UTA
R-6235) captured on 16 July 1976 in a pine-oak forest at 2652 m,

THE NATURAL HISTORY OF MEXICAN RATTLESNAKES 43

located 17.2 km E San Antonio de las Alazanas, gave birth to four
young the following day (UTA R-6175-77, 6236).

The habitat type preferred by C. p. miquihuanus is the Temper-
ate Pine-Oak association (Klauber 1956).

Crotalus pusillus Klauber

Crotalus pusillus is a small rattlesnake found in intermittent
montane habitats in southern Jalisco and westcentral Michoacan
(Figs. 26, 27). Klauber (1972: 163) considered this taxa to be one
of the most primitive Mexican mountain rattlesnakes. Crotalus
pusillus inhabits the pine-oak forests at elevations from 1525 m to
at least 2380 m (Sierra de Coalcoman), and resides within the
Upper Sonoran-Transition Zone as defined by Goldman (1951). It
occurs sympatrically with C t. triseriatus throughout its range with
the exception of the Sierra de Coalcoman where possibly only C.
durissus culminatus and C b. hasiliscus live in close proximity.

Crotalus pusillus is locally common, and 17 specimens were ob-
served by us (UTA R-4530-1, 5846, 6119). We found them to be
quite abundant in the Sierra de Coalcoman, in cleared areas of
humid pine-oak forest (Fig. 28). Some specimens were taken in the
early morning, but this animal was more easily located around
1200 hours, after the sun had burned off the low morning clouds.
Duellman (1961: 658) suggested that this species was partially noc-
turnal. Several snakes were secured in cultivated fields where corn
was planted between rock outcroppings. Others were located on
and under fallen logs, and some were found in a rocky, grassy,
agave situation. J. R. Dixon (pers. comm.) secured specimens from
14.4 and 20.9 km W Atenquique, Jalisco, under fallen bark in an
oak forest between 1829-2164 m elevation.

Crotalus pusillus exhibits a typical behavioral pattern of rattle-
snakes from high, humid, mountain areas. It will often reveal itself
by rattling furiously when approached, but is comparatively less
nervous than C. lepidus in similar encounters. Many of our speci-
mens defended themselves vigorously upon discovery, and often
would bite themselves or the tongs which were used to capture
them. Ritualized combat between captive males has been observed
by us. The stomach of an adult (U1"A R-4530) contained an
orthopteran.

A female collected near Dos Aguas, Michoacan, gave birth to
one live and four stillborn young (KU 155545-548), and one infertile
egg on 23 January 1974. Range of variation is as follows: total
length 165-179 mm, mean 171; S-V 150-162 mm, mean 153; weight
3.0-6.1 g, mean 4.0. The surviving juvenile was maintained in cap-

44 SPECIAL PUBLICATION-MUSEUM OF NATURAL HISTORY

Fig. 26. Crotalus pusillus. Specimen from near Dos Aguas, Sierra de
Coalcoman, Michoacan, Mexico. (Photograph by John H. Tashjian. )

Jf^i/

^lA t£*' V

Fig. 27. Crotalus pusillus. Juvenile specimen from near Dos Aguas, Sierra
de Coalcoman, Michoacan, Mexico. (Photograph by John H. Tashjian.)

THE NATURAL HISTORY OF MEXICAN RATTLESNAKES

45

:»>trrA«r.

Fig. 28. Habitat of Crotalus pusillus near Dos Aguas, Sierra de Coalcoman,
Michoacan, Mexico, July 1974. Extensive limestone outcropping bordered by
pine-oak forest. Snakes appeared more abundant in areas previously cleared
for cultivation. (Photograph by Jonathan A. Campbell.)

tivity and bred, and gave birth to three young on 15 July 1976.
Two of the young were stillborn within a single fetal membrane,
and the mean weight and measurements for both are as follows:
total length 111 m, S-V 98 mm, weight 1.1 g. The third neonate
was alive at birth, measured 179 mm total length (156 mm S-V) and
weighed 6.0 g (UTA R-7164).

On 24 March 1975 a pair of captive C. pusillus was placed to-
gether and a complete mating sequence was observed.

1130h Male placed in cage with female. Female had previously shed and
defecated. Female immediately began twitching.

1130-1159 Male began spasmodic forward jerking (2 per sec) and tongue-
flicking (2 per sec). Male loosely draped his coils over dorsum of
female, bent head laterally at 90° angle to longitudinal axis of his
body and, by contracting vigorously, pulled his head approximately
2.5 cm posteriorly along body of female. This action was performed
repeatedly. Female raised tail 11 cm vertically and opened cloaca.
Male became excited and investigated female's cloacal region. This
entire sequence was repeated 34 times. Female twitched only
occasionally.

1200 Male wrapped tail around female's head.

1201 Male released female's head and brought his cloaca to vent of female.
Mating was not accomplished.

1202-1220 Both snakes remained quiescent.

1221 Female moved head, male lost grip and became excited.

1222-1400 Same pattern as above.

46 SPECIAL PUBLICATION-MUSEUM OF NATURAL HISTORY

1401 Male and female simultaneously raised tails vertically, tightly
wrapped tails, held position for 1 minute, then released.

1410 Female raised tail vertically (11 cm) for 5 minutes. Male con-
tinued to investigate.

1412 Both male and female raised tail 11 cm with cloacae juxtaposed
giving appearance of a lyre. Distance between rattle matrix of
both snakes was 6 cm. Male released position, then slid tail ap-
proximately 2 cm to cloaca of female. Male's tail was encircled
around base of female's tail. Male slid tail rapidly and repeatedly
along longitudinal axis of female's tail.

1417 Male's tail encircled base of female's tail twice.

1419 Male lost grip with his tail.

1424 Female raised tail.

1431 Female continued to raise tail, male inserted right hemipenis. A
noticeable bulge was apparent eight ventral scale rows anterior to
female's cloaca. Male continued to push against resistance of female
which caused the female's tail to bend backwards over her body un-
til the matrix of her tail was 3.5 cm above her vertebral column.

1443 Four quick pulsations at cloacal area of male.

1446 Male began undulations (2 per sec) which were within a vertical
plane and were 5 cm anterior to the vent. During this period, the
male constantly pushed the female's tail backward.

1500 Male tapped mental area of his lower jaw in a vertical plane on
dorsum of female 39 times in a 6 minute period. No tongue flicks
accompanied this behavior unless the male shifted position.

1509 Male tapped his lower jaw on dorsum of female 17 times, no
tongue flicks.

1534 Pair still joined, diameter of female's cloaca was 12.5 mm which
was distended due to male's hemipenis.

1740 Approximately 6 mm of base of hemipenis visible.

1800 Male pushed tail against female's tail, writhed for 3 seconds.

1834 Same as above.

1841 Both tails moved slowly for 10 sec.

1909 Observations discontinued.

The next day at 0800 hours the snakes had separated.

On 22 April 1975 the same male specimen was placed with a
different female. The same behavioral patterns were observed, in-
cluding upraised tails, bending of the female's tail backwards over
her body, and pulsations. The head tapping behavior of the male
was also observed. The raised tail of the female stimulated the
male noticeably.

The habitat preferred by C. pusillus is mixed pine-oak forest
and volcanic rock (Klauber 1956) and Pine-Oak Forest at an alti-
tude of 1550 to 2300 m (Duellman 1965).

Crotalus ruber Cope

The red diamond rattlesnakes, C. ruber, of extreme southwestern
California and adjacent Baja California, are among the largest and
generally the most placid rattlesnakes we have observed. Partly
because of their range and habitat preference, these snakes have
been collected by one of us (BLA) every mondi of the year. Their
peak period of activity is in late spring, and they are relatively

THE NATURAL HISTORY OF MEXICAN RATTLESNAKES 47

common throughout their range. In southern California, the status
of C. r. ruber populations is becoming precarious as more and more
developments claim inland valley habitats.

Crotalus ruber ruber. The northern subspecies, C. r. ruber,
ranges from southeastern Los Angeles and Orange, San Diego,
western San Bernardino, and Riverside counties, and extreme south-
western Imperial County in southern California, south to Loreto,
Baja California del Sur where it intergrades with C. r. lucasensis
(Klauber 1949a: 46). Klauber (1972: 46) also reported individuals
from Monserrate, San Marcos, Pond, Angel de la Cuarda, and
South San Lorenzo islands in the Gulf of California. It seems
reasonable to assume that this rattlesnake will eventually be found
on other, smaller islands that have not yet been thoroughly ex-
plored. It is interesting to note that a series of C. ruber, now known
as C r. lorenzoensis, from South San Lorenzo Island in the Univer-
sity of Colorado collection, show tendencies toward rattle loss
(Radcliffe and Maslin 1975). This condition was seen on approxi-
mately 50 percent of the specimens. C. r. ruber obtained by us
have been observed from various locations in five counties in south-
ern California, and the following localities in Baja California: 6.4
km S Tijuana, the vicinity of Ensenada, the Hamilton Ranch (20
specimens) near Colonet, San Quintin, El Socorro (5 specimens),
El Rosario, 16 km N Laguna Chapala, San Ignacio, 17.6 km W
Santa Rosalia, 22.5 km S Mulege. Crotalus r. ruber appears to be
absent from the desert east of the Sierra de Juarez in Baja California
del Norte.

In southern California, C. r. ruber is a resident of the hot inland
valleys which include the Borrego Desert of San Diego County.
It is found generally in areas of granite rock outcroppings, espe-
cially during winter months. During these months we have had
success locating numbers of these snakes in the hottest part of the
day. They are often coiled at the entrance of a gopher burrow, or
in the midst of the rubble of a dilapidated barn. Rarely are they
located any distance from some type of shelter. This rattlesnake is
extremely lethargic during winter months, and defensive behavior
is seldom observed even when it is provoked. However, during
the late spring and summer months, C. r. ruber becomes increasingly
nocturnal, and invades the grasslands between rock outcrops where
it probably feeds to a large extent on cottontail rabbits, SyJvilagus
auduboni sanctidiegi, and ground squirrels, Citellus b. beecheyi,
both of which are abundant. In the true desert regions of its range,
this snake is found near rocks, and seldom wanders from protective
retreats. In the early spring, pairs of red diamond rattlesnakes are
often observed basking together in full sunlight. Mating was ob-
served in early April between a large pair in Railroad Canyon, just
east of Lake Elsinore, California. The snakes were not collected.

48 SPECIAL PUBLICATION-MUSEUM OF NATURAL HISTORY

Wagner (1964) characterized the vegetational characteristics of
the area inhabited by C. r. ruber as California Chaparral, Seasonal
Formation Series, and Cactus Scrub.

Crotalus ruber lucasensis. The southern subspecies of the red
diamond rattlesnake, C. r. lucasensis, has a geographic range from
the fishing village of Loreto to the Cape region of Baja California
del Sur. Klauber (1972: 46) reported specimens of this race from
Santa Margarita and San Jose islands. As with C. r. ruber, C. r.
lucasensis may someday be found on other islands when they are
thoroughly explored. Specimens obtained by us from the mainland
were collected at El Triunfo, 8 km N San Jose del Cabo, and Buena
Vista (B. Tomberlin, pers. comm.).

Barney Tomberlin (pers. comm.) reported that C. r. lucasensis
was the most common snake in the Cape region of Baja California.
Most of his snakes were taken in September when that area re-
ceived a major portion of its annual rainfall. At that time they were
nocturnal generally, and specimens were easily found by night
driving. Although this rattlesnake can be found in the desert and
open arid plains of southern Baja California del Sur, it is most com-
mon in heavy brush where rocks and rocky outcrops are prevalent.

This subspecies inhabits the Seasonal Formation Series and
Cactus Scrub as defined by Wagner (1964).

Crotalus scutulatus (Kennicott)

Crotalus scutulatus was once confused with many of the other
prairie dwelling rattlesnakes (Klauber 1972: 541), and therefore,
much that has been written about it may, in part, be erroneous.
It is a rather large, heavy-bodied snake that has a range similar to
that of C. molossus. Crotalus scutulatus does not, however, inhabit
the multiple habitats within its range as does C. molossus. C.
scutulatus is found from the Mojave Desert and adjacent Nevada,
south through Arizona, southwestern New Mexico, and Texas, and
thence south into the Mexican central plateau.

Crotalus scutulatus scutulatus. Crotalus s. scutulatus is a com-
mon rattlesnake within its range in the United States. Klauber
(1972: 47, 541) gave an excellent summary of the localities and
field data for this race. In Mexico, we have found C. s. scutulatus
throughout the flat, arid central plateau, but nowhere did it seem
particularly common. Snakes captured and observed by us (approx.
30) were recorded from: CHIHUAHUA: the valley between the
Sierra del Nido and Sierra de Santa Clara at 1982 m (UTA R-4554);
SONORA: 12.8 and 17.6 km S Agua Prieta; DURANGO: La Zarca,
12.8 km N Rodeo, San Juan del Rio; NUEVO LEON: Santa Fe
(UTA R-4595), 12.8 km S Galeana, 8 km E San Roberto Junction;
COAHUILA: 3.2 km W San Antonio; SAN LUIS POTOSI:

THE NATURAL HISTORY OF MEXICAN RATTLESNAKES 49

Matehuala, 4.8 km E Matehuala; ZACATECAS: 13.3 km E Junc-
tion of Mexican Hwy. 45 and 49 (UTA R-2715).

Miller and Stebbins (1964) noted that throughout the northern
portion of its range, C. s. scutulatus appeared to be extremely noc-
turnal in behavior. Most of the specimens obtained by us were a
result of night driving. In spring, it was not uncommon to find a
specimen coiled at the base of a palo verde or creosote bush in
early morning. During summer months, however, these snakes
were rarely encountered during daylight hours. Crotalus s. scutu-
latus seems to prefer the open mesquite-creosote-cacti habitat within
its range. It usually avoids true sand desert where Crotalus cerastes
is the dominant rattlesnake.

In southern Arizona and adjacent Mexico, C. s. scutulatus is
very common in prairie valleys (1220-1525 m) between the numerous
forested mountain ranges of that region. Southward within the
Mexican plateau this snake becomes crepuscular due to the higher
elevations of the open, arid habitat. One individual obtained by
us from grassland, juniper plain (1982 m elevation) near Santa Clara,
Chihuahua, was located crossing a dirt road at 1100 hours in early
July. In central Durango near the town of La Zarca, C. s. scutulatus
was often seen in mid-afternoon crossing the highway or basking
at the edge of the pavement. This area is approximately 2440 m
elevation, and the daytime temperature is somewhat cool. Juniper
and grassland are the dominant vegetation in this wind-swept re-
gion. Individuals from the eastern part of San Luis Potosi, espe-
cially around Matehuala, 1372 m, were also crepuscular in activity
despite a lower elevation than at the Durango localities. Reese
(1971) found C. s. scutulatus at a locality 77.2 km S San Roberto
Junction in Nuevo Leon, and Banta (1962) found it at Caiiada
Honda, Aguascalientes. One adult snake obtained by us (UTA
R-4554) contained rodent hair.

The Temperate Mesquite-Grassland and Desert as defined by
Leopold (1950) are typical environments for C. s. scutulatus. See
Gloyd (1937) and Lowe (1964) for a description of the habitat in
Arizona.

Crotalus scutulatus salvini. Crotalus s. salvini (Fig. 29) is also
a resident of open, high interior plains. It is found in Tlaxcala,
Puebla, and west-central Veracruz, and its range is doubtless de-
creasing as these plains are cleared for cultivation. The type lo-
cality for this race, Huamantla, Tlaxcala (Klauber 1972: 147), at an
elevation of over 2440 m, may now possibly be devoid of enough
suitable habitat to sustain a population of these rattlesnakes. All
of our specimens were collected within the lava beds near Perote,
Veracruz, and Tehuacan, Puebla (Fig. 30). Both localities support
luxuriant desert plant growth such as yucca, palmettos, and the
giant, slender Neobuxbaumia, which are the most common plants.

50 SPECIAL PUBLICATION-MUSEUM OF NATURAL HISTORY

Witliin the relatively undisturbed areas of its limited range, C. s.
salvini remains a rather common snake, and, unlike C. s. scutulatus,
is found in association with rocks. The lava beds of Perote seem
to provide this rattlesnake excellent habitat. It may, however, be
common there only because of man's encroacliment upon the roll-
ing, open, rockless plains where C. s. salvini was once, no doubt,
the dominant member of the genus. Specimens of C. s. salvini ob-
tained by us were all located during mid-afternoon in July and
August, and were basking near porous volcanic rocks.

This rattlesnake is one of the most aggressive species we have
encountered. Several struck so violently that their entire body
appeared to be momentarily air borne. It is common behavior for
them to strike even after being constrained in cloth bags for several
weeks. C. s. salvini inhabits the Temperate Pine-Oak and Mesquite-
Grassland vegetational areas as defined by Leopold (1950).

Crotalus stejnegeri Dunn

Crotalus stejnegeri is a primitive rattlesnake found in the moun-
tains of southeastern Sinaloa and southwestern Durango. Klauber
(1972: 542) reported two specimens that ". . . were found on the
border of a pine forest, at the upper edge of a canyon dissecting a
plateau." McDiannid et al (1976) reported an individual found
on a road which was 3.3 km west of a pine forest, probably in sub-
tropical dry forest (Hardy and McDiarmid 1969). Road temperature
was about 27 °C and air temperature ca. 24 °C where the snake was
collected. A small male (UTA R-5926) and female were collected at
Plomosas, Sinaloa, at 1067 m on 3 August and 27 August 1976,
respectively. The snakes were found in a transition area between
pine-oak and tropical deciduous forest (Fig. 4). Vegetation in this
area includes silk -cotton trees, morning glory trees, Enterolobium,
Coccoloba, nanche (Byrsonirna crassifolia), cassias, acacias, and
mimosas (Goldman 1951). The male was found in a rock sHde at
the base of a bluff, and the female was secured in a small rodent
burrow in a semi-open field. A large male (UTA R-6234; Fig. 31)
was found on 15 August at Ejido Tebaira, Sinaloa, at 1067 m in a
tropical deciduous forest. This individual measured 638 mm total
lengtli, 86 mm tail length, and is larger than the largest snake re-
ported by McDiarmid et al. (1976).

Crotalus tigris Kennicott

Crotalus tigris is a medium to small-sized desert dwelling rattle-
snake with a proportionately small head and large rattle. This
species has a rather limited range, and is known only from the
states of Arizona and Sonora. Wright and Wright (1957: 1001) con-
sidered this species rare.

Crotalus tigris is a resident of rocky foothills within the Sonoran

THE NATURAL HISTORY OF MEXICAN RATTLESNAKES 51

■. i^

Fig. 29. Croialus scutulatus salvini. Specimen from 1.6 km E El Limon
Totalco, Veracruz, Mexico. (Photograph by John H. Tashjian. )

v^ip

Fig. 30. Habitat of Crotaltis scutulatus salvini and Sistrurus ravus, near
El Limon Totalco, Veracruz, Mexico. Area is lava flow characterized by
cactus, agave and grasses. (Photograph by R. Terry Basey.)

52 SPECIAL PUBLICATION-MUSEUM OF NATURAL HISTORY

Fig. 31. Crotalus stejnegeri. Specimen from Ejido Tebaira, Sinaloa,
Mexico. (Photograph by Jonathan A. Campbell.)

Desert of Mexico and the United States, and ranges from near sea
level at La Posa, Sonora, to at least 1465 m elevation (Humphrey
1936). We found an individual snake at the same elevation on the
western slopes of the Santa Rita Mountains, Arizona. Van Den-
burgh (1922) recorded the maximum elevation for this species at
2440 m. C. tigris is rarely found far from rocky retreats. Lowe
(1964: 173) considered this rattlesnake to be strictly a rock dweller
in rocky canyons, and on the hillsides and bajadas of desert ranges.
Three snakes obtained by us from 11.2 km NE Scottsdale, Arizona
(UTA R-6943, KU 155525) were taken at night in early July just
after a thunderstorm. They were active among granite outcrops
where cacti, palo verde, and creosote were abundant. This snake
is nocturnal and rarely ventures forth before summer rains begin.
Taylor (1936: 49) found two C. tigris at night, south of Hermosillo,
Sonora, coiled in isolated rock outcrops in low mountains. We ob-
served a snake of this species from the Tucson Mountains, Arizona,
at night, as it was investigating a pack rat nest. This area is very
rocky, with much desert vegetation. Dixon et al. (1962: 99) de-
scribed the habitat of C. tigris in southern Sonora as scrub desert.

Crotalus tigris is behaviorally unpredictable. Two specimens
from near Scottsdale were located when they rattled as we passed
within 3 m of them. This observation occurred on an extremely
warm, humid evening, after a thunderstorm. A specimen from the
Tucson Mountains never rattled, even while being captured.

THE NATURAL HISTORY OF MEXICAN RATTLESNAKES 53

Klauber (1972: 453) felt that this rattlesnake was generally inoffen-
sive, and not prone to rattle or strike.

The Seasonal Formation Series and Cactus Scrub as defined by
Wagner (1964) are typical environments inhabited by C. tigris.

Crotalus tortugensis Van Denburgh and Slevin

Klauber (1972) characterized Tortuga Island off Baja Calffornia
as rocky and barren, more arid than the mainland, with sparse
brush and cacti. Three specimens of C. tortugensis obtained by
us (KU 174832) were collected on 27 October 1975 in an area of
lava boulders (D. Brown, pers. comm.). Wagner (1964) character-
ized the vegetational character of Tortuga Island as Seasonal For-
mation Series.

Crotalus transversus Taylor

Crotalus transversus is, biologically, a poorly known rattlesnake
both in the field and in captivity. Its type locality was listed by
Smith and Taylor (1950) as Tres Cumbres, Morelos, but Davis and
Smith (1953) reassigned the type locality to nearby Laguna Zem-
poala, Mexico. The late E. H. Taylor (pers. comm.) informed us
that the type locality is Tres Cumbres. Further collecting may
reveal the presence of C. transversus in northwestern Morelos and
southwestern Distrito Federal due to the topographical similarity
of these regions to that of the type locality.

Only three individuals of this species have been recorded from
the time of its discovery in 1942 until 1971. Since then nine speci-
mens of C. transversus have been secured, six of which are dis-
cussed here (Figs. 32, 33).

On a trip to southern Mexico in the summer of 1973, Charles
Radcliffe and one of us (BLA) succeeded in collecting five of these
snakes (UCM 51421-3, KU 159361-2). An additional specimen was
collected in May 1975 (UTA R-3988). All were taken at Laguna
Zempoala, Mexico (Fig. 34) between 1 August and 5 August at
elevations ranging from 2896 to 3293 m. The habitat was temperate
boreal forest as defined by Leopold (1950). Five specimens were
observed basking on south-facing slopes on volcanic rocks in the
early afternoon. None of the specimens was basking in direct sun-
light. The snakes were found after morning rain-showers which
occur almost daily at this locality during summer months. Air
temperature at the time of collection was between 16-20 °C.

C. transversus is a very inoffensive rattlesnake, and retreats
rather quickly when discovered. None of the captive specimens
maintained by us rattled or struck, unlike C. t. triseriatus with
which this species is sympatric. T. Walker (pers. comm.) observed
that only one of his three captive specimens ever rattled, and be-
lieves that this species is not rare, but is only thought to be so

54 SPECIAL PUBLICATION-MUSEUM OF NATURAL HISTORY

Fig. 32. Crotalus transversus. Specimen from near Laguna Zempoala,
Mexico, Mexico. Reddish color phase widi distinct cross-bands. (Photograph by
John H. Tashjian. )

•%-

Fig. 33. Crotalus transversus. Specimen from near Laguna Zempoala,
Mexico, Mexico. Grayish color phase with indistinct cross-bands. (Photograph
by John H. Tashjian.)

THE NATURAL HISTORY OF MEXICAN RATTLESNAKES 55

Fig. 34. Habitat of Crotalus transversus and C triseriatus triseriatus near
Laguna Zempoala, Mexico, Mexico. Open fir-pine-oak forest with understory
of bunchgrass. (Photograph by M. Granger.)

56 SPECIAL PUBLICATION-MUSEUM OF NATURAL HISTORY

because of its limited activity periods in its natural habitat. An
abundance of lizards and mice in the preferred habitat of C. trans-
versus probably results in the need for only limited foraging ac-
tivity. Klauber (1972) mentioned that one individual contained
lizard scales, and one of our snakes defecated lizard scales, probably
ScelopoTus aeneus.

A number of food items were offered to captive C. transversus:
treefrogs (Hyla), lizards (Sceloporus, Gerrhonotus, Uta, Phyllo-
dactylus, Xantusia, Cnemidophorus, Anolis), wild rodents (Mus
musculus), snakes (Thamnophis, Nerodia), newborn laboratory rats,
sparrows, and domestic gray crickets (Gryllus); all were refused.
The rodents were killed by tlie snakes, but the other potential prey
items were ignored.

Captive snakes of this species were maintained by us at the
following thermal levels: (1) 26.5°C during the day and dropping
to 10°C at night; (2) 25-27°C during the day, 20°C at night; (3) low
20°C temperature at all times. These snakes may have been affected
by the change in elevation from their native habitat to captive con-
ditions. They fared badly at all three thermal levels.

Crotalus triseriatus (Wagler)

The dusky rattlesnakes of south-central Mexico are, taxonomic-
ally, one of the most complicated groups of rattlesnakes yet de-
scribed. We have made over 100 observations of both subspecies,
and this field work has shed some light on their relationships. It
has become apparent to us that the two currently recognized sub-
species, C. t. triseriatus and C. t. aquilus, require a complete review,
and such work is underway by several of our colleagues.

Crotalus triseriatus triseriatus. Crotalus t. triseriatus is a mon-
tane race reaching the highest altitude (4572 m on Mt. Orizaba,
Veracruz) at which any member of the genus Crotalus has been
recorded (Klauber 1972: 516, 542). Although found at such high
altitudes, C. t. triseriatus is more commonly encountered at eleva-
tions between 2743 m and 3353 m. Specimens have been recorded
by us from: JALISCO: the mountain valleys N Nevado de Colima
(UTA R-4909) (see Campbell, 1979); MEXICO: Laguna Zempoala;
DISTRITO FEDERAL: La Cima; MORELOS: Tres Cumbres,
the lava beds N Cuemavaca, Huitzilac (Fig. 35).

Crotalus t. triseriatus probably occurs in the Mexican state of
Puebla, but we have no records as yet from that area. It also may
be found on Cerro La Malinche, a 4267 m peak in Tlaxcala. Resi-
dents in this area described to us a snake which appeared to be
C. t. triseriatus, but no specimens have been secured. In general,
the range of C. t. triseriatus follows a narrow east-west belt be-
tween the 18th and 20th parallels. This range includes Cordillera
Volcanica of southern Mexico.

THE NATURAL HISTORY OF MEXICAN RATTLESNAKES

57

Crotalus t. triseriatus is one of the most common forms of the
small Mexican montane rattlesnakes. Due to the high elevations
at which it occurs, this subspecies is almost entirely diurnal in
activity. Even during the warmer months of the year, temperatures
fall to levels at night which would not permit activity by this snake.
Consequently, this rattlesnake is most easily observed in mornings
and afternoons when the sun has sufficiently warmed its habitat.
These animals are also active during the rainy season, and it is
not uncommon to find several following an afternoon shower, bask-
ing on rocks that are still warm from the earlier rays of the sun.
Davis and Smith (1953: 141) found C. t. triseriatus common near
Huitzilac, Morelos, along streams that were bordered by a luxuriant
growth of bunch grass. Our observations agree with theirs; this
-snake is rarely found any distance from a rocky, grassy environ-
ment. We have obtained snakes on rock slides near Laguna Zem-
poala, Mexico, but they are not nearly so common there. Duellman
(1965) mentioned this species as an inhabitant of the Pine-Oak
Forest at 1600-3270 m altitudes.

Crotalus t. triseriatus exhibits behavior that is comparable to
that of other rattlesnakes in montane habitats. It is alert and ready
to retreat when annoyed, yet curious enough to expose itself to
capture. This subspecies is relatively unaggressive even when be-
ing captured. We have observed ritualized combat between
captive males. C. t. triseriatus feeds on lizards and small rodents

Fig. 35. Habitat of Crotalus triseriatus triseriatus and Sistrurus ravus.
La Cima, Distrito Federal, Mexico. Old lava flow covered with bunchgrass,
formerly a pine forest. (Photograph by M. Granger.)

58 SPECIAL PUBLICATION-MUSEUM OF NATURAL HISTORY

which are plentiful in its preferred habitat. Cursory examination of
stools from freshly caught specimens supports this diet preference.

On 15 July 1970, a female from the northern drainage of Nevado
de Colima, Jalisco, was located in a partially cleared pine-oak
forest under a log, along with six new-born young. A captive
female passed a partially formed embryo on 1 July 1976. Four
young were bom on 30 October 1975 to a captive female collected
at Laguna Zempoala. Range of variation is as follows: total length
159-178 mm, mean 168; S-V 141-161 mm, mean 150; weight 4.8-
5.1 g, mean 4.9. A captive pair was observed in copulo on 24 April
1975 at 0800 and were separated at 1400. The same pair also bred
on 8 September 1977.

Crotalus triseriatus aquilus. The northern subspecies of this
rattlesnake, C. t. aquilus (Figs. 36, 37), does not reach the high
altitudes inhabited by C. t. triseriatus. It seems to prefer the open,
grassy habitats north of the Cordillera Volcanica. Specimens as-
signable to this subspecies have been recorded by us from:
MICRO AC AN: 16 km W Jiquilpan; SAN LUIS POTOSI: the
vicinity of Alvarez; HIDALGO: Jacala, San Vicente, Durango, El
Chico (KU 155549-553, 155556; UTA R-4540, 6941), La Estanzuela
(UTA R-4675, 6115).

This rattlesnake is a common montane snake, but, as previously
stated, does not usually occur in extremely high-altitude habitats.
Klauber (1972: 517) gave 2438 m elevation, near Jacala, Hidalgo,

Fig. 36. Crotalus triseriatus aquilus. Juvenile specimen from near El
Chico, Hidalgo, Mexico. (Photograph by John H. Tashjian.)

THE NATURAL HISTORY OF MEXICAN RATTLESNAKES 59

n

Fig. 37. Crotaltis triseriatus aquilus. Specimen from near El Chico,
Hidalgo, Mexico. (Photograph by John H. Tashjian. )

as the highest point where C. t. aquilus has been collected. How-
ever, we have located a population near El Chico, Hidalgo (Fig.
38) where these rattlesnakes have been found up to 3110 m in a
fir forest. The snakes from this population, while falling well within
the known geographic range of C. t. aquilus, are quite different
from our other specimens of this race. They are larger and exhibit
a greenish hue, with males far exceeding females in size and
brightness of color. These specimens tend to be heavier bodied
than typical C. t. aquilus and they are extremely alert and ag-
gressive. In many characteristics they resemble C. lepidus. Further
study of these specimens is needed in order to understand their
status in relation to other populations of C. t. aquilus.

Generally, C. t. aquilus is an inhabitant of jDine-oak forests and
open mesquite-grasslands of the southern central Mexican plateau
(Fig. 39). Snakes from Michoacan, the southernmost portion of the
range of C. t. aquilus, usually inhabit mesquite-grassland. North-
ward to Hidalgo and San Luis Potosi, this race tends to occur
more commonly in the pine-oak forests where there is an abundance
of rock outcroppings.

The behavior of C. t. aquilus in the field is much the same as
that of C. t. triseriatus, but individuals of the former tend to be
somewhat more aggressive and easily agitated. These snakes often
reveal their presence by rattling when alarmed, as is typical of

60 SPECIAL PUBLICATION-MUSEUM OF NATURAL HISTORY

Fig. 38. Habitat of Crotalus triseriatus aquilus, near El Chico, Hidalgo,
Mexico. Fir forests cover hills reaching elevations exceeding 3110 m. Snakes
most commonly found in open rocky outcrops. (Photograph by Jonathan A.
Campbell. )

Fig. 39. Habitat of Crotalus triseriatus aquilus, near San Vicente, Hidalgo,
Mexico. Steep hillsides are covered with pine-oak forest. Snakes most com-
mon in open limestone bedrock. (Photograph by R. Terry Basey.)

THE NATURAL HISTORY OF MEXICAN RATTLESNAKES 61

C. lepidus. This race is active in the mornings and afternoons, but
unHke C. t. triseriatus, it may be found foraging at night. We have
found them while driving at night in the vicinity of Durango,
Hidalgo, a somewhat humid area where night temperatures are
occasionally favorable for activity. Ritualized combat between
captive males has been observed by us.

Two females of C. t. aquilus from El Cliico, Hidalgo, gave
birth to six and seven young on 7 June 1974 and 29 July 1974, re-
spectively. Range of variation of the two broods is as follows:
total length 168-192 mm, mean 178; S-V 153-177 mm, mean 163;
weight 5.0-8.8 g, mean 6.4. Another female, collected 16 km W
Jiquilpan, Michoacan, gave birth to three young on 29 June 1974.
Range of variation is as follows: total length 164-171 mm, mean
167; S-V 150-156 mm, mean 152; weight 5.0-6.4 g, mean 5.5. A
captive pair of C. t. aquilus from El Chico was observed copulating
on 28 April 1975. One of these snakes later gave birth to two
viable and one deformed young on 27 July 1977, and the measure-
ments and weights are as follows: total length 120-181 mm, mean
150; weight 4.9-7.8 g, mean 6.3. The deformed snake exhibited a
fusion of the ventral surface in the lower cervical area (UTA R-
6940). Another pair of C. t. aquilus was joined in mating at 0830
hours on 26 October 1977. The female's cloaca was distended and
the male's body pulsated (twice per second) near the vent during
coitus. The uplifted tails of the snakes produced a venter-to-venter
configuration in the region of the cloaca. Separation occurred at
0901, and detumescence of the hemipenis occupied 7 seconds.

Duellman (1965) characterized the vegetational character of the
habitat of this rattlesnake as Mesquite-Grassland and Pine-Oak
Forest at 1600-2000 m altitudes. This race is found in the Temperate
Mesquite-Grassland and Pine-Oak Forest as defined by Leopold
(1950).

Crotalus viridis (Rafinesque)

Crotalus viridis caliginis. Crotalus v. caliginis is a race found
on South Coronado Island off the northwest coast of Baja Cali-
fornia. This area is rocky with brush and cacti, and two adult
specimens (KU 174835, UTA R-6945) were collected at the north
end of the island on 25-26 June 1976 (D. Brown, pers. comm.). The
female passed one partially formed embryo on 14 August 1976.

This subspecies is found in the California Chaparral as defined
by Wagner (1964).

Crotalus viridis helleri. Crotalus v. Jielleri is a subspecies found
in southern California and northern Baja California, and is a resi-
dent of almost eveiy habitat witliin its range. As with C ruber,
with which it shares a great deal of its range, a number of C v.
helleri have been observed by us every month of the year. These

62 SPECIAL PUBLICATION-MUSEUM OF NATURAL HISTORY

snakes vary greatly in color and pattern within a given population.
As individuals of this rattlesnake age, they apparently lose their
pattern or it fades greatly, a common trait of all races of C. viridis.
Most larger specimens of C. v. helleri are black or very dark with
little pattern. Many young of this race have distinctly patterned
heads, but this fades at an age of two years on most individuals.

C V. helleri ranges from San Luis Obispo and Kern counties
south through southern California (avoiding only the true deserts
in the eastern part of the state) and continues into Baja California
as far south as Playa Maria Bay on the west coast, and near Bahia
de Los Angeles on the east coast (Klauber 1949b: 83). Murray
(1955: 47) reported a specimen 32 km SE Mesquital, some 88 km
south of Klauber's (1949b: 83) soutliernmost record. Smith et al.
(1971) found two specimens 69 km WNW Bahia de Los Angeles
and 16 km S El Arco. Specimens from these southenmiost locations
are rare, although it is possible for C. v. lielleri to inhabit the area
as far south as San Ignacio in central Baja California. Rattlesnakes
obtained by us come from many localities in southern California,
but we have few records from Baja California. Individual speci-
mens from Mexico have been observed from Punta Camalii, north
of Ensenada, and the western slopes and interior peaks within the
Sierra Juarez.

Crotalus v. helleri is common within its range, although its
numbers seem to decrease in the southern portions. On the lower
slopes of the San Gabriel Mountains in Los Angeles County, Cali-
fornia, the populations of this race are as concentrated as any of
the genus known to us. This is a nigged, rocky area with chaparral,
and the rainy season occurs in the winter and early spring. Rattle-
snakes have been found from the edges of the San Gabriel River
to the hot, sparsely vegetated, south-facing slopes and pine-oak
slopes facing the north. Although individuals of C. v. helleri in this
area do not den or hibernate as such, they do congregate in rock
slides or rock outcrops which face the south or east during rainy,
winter months. To the south in Orange, Riverside, and San Diego
counties, this subspecies is found from the Pacific coast in rolling,
grassy savannahs to at least 3200 m in a pine-fir forest on Mt. San
Jacinto. Throughout most of these southern California counties,
C. mitchelli pyrrhus and C. r. ruber live sympatrically with C. v.
helleri. We have observed all three rattlesnakes within 45 m of
each other near Warner Hot Springs, just north of the Borrego
Desert. In that particular area, all three species seem equally
abundant. Elevation in this area is approximately 1068-1220 m.
In 1967 one of us (BLA) found an apparent hybrid, C. v. helleri X C.
r. ruber, on the road in this same general area. Reports of hybrids
from this area are not uncommon.

In Baja California, C. v. helleri is much less common and is

THE NATURAL HISTORY OF MEXICAN RATTLESNAKES 63

apparently replaced by C. enyo furvus, C. r. ruber, and C mitchelli
pyrrhus. A snake from Camalu was observed in scrubby, coastal
vegetation approximately 9 m from the shoreline. Another example
from the western slopes of the Sierra Juarez was found basking in
diffused sunlight under a manzanita bush. Klauber (1949b: 83) re-
ported a specimen 16 km NW Bahia de Los Angeles in desert
mountains in an elephant tree association.

Crotalus v. lielleri tends to be a rather aggressive form and is
easily agitated, behavior reported by Klauber (1972: 454). Most
adults will defend themselves vigorously.

Wagner (1964) characterized the vegetational character of the
habitat of C. v. lielleri as California Chaparral and Montane For-
mation Series, and Leopold (1950) characterized it as Pine-Oak
Woodland and Pifion-Juniper Woodland.

Crotalus toillardi Meek

The ridgenose rattlesnake, C. toillardi, is an example of a small,
montane species whose biology has remained somewhat obscure,
no doubt due to difficulties encountered in reaching possible Mexi-
can collection localities. Little is known of its habits, but it does
not appear to be rare within its range. The four races of this spe-
cies collectively inhabit the Sierra Madre Occidental from south-
eastern Arizona and southwestern New Mexico, southeasterly to
southern Durango and western Zacatecas (Klauber 1949c: 125).

Crotalus willardi willardi. The northern subspecies, C. w.
willardi, is a resident of the pine-oak forests of the Huachuca and
Santa Rita Mountains in Arizona, and the Sierra de Ajos and Cerro
Azul in Sonora, Mexico. This rattlesnake seems to prefer the more
humid canyon bottoms of pine-oak habitats as opposed to exposed
arid slopes where C. lepidus and C. pricei are common. B. Tomber-
lin and R. T. Basey (pers. comm.) reported that individuals from
the Sierra de Ajos (UTA R-6689, R-6928-30) were common in the
wide, rocky stream beds where pools of water remained after sum-
mer rains. This was between 1982 m and 2287 m in elevation. Two
lizards {Sceloporus jarrovi and S. undulatus) were common in this
stream bed habitat, and no doubt contribute a major portion to
the diet of this snake. Klauber (1972: 642) mentioned a specimen
that contained a brush mouse (Peromyscus boyli). Although pre-
ferring the cooler, humid canyons, C. tv. willardi is in no way re-
stricted to this habitat. A snake from upper Madera Canyon at
2593 m in the Santa Rita Mountains was observed on an east-facing
slope amidst broken rock and scrub oak. It would have been over-
looked had it not rattled, for the area where it was found is steep
and the scrub oak nearly impenetrable. A specimen of C. pricei
was located nearby. C. w. willardi is most active on warm, humid
days when the sunlight is intemiittent. It is especially common

64 SPECIAL PUBLICATION-MUSEUM OF NATURAL HISTORY

before and after an afternoon shower when the temperature is
between 24-29°C.

Crotalus w. willardi is typical of the small, montane rattlesnakes
in that it is nervous and irritable, although not to the degree ex-
hibited by C. lepidus or C. pricei. At times it reveals its presence
by rattling, and will crawl to safety upon the least provocation.
Captured specimens are more apt to turn and bite, as opposed to
typical rattlesnake striking behavior. Kauffeld (1943b: 357) and
Stebbins (1954: 484) also found this to be true. After a short period
of time, captive specimens of C. w. willardi generally calm down
and maintain a peaceful disposition.

Martin (1975a, 1975b) discussed reproduction in C. w. willardi.
On 18 April 1971 a captive pair was observed by us in copulo at
0800 hours, and they were still joined at 1700 hours when observa-
tions were discontinued. There were no results from this mating.

Crotalus w. willardi is found in the Montane Fomiation Series
and Montane Thicket as defined by Wagner (1964) and Pine-Oak
Forest and Pine-Oak Woodland as defined by Leopold (1950). ' See
Gloyd (1937) and Lowe (1964) for a description of the habitat in
Arizona.

Crotalus willardi amabilis. Five adult (KU 178794; Fig. 40)
and four subadult C. w. amabilis (UTA R-7162) were taken in
Arroyo Mesteno, Sierra del Nido, Chihuahua, at 2440 m on 18, 19,
21 July 1977 (L. A. Mitchell and D. G. Barker, pers. comm.). Most
of the snakes were located at the bases of small trees in piles of
twigs and leaf litter near streams; one was found in a shin-oak
association. The area had been logged three times in twelve years,
and the last logging had occurred four years earlier. All snakes
were visible, some coiled and others crawling, and only two rattled
when captured. Intemiittent thunderstorm activity had occurred
for two weeks prior to 18 July, and occasional showers occurred
during the collecting period. The ambient temperature was ca.
21 °C, and the snakes were found throughout the day. Two C. p.
pricei, one within 5 m of an C. iv. amabilis, and two C. lepidus
klauberi, one in scrub-oak and the other in a talus slope near shin-
oak, were also found. Two C. iv. amabilis defecated rodent hair in
the laboratory. R. K. Guese (pers. comm.) observed a captive male
courting a female on 8-9 September 1977. The female remained
in a resting coil and the male directed head-bobbing and tongue-
flicking across the female's dorsum (3-5/5 seconds). The male
moved his uplifted tail in both a horizontal and vertical plane with
an undulating motion. Physical contact with the side of the en-
closure or the substrate stimulated this behavior which we interpret
as Tail Search Copulatory Attempts. The sequence lasted 5 minutes.

Crotalus willardi meridionalis. The southernmost subspecies,
C. w. meridionalis, inhabits a limited area in southern Durango and

THE NATURAL HISTORY OF MEXICAN RATTLESNAKES 65

extreme southwestern Zacatecas, and few specimens are available
for study (Gloyd 1940; Klauber 1972). Like its sister races, C. iv.
meridionalis is a montane inhabitant rarely found below 2440 m.
This subspecies is not generally a canyon-bottom dweller. The
area inhabited by it is high (2440 m) pine-covered plateaus where
open meadows provide breaks in the forest. Five individuals ob-
tained by us from near Llano Grande, Durango (Fig. 23; UTA
R-5639-41, 6124-25), were collected on 4 August 1975 on a partially
open hillside which sloped into a grassy meadow. The slope had
a northern to northeastern exposure. All specimens were located
at ca. 2560 m at a temperature of 21-24°C between 0830 and 1000
hours. It had rained the previous evening and continued until
0600 when the sun appeared. The environment was humid when
the rattlesnakes were captured, and none was found after the high
clouds had disappeared. The habitat, although not a canyon, was
similar to that occupied by other races of C icillardi. It was com-
posed of sparse short-needled pines mixed with equally sparse oak
and madroiio. Rocks were scattered throughout the scrub-oak, and
grassy ground cover provided excellent retreats. All five specimens
were newborn young and had shed their skin at least once. They
exhibited gentle dispositions and refused to bite upon capture. An
adult male C. lo. meridionalis was found on 18 July 1976 at the
crest of a partially denuded hill with a low dense covering of
scrubby brush. It was basking in direct sunlight at 1100 hours. An
adult female, opaque-colored prior to shedding, was discovered at

Fig. 40. Crotalus willardi amabilis. Specimen from Arroyo Mesteno, Sierra
del Nido, Chihuahua, Mexico. ( Photograph by Jonathan A. Campbell. )

66

SPECIAL PUBLICATION-MUSEUM OF NATURAL HISTORY

1000 hours on 12 July 1977 during an intermittent rainstorm. Heavy
fog shrouded the area in the early morning. The ambient tempera-
ture was ca. 21°C, and the snake was resting in shade. Seven addi-
tional snakes (one preserved, KU 178975) were taken in the upper
regions of the mountains at a ranch near the city of Durango (S.
Prchal, pers. comm.).

This snake inhabits the Montane Formation Series as defined by
Wagner (1964).

Crotalus willardi silus. Crotalus to. silus (Fig. 41) is also a resi-
dent of the temperate, pine-oak forests in the Animas Mountains
of New Mexico (Bogert and Degenhardt 1961), the adjacent Sierra
de San Luis, the Sierra de la Purica (Sierra de Nacozari) in Sonora,
and the mountains to the immediate east and south of Chihuahua
(Fig. 42). Crotalus w. silus is very similar to C. w. willardi in its-
habitat preference. Snakes from the Animas and San Luis ranges,
however, exist in a much drier climate since these mountains re-
ceive less rainfall than mountains which support other C. tvillardi
populations. Three individuals observed by us in the Animas
Mountains were found in late October and early May when it was
very dry. All three specimens were located at an elevation of 2120
m on east-facing slopes where pines, scrub oak, and manzanita
comprised the major vegetation. None of the snakes rattled or
tried to escape, unlike other C iv. silus from humid habitats.

We observed approximately 40 C. w. silus between 1970 and
1974 in the Sierra de la Purica in northern Sonora (KU 155554-555,

Fig. 41. Crotalus tvillardi silus. Specimen from Sierra de Nacozari, Sonora,
Mexico. (Photograph by John H. Tashjian.)

THE NATURAL HISTORY OF MEXICAN RATTLESNAKES

67

158562, UTA R-6942). No other member of the genus has been
observed in this range, which contains perfect C. willardi habitat
consisting of long, large canyons with a south to north stream-flow
where water is present throughout the year. Pines and oaks domi-
nate the vegetation with sycamores in stands along the wide can-
yons. Ferns and poison oak provide ground cover in the rocky
stream beds, where C. w. silus is most prevalent. Snakes have been
seen from April through October, with the greatest numbers being
observed in the July-August rainy season. At this time, generally
cloudy skies and high humidity provide excellent conditions for
these snakes. On one morning in early July we observed eleven
specimens in a two hour period while hiking in a single canyon at
elevations of 1630 m to 2300 m. Most of the snakes appeared to
be foraging, since only one of the eleven was observed in a coiled,
basking position. Of the four specimens collected that morning,
one gravid female gave birth to five viable young on 7 August 1970.
Another contained four fully developed embryos.

Ten females and one male were observed in the Sierra de la
Purica between 15-23 July 1978. One pair was copulating on 15
July, and two adult females appeared as though they had just given
birth. Another female gave birth to four young (KU 179025) on
10 August 1978. Total length of the four ranged from 146-218 mm,
mean 190; S-V 129-193 mm, mean 168; weight 4.9-6.9 g, mean 6.0.

Fig. 42. Sierra de la Purica (Sierra de Nacozari), Sonora, Mexico. Habitat
of Crotalus willardi silus. Canyon bottoms run south to north, pine-oak forest
with ferns, poison oak, sycamore trees. Aspens in upper regions. (Photograph
by R. Terry Basey. )

68 SPECIAL PUBLICATION-MUSEUM OF NATURAL HISTORY

On 17 April 1971, captive breeding was observed. The female
was placed with the male at 0800 hours. The male appeared to
recognize the presence of tlie female within 10 seconds, began
twitching (1 twitch per second), and tested the entire dorsal surface
of female's body with his tongue (2 tongue-flicks per second). The
male rubbed along the dorsal surface of the female for approxi-
mately 5 minutes by holding his head at 30° angle and sliding the
area of the mental scale forward for 1 cm. Other mating dates
recorded were 19 April 1971 and 29 January 1973. The matings
were unsuccessful.

An interesting observ^ation was made by us concerning the
brown coloration of individuals of C. w. silus from the Sierra de
la Purica. Those specimens collected or observed above the water-
falls (2070 m) tended to be light brown, resembling the dominant
pine needle litter. Other specimens from below the waterfalls
tended to be a much darker brown, resembling the more dominant
oak leaf litter. Specimen size was apparently not a factor in this
difference in color.

Sistrurus ravus (Cope)

Sistrurus ravus is a resident of the tableland of south-central
Mexico. It occurs in a variety of habitats within that tableland,
ranging from nearly 3050 m in pine-oak forest near Huitzilac,
Morelos, to about 1525 m in agave-cacti habitat of the valleys
which surround the city of Oaxaca in central Oaxaca. Specimens
have been recorded by us from: PUEBLA: Zacapoaxtla, Cacaloa-
pan, Puebla (J. A. Campbell, pers. comm.); TLAXCALA: near San
Dionisio; DISTRITO FEDERAL: La Cima; MORELOS: Huitzilac,
the lava beds north of Cuemavaca; OAXACA: near Mitla, Ixtlan
de Juarez.

Within its known range, S. ravus is a very common rattlesnake.
It seems to be rather gregarious, at least in mountain valley habitats.
We have observed over 100 S. ravus under many varied conditions,
and it appears to be a very adaptable species. For example, near
Huitzilac, Morelos, a large forested area had been cleared, perhaps
30 years ago, and now consists of tall bunch grass (Fig. 43). Sistrurus
ravus has occupied this area and has become so common that the
local people are extremely aware of its presence. The population
from this particular area (at an elevation of 2866 m) contains the
largest examples of this snake of all the localities where we have
collected them. Several males measured 76 cm and engaged in
ritualized combat in captivity. Many of these males tend to be
melanistic, a characteristic that Klauber (1952: 114) noted in these
snakes from Tres Cumbres, Morelos.

Although S. ravus seems to prefer grassy mountain valleys, it

THE NATURAL HISTORY OF MEXICAN RATTLESNAKES

69

is not restricted to that habitat. It is common in the lava beds north
of Cuemavaca, Morelos, where it is sympatric with C. t. triseriatus
and C. molossus nigrescens. S. ravus from this area are much smaller
than the Huitzilac specimens. In parts of Puebla, Veracruz, and
adjacent Oaxaca, S. ravus is found at lower elevations in areas of
grass, agaves, cacti, and small, stunted oaks (Fig. 30). Individuals
in this habitat appear to be more dispersed and were encountered
less frequently.

In the spring of 1974, Jonathan A. Campbell and one of us (BLA)
located an unreported disjunct population of Sistrurus in the central
highlands of Guerrero. This population is removed some 330 air
kilometers from the range of S. ravus as plotted by Klauber (1956:
58; 1972: 64), and is isolated from other populations by the Rio
Balsas basin to the north and the Chilpancingo gap to the east.
A complete description of this Guerreran population is forthcom-
ing (Campbell and Armstrong, 1979). However, we think it proper
to describe here the habitat and habits of these snakes. They
are residents of tlie pine-oak forests of the Sierra Madre del Sur,
and all of our specimens were secured in large, brushy flood plains
at various elevations between 2975-3105 m. The first snakes were
secured in mid-May during the warmer parts of the day. This re-
gion receives ca. 1500 mm of rainfall from May through October
(Page 1930). According to the local inhabitants, frost does not occur
at these elevations in the Sierra Madre del Sur, a large mountain

Fig. 43. Huitzilac, Morelos, Mexico. Habitat of Sistrurus ravus. Snakes
extremely abundant in bunchgrass meadows bordered by fir-pine-oak forest,
(Photograph by M. Granger.)

70 SPECIAL PUBLICATION-MUSEUM OF NATURAL HISTORY

range that is rugged and steep, and would seem to offer little
suitable habitat for grassland dwellers such as rattlesnakes of the
genus Sistrurus.

The habits of Guerreran S. ravus are comparable to those from
the southern central plateau. They are alert and quick to retreat
when encountered, but will stand their ground when escape is im-
possible. Like most Sistrurus, they are more apt to turn and bite
rather than coil and strike. Stools of some specimens contained
rodent hair.

A female of this species from the vicinity of Huitzilac, Morelos,
gave birth to seven young on 1 June 1974. Two other females
from the same locality gave birth on 28 May 1975 to nine and three
young respectively. The range of variation of the three broods is
as follows: total length 160-208 mm, mean 183; S-V 138-192 mm,
mean 166; weight 3.9-7.4 g, mean 5.4. A captive pair from Huitzilac
was observed in copulo on 1 May 1976.

This rattlesnake inhabits the Temperate Pine-Oak Forest as de-
fined by Leopold (1950).

DISCUSSION

The following observations and discussion present our views on
various aspects of the biology of Mexican rattlesnakes, and indicate
areas where research is needed concerning the ecology and be-
havior of these animals.

Habitat. Most montane species from Mexico were closely asso-
ciated with rocky areas and such microhabitats allowed maximum
protection. Crotalus I. lepidus, C. I. klauberi, C. p. pricei, and C. m.
molossus were often located in talus slopes. Other montane rattle-
snakes were found in pine-oak forests, mesquite grasslands and
highland deserts but usually rocky outcroppings were the dominant
feature. Crotalus transversus occurred in areas of volcanic rock,
whereas C. pricei miquihuanus was found in situations with lime-
stone bedrock. Some of the lowland taxa such as C. durissus culmi-
natus inhabited arid tropical scrub forest with rocky outcroppings.
Rattlesnakes were rarely found in cloud forests.

When first discovered, smaller montane taxa such as C inter-
medius, C. lepidus, C. pricei, C. pusillus, C. transversus, and C.
willardi would often rattle and quickly crawl to safety within rocks.
Many of these rattlesnake taxa seemed to utilize specific retreats,
for in some cases the snakes crawled toward the collector in their
endeavors to escape, when suitable retreats were available in the
opposite direction.

Our field experience indicated that Mexican rattlesnakes from
higher elevations were most often encountered in microhabitats
with a southern exposure (C. i. interrnedius, C. lepidus klauberi,
C. I. moTulus, C. m. molossus, C. p. pricei, C. transversus). A few

THE NATURAL HISTORY OF MEXICAN RATTLESNAKES 71

taxa that appeared to vary from this mode were more frequently
found in microhabitats with southeastern (C. lepidus maculosus,
C. m. molossus, C. viridis helleri), eastern (C. pricei miquihuanus,
C. w. willardi, C. to. silus), northern and northeastern (C. willardi
meridionalis), and western exposures (C. intermedius gloijdi).

Nearly all montane rattlesnakes were found above a 1800 m
contour. Lowland taxa which invade the Mexican highlands in-
cluded C. durissus, C. molossus, and C. hasiliscus. Crotalus molos-
sus and C. scutulatus inhabited lower elevations in the northern
part of the range. A narrow vertical distribution at relatively low
elevation characterized the habitat of C. stejnegeri (and perhaps
C. lannoTni). Sistrurus ravus avoided steep hillsides, but was found
on gentle slopes or in valleys. It was not necessarily associated
with rocky outcroppings, and was found also in lava beds, bunch
grass bordered by fir forests, pine-oak forests, and areas of grass,
agave or cactus.

Degree of slope inclination delineated the habitat of some Mexi-
can rattlesnakes. Crotalus pricei, C. pusillus, C. intermedius, C.
transversus and C. lepidus often inhabited extremely steep slopes
whereas C. willardi was found in mountain valleys, usually with
permanent water. Crotalus polystictus and Sistrurus ravus preferred
relatively flat areas.

Ground cover is evidently a significant characteristic in the
habitat of some Mexican rattlesnake taxa, and varied from light
(C. polystictus) to intermediate (C. pusillus, C. triseriatus, S. ravus).
Crotalus transversus and C. triseriatus (in some areas) inhabited
relatively open situations with a heavy canopy.

Daily activity. Daily emergence of Mexican rattlesnakes from
retreats, particularly montane taxa, was associated with diffused
sunlight and high atmospheric humidity. Immediately following
thunderstonn activity, rattlesnakes began to emerge and were
often encountered basking on rocks which were still moist. In
general, highland forms tended to be diurnal and the lowland
taxa, especially during summer months, were nocturnal. The fol-
lowing Mexican rattlesnake taxa were found abroad by day: C. b.
hasiliscus, C. h. oaxacus, C. cerastes, C. durissus tzabcan, C. i. inter-
medius, C. i. gloydi, C. i. omiltemanus, C. I. lepidus, C. I. klauheri,
C. I. maculosus, C. I. morulus, C. mitchelli muertensis, C. m. molos-
sus, C. m. nigrescens, C. polystictus, C. p. pricei, C. p. miquihuanus,
C. pusillus, C. r. ruber, C. s. scutulatus, C. s. salvini, C. stejnegeri,
C. tortugensis, C. transversus, C. t. triseriatus, C. t. aquilus, C.
viridis lielleri, C. w. willardi, C. w. amabilis, C. w. meridionalis,
C. w. silus, and Sistrurus ravus. The following Mexican rattlesnake
taxa were found as they were prowling by night: C. atrox, C.
cerastes, C. d. durissus, C. d. culminatus, C. d. totonacus, C. e. enyo,
C. e. furvus, C. I. lepidus, C. m. mitchelli, C. m. pyrrhus, C. m.

72 SPECIAL PUBLICATION-MUSEUM OF NATURAL HISTORY

molossus, C. polijstictus, C. pricei miquihuanus, C. r. ruber, C. r.
lucasensis, C. s. scutulatus, C. tigris, and C. triseriatus aquilus.

Adult snakes of three taxa (C. intermedius omiltemanus, C. I.
lepidus, C. t. triseriatus) remained in the same location for a period
of over two weeks and utilized the same rock for basking daily.
Juveniles of these taxa were encountered prowling by night, per-
haps to avoid predation while dispersing.

During the rainy season, pairs of C. intermedius oiniltemanus
were encountered in the field.

Seasonal Occurrence. The onset of the rainy season in Mexico
stimulates rattlesnake seasonal activity. For example, 55 speci-
mens of Crotalus polijstictus from southwestern Jalisco were ob-
served during a three day period in the rainy season, whereas only
three snakes were seen during a comparable period in May before
the beginning of the rains. Seventeen specimens of C. lepidus
morulus were found during one day after the first major rain near
Galeana in the Sierra Madre Oriental; only three individuals were
seen during June. The seasonal activity of most other Mexican rattlfe-
snake taxa corresponded to the pattern noted above. Exceptions
included C. I. lepidus and C. I. klauheri which were found through-
out the year if the temperature was warm. These two taxa have
adapted to a more arid environment.

Faunal and Climatic Assemblages. Kendeigh (1954) and Duell-
man (1965) reviewed methods used in interpreting faunal assem-
blages and discussed the limitations inherent in these concepts. The
Life-Zone Concept of Merriam (1890, 1894) was envisioned as a
latitudinal transcontinental belt or vertical zonation belt in moun-
tainous areas which incorporated faunal and floral information
with additional climatic data, particularly temperature. The Life-
Zone Concept was used by Goldman (1951) and applied to the
zoogeographic characteristics of Mexico in order to evaluate the
distribution of birds, mammals, and the flora community. Stuart
(1964) felt that Goldman's summary tended to reflect the Biotic
Province Concept. The Biotic Province (Vestal 1914; Dice 1943,
1952) was defined by Duellman (1965) as "... a considerable and
continuous geographic area that is characterized by the occurrence
of one or more important ecological associations that differ, at least
in proportional area covered, from the associations of adjacent
provinces." Smith (1939), utilizing the lizard genus Sceloporus, at-
tempted to map biotic provinces in Mexico. Later (1940, 1949) he
modified his position concerning the application of the biotic prov-
ince concept (see also Peters 1955). Smith (1960) noted, ". . . ex-
tensive agreement of range limits of numerous species is a neces-
sary assumption in acceptance of the validity of the biotic province
concept." It is unlikely that any two plant or animal species are
affected identically by all environmental components; consequently,

THE NATURAL HISTORY OF MEXICAN RATTLESNAKES 73

no two species or subspecies can be expected to have identical
ranges.

Bearing these difficulties in mind, we attempted to delineate
the patterns of distribution of 42 taxa of Mexican rattlesnakes by
consulting two faunal and two climatic assemblages: Life-Zone
Concept of Goldman (1951), Biotic Province Concept (Stuart 1964),
Temperature and Rainfall Provinces (Page 1930), and the Koeppen
climatic classification system. The last system, outlined by Koeppen
(1900, 1948), Koeppen and Geiger (19'30-39) and summarized by
Vivo (1964), roughly delineates the climatic types and utilizes data
from annual and monthly averages of temperature and precipita-
tion, including the variable effect upon plant growth through
evaporation.

Frequency of occurrence for distributional overlap is presented
in Table 1. Two taxa (C. atrox, C. molossus nigrescens) occurred
in six Koeppen provinces. Crotalus atrox and C. d. durissus in-
habited six and five biotic provinces, respectively. Four Life-Zone
Provinces were applicable to C. t. triseriatus. Crotalus molossus
nigrescens and C. s. scutulatus occurred in five rainfall provinces,
and C. atrox inhabited five temperature provinces.

The validity of physiographic, climatic or biotic provinces, at
least when used to explain the distribution of rattlesnakes in terrain
as complex physiographically as Mexico, is highly tenuous. The
discrepancies failed to produce an explanation of distribution that
can be considered to be either valid or useful. Hence, an analysis
of the ecological requirements of rattlesnakes must be concerned
primarily with the microhabitat or habitat niche (Dammann 1961).

Food. Our field experience suggested that the occurrence of
montane rattlesnakes in a given habitat was related to the avail-
ability of members of the lizard genus Sceloporus. The density of
rattlesnake populations was considerably greater when an area
supported a large number of lizards, especially species of Sceloporus
which are largely saxicolous. We suspect that the feeding patterns
of the smaller montane rattlesnakes are correlated with the diurnal

Table 1. Frequency of occurrence for distributional overlap in 42 taxa of
Mexican rattlesnakes according to biotic or climatic classifications. Percentage
followed by number of taxa in parentheses.

Number of Biotic or Climatic Provinces In

Which Taxa

Occur

Classification Sijstem

1

2 3

4 5

6

Temperature (Page 1930)

67(28)

24(10) 5(2)

0(0) 5(2)

0(0)

Rainfall (Page 1930)

50(21)

19( 8 ) 21(9)

5(2) 5(2)

0(0)

Life-Zone (Goldman 1951)

74(31)

19(8) 5(2)

2(1) 0(0)

0(0)

Biotic Province (Stuart 1964)

52(22)

19( 8 ) 21(9)

2(1) 2(1)

2(1)

Climate (Koeppen 1900, 1948)

33(14)

33(14) 14(6)

12(5) 0(0)

7(3)

74 SPECIAL PUBLICATION-MUSEUM OF NATURAL HISTORY

activity patterns of various species of Sceloporus. On one occasion
in the early afternoon at Llano Grande, Durango, a Sceloporus
poinsetti darted from a clump of rocks and died a few minutes later.
After five minutes, an adult C. p. pricei emerged, trailed the lizard
and consumed it.

Captive C. pricei, C. intermedins, C. lepidus, C. triseriatus, and
C. willardi showed a marked preference for lizards rather than
rodents. The snakes usually struck the lizards in the thoracic re-
gion and held onto them until movement ceased. When rodents
were struck, they were, in most cases, released. Captive juvenile
examples of the larger lowland rattlesnake taxa preferred appro-
priate sized laboratory mice.

Ritualized Male Combat. Crotahis atrox, C. intermedins gloydi,
C. i. omiltemanus, C. lepidus klauberi, C. pusillus, C. t. triseriatus,
C. t. aquilus, and Sistrurus ravus engaged in ritualized male com-
bat in captivity. A detailed description has been published for
C. lepidus (Carpenter et ah 1976) and a comparative analysis of
the other taxa will be forthcoming (manuscript in prep.).

Courtship and Mating Behavior. The following Mexican rattle-
snakes were observ^ed engaging in courtship and/ or mating be-
havior in captivity: Crotalus atrox, C. catalinensis, C. durissus cul-
minatus, C. d. tzahcan, C. durissus ssp., C. e. enijo, C. intermedins
omiltemanus, C. I. lepidus, C. I. klmiheri, C. I. morulus, C. m.
mitclielli, C. in. muertensis, C. molossus nigrescens, C. p. pricei,
C. pusillus, C. t. triseriatus, C. t. aquilus, C. w. loillardi, C. w.
amahilis, C. w. silus, and Sistrurus ravus.

The general pattern of courtship and copulation is summarized
in the following account. Shedding by the female often stimulated
male courtship activity and the female frequently defecated, pre-
sumably to evacuate the cloaca. The male directed head-bobbing
and tongue-flicking sequences upon the head and dorsum of the
female. If the female was in a resting coil, the male might vigor-
ously press his lower jaw against her body in order to stimulate
her to crawl. This action allowed for alignment of the bodies. The
head-neck angle of the male varied between 30-45° and the male's
mental region was pressed on the female's dorsum. The dorsal ad-
vance movements by the male, either sagittally, para-sagittally or
laterally, included a head-bobbing motion which varied interspe-
cifically from no lateral movement to a pronounced side-to-side
motion. Female C. atrox assumed a raised head and anterior trunk
stance on occasion; possibly this was a rejection posture (manuscript
in prep.). As the male directed courtship activities toward the fe-
male, she raised her tail and gaped the cloaca (as in C. atrox, C. d.
durissus, C. pusillus) which often stimulated the male. The male en-
circled the female's tail with his tail and rapidly slid his caudal con-
figuration in anteriorly and posteriorly directed movements (as in C.

THE NATURAL HISTORY OF MEXICAN RATTLESNAKES 75

durissus tzabcan, C. d. durissus, C. e. enijo, C. pusiUus). This be-
havior has been termed the "stroke-cycle" (Chiszar et al. 1976).
The male draped large-radius coils over the female's body and at-
tempted to place the cloacae in apposition by sliding his tail under
the tail of the female. The male might push backwards against
the resistance of the female's tail as he inserted (as in C. d. durissus,
C. pusillus). Only one hemipenis was inserted, and eversion of
the hemipenis prior to insertion was never observed. The tactile-
chase and tactile-alignment patterns varied from 4 minutes in
C willardi to over 3 hours in C. pusiUus. The range of variation
might be due to female receptivity. Coitus varied from 2 hours
in C. atrox to 9 hours in C. w. willardi. Jerking and twitching gen-
erally ceased when intromission occurred, but often the male
maintained a steady pulsation of the body near the tail (as in C.
atrox, C. pusillus). The female might drag the male backwards
by the attached hemipenis (observed in C. atrox, C. d. durissus)
until separation occurred.

Reproduction. The rainy season in Mexico usually occurs be-
tween May and October, and the dry season between December
and April. In parts of southern Mexico, a short dry season occurs
during the summer rainy period. During late June and early July,
the rainy season normally begins in northern Mexico. In late July
and September, the rainy season starts in central and southern
Mexico. The northwest area of Baja California receives winter
rains whereas the southern section receives summer rains.

In compiling literature references and our observations on
parturition in wild Mexican rattlesnakes, we accumulated the fol-
lowing data on broods: 11 litters were born in June, 12 in July,
16 in August, 2 in September, 1 in October, and 1 in January.
Compiled obser\^ations on our captive individuals revealed 5 litters
born in June, 4 in July, 8 in August, 4 in September, and 1 in
October. Data on these rattlesnake taxa are listed in Table 2.
Parturition in Mexican rattlesnakes studied to date coincides with
the rainy season, possibly due to a greater abundance of potential
food items available at this time.

Warning Mechanisms and Defensive Actions. In addition to
the use of the rattle as a warning mechanism and the striking coil,
a number of other defensive behaviors were observed by us in
Mexican rattlesnakes. When seized by collecting tongs, many of
the smaller montane taxa turned and bit in lieu of assuming a
striking coil. Some, especially C. inter77iedius, began spinning on
a longitudinal axis. Adult C durissus tzabcan raised the anterior
portion of the body in an almost vertical column with a lateral
bend in the neck, and faced the intiiider. Captive juvenile C. d.
tzabcan flattened and rotated their head and cervical region as
much as 90° so that the dorsal aspect, with the contrasting pattern

76 SPECIAL PUBLICATION-MUSEUM OF NATURAL HISTORY

on the head and the dark paravertebral neck stripes, was visible.
Young captive C. durissiis totonacus inflated their trunk, exhibited
convulsive movements and, on rare occasions, flipped over upon
the dorsum when confronted by a human observer. Crotahis poly-
stictus usually thrashed wildly with pronounced lateral undulations
when handled; this snake sometimes returned to a striking coil
when cornered. An adult C. s. scutulatus and some examples of
Sistrurus ravus repeatedly flattened their heads and trunks on the
substrate, then lifted the tails vertically and waved them slowly
in response to the presence of humans. The snakes did not attempt
to rattle. Some of the smaller rattlesnakes responded to the pres-
ence of live mice and rats by launching into a striking coil, then
struck the rodents with closed mouth. Rattlesnakes reacted to
kingsnakes and other ophiophagous snakes by forming pronounced
body-bridges which were used to strike the predators. Dermal
substances of ophiophagus snakes elicited body-bridging responses
from a number of Mexican rattlesnake taxa (Weldon and Burghardt,
1979). Crotalus tortugensis, when touched, used vertical body-
bridges or lateral flexures to strike an annoying object. Frequently,
the force was great enough to knock freshly killed rodents from
the feeding forceps.

Crotalus stejnegeri vertically elevated its tail and a portion of
the posterior trunk and held them rigidly as it assumed a striking
coil. Although Klauber (1956) was unaware of an open mouth

Table 2. Data on rattlesnake litters with viable young showing number of
young, length of young, and weight of young. Numbers in parentheses are

Number

Total length

Weight

Taxon

of young

(mm)

(g)

c.

atrox

6-25(14.3)

—

-

c.

durissus ssp.

10

—

-

c.

d. tzabcan

21

290-350(316)

18.4-26.8(23.4)

c.

e. furvus

7

206-222(213)

-

c.

intermedius omiltemanus 5

194-212(205)

4.9- 5.4( 5.2)

c.

/. lepidus

6

165-190(177)

5.0- 5.7( 5.2)

c.

I. klauberi

3-5(4)

181-196(191)

6.3- 7.0( 6.6)

c.

I. morulus

4

-

-

c.

m. mitchelli

1

275

17.5

c.

771. 77iuert crisis

4

143-179(167)

3.3- 7.0( 4.9)

c.

molossus nigresce7\s

5

291-316(304)

25.4-27.4(26.6)

c.

polystictus

5-12(8)

155-287(214)

2.4-11.5( 8.9)

c.

p. pricei

3- 9(5.3)

152-185(164)

2.7- 5.6( 4.1)

c.

p. 7i}iquihuanus

4- 5(4.5)

130-143(135)

2.6( 2.6)

c.

pusilhis

3- 5(4)

165-179(171)

.3.0- 6.1 ( 4.0)

c.

t. triseriatus

4- 6(5)

159-178(168)

4.8- 5.1( 4.9)

c.

t. aquilus

2- 7(4.7)

120-192(168)

4.9- 8.8( 6.1)

c.

willardi silus

4- 5(4.5)

14^218(190)

4.9- 6.9 ( 6.0)

Sistrurus ravus

3- 9(6.3)

160-208(183)

3.9- 7.4( 5.4)

THE NATURAL HISTORY OF MEXICAN RATTLESNAKES 77

threatening posture in rattlesnakes, three C. molossus nigrescens
obsei'ved by us repeatedly exhibited this behavior for periods as
long as five minutes. One C. polystictus also assumed this posture.
Crotalus atrox, C. lepidus, C. molossus, C. scutulatus, and Sis-
triirus ravus emitted musk upon capture. An adult C. durissus dis-
charged a stream of musk while being restrained.

RESUMEN

Se presenta informacion sobre la ecologia e historia natural de
45 taxa de cascabeles mejicanos, generos Crotalus y Sistrurus.

Observaciones de la fisiografia, geologia, clima, y la vegetacion
de Mexico son registradas en lo que se refiere a los cascabeles de
esa region. Datos de observaciones tomadas directas de poblaciones
naturales y ejemplares captivos son listados, incluyendo los predilec-
ciones de habitacion, extenciones de area y elevacion, reproducion,
periodos de actividad, y comportamiento.

La mayoria de los cascabeles monteses habitan bosques de
pino y encino, llanos cubierto de hierba, y desiertos de altura.
Generalmente los afloramientos rocosos son un factor determinante.
Algunas taxa de tierra baja se encuentran en bosques tropicales
aridos con afloramientos rocosos. Refugios especificos son a menudo
utilizados por cascabeles monteses. Microhabitaciones con orien-
tacion hacia el sur son habitadas con mas frequencia por cascabeles
de los altos, sequido por el sudeste, este, norte, noreste, y oeste
en el orden de preferencia. Casi todas las taxa monteses se
encuentran a 1800 metres o mas de altura. Una distribucion
limitada en elevaciones relativamente bajas caracterizan la habi-
tacion de Crotalus stejnegeri (y talvez C. lannomi). Sistrurus ravus
es una excepcion ya que este se encuentre en asociaciones variadas.
El grado de inclinacion de la falda es importante. La densidad de
la vegetacion va de leve a moderado.

Rayos del sol difusos y alta humidad atmosferico contribuyen
a la actividad diaria, especialmente en las culebras que habitan
regiones montaiiosas. Especies de tierras altas tienden a ser diumas
y las de tierras bajas noctumas, especialmente en el verano. Treinta
dos taxa fueron encontrados activas durante el dia y diez y ocho
fueron cazadas de noche. Serpientes adultos (C. intermedius, C.
lepidus, C. triseriatus) pueden permanecer en el mismo lugar por
lo menos dos semanas y utilizar la misma pena para tomar el sol.
Ejemplares juveniles de estas taxa fueron encontradas de noche.

La actividad de cascabeles en Mexico depende de la epoca
lluviosa. Pocas culebras se encuentran durante la epoca seca.

Los patrones de distribucion de cascabeles mejicanos no se
explican por uso de asambleas faunales ni climaticas. La frecuencia
de traslape distribucional involucraben tanto como seis provincias

78 SPECIAL PUBLICATION-MUSEUM OF NATURAL HISTORY

faunales o climaticos para una forma de cascabel. Estas discrepen-
cias sugieren que los requisites ecologicos de los cascabeles tienen
que ser primariamente evaluados con un analisis de microhabitacion
o nicho habitacional.

La ocurrencia de las culebras montaneses se relaciona, en
parte, a la disponabilidad de especies de Sceloporus. Muchas de
las culebras pequenas de altura muestran una decidida preferencia
por lagartijas como comida y las cascabeles jovenes de tierra baja
prefieren roedores pequefios.

Combate ritualizado entre machos fue observado en C. atrox,
C. intermedius gloydi, C. i. omiltemanus, C. lepidus klauheri, C.
pusillus, C. t. triseriatus, C. t. aquilus y Sistrurus ravus.

Los aspectos de cortejo y/o brama fueron registrados en
diecinueve distintas cascabeles captives. Un patron generalizado
de comportamiento fue aparente y esta descrito.

Parturacion en los cascabeles mejicanos estudiados hasta la
fecha coincide con la epoca lluviosa. Informacion reproductiva de
19 taxa es presentada.

Comportamiento de advertencia, adicional al enrollamiento,
ataque, y resonamiento, incluye lo siguiente: torneando y mor-
diendo, rotando en su axis longitudinal, rotacion de la cabeza y
cuello, elevacion vertical del cuerpo anterior, inHacion del cuerpo,
movimientos convulsos, aplastamiento en el suelo y undulacion
lente de la cola, lanzando con boca cerrada, amenanza con boca
abierta, elevacion vertical rigida de la cola y emision de almizcle.

APPENDIX

Range or altitude extensions for Mexican rattlesnake taxa reported

in text.

Crotalus b. hasiliscus MICHOACAN: near Dos

Aguas in the Sierra de
Coalcoman (2225 m)

Crotalus hasiliscus oaxacus OAXACA: 35.5 km NW

Telixtlahuaca

Crotalus durissus culminatus MICHOACAN: 7 km W

Morelia

Crotalus durissus totonacus TAMAULIPAS: 20.8-48 km N

Soto la Marina

Crotalus i. intermedius PUEBLA: near Cacaloapan

Crotalus intermedius gloydi OAXACA: near Cerro Machin

(Sierra de Juarez), Suchixtepec
(Sierra de Miahuatlan)

THE NATURAL HISTORY OF MEXICAN RATTLESNAKES 79

Crotalus intermedius omiltemanus GUERRERO: 1.6 km SW Filo

de Caballo, San Vicente, 1 km
N Puerto del Gallo

Crotalus lepidus morulus NUEVO LEoN: 24-32 km NW

Galeana

Crotalus stejnegeri SINALOA: Ejido Tebaira

Crotalus triseriatus aquilus HIDALGO: El Chico (3110 m)

Sistrurus ravus GUERRERO: mountains of

Central Guerrero

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1966. Ecological relationships of rattlesnakes in southeastern Arizona
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1974. Squeeze box technique for measuring snakes. Herp. Review,
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1975. A new subspecies of the red rattlesnake, Crotalus ruber, from
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1971. Notes on a small herpetological collection from northeastern
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1960. Evolution of a peninsular herpetofauna. Syst. Zool., 9:184-212.
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1946. Informe preliminar sobre algunos estudios fitogeograficos efectuados

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THE NATURAL HISTORY OF MEXICAN RATTLESNAKES 85

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INDEX TO HERPETOLOGICAL SCIENTIFIC NAMES

PAGE

Agkistrodon piscivoriis 37

Anolis 56

Bothrops barbouri 22

Bothrops undulatus 22

Cnemidophorus 10, 56

Crotalus 6,12, 56, 77

atrox 4, 71, 73, 74, 75, 76, 77, 78

basiliscus -— _ 4, 6, 31, 71

basilisciis basiliscus 6, 7, 8, 43, 71, 78

basiliscus oaxacus 6, 8, 71, 78

catalinensis 8, 74

cerastes 9, 10, 49, 71

cerastes cercobombus 9, 10

cerastes laterorepens 9

durissus 10, 12, 13, 14, 15, 71, 73, 74, 75, 76, 77

durissus culminatus 7, 11, 12, 14, 43, 70, 71, 74, 78

durissus curnanensis 11

durissus durissus 10, 11, 14, 71, 73, 74, 75

durissus totonacus 10, 12, 13, 14, 71, 76, 78

durissus tzabcan 14, 15, 16, 71, 74, 75, 76

enyo - - 16

enyo enyo 16, 17, 30, 71, 74, 75

enyo furvus 17, 18, 63, 71, 76

intermedins — - 18, 70, 71, 74, 75, 77

intermedins gloydi 8, 19, 20, 21, 71, 74, 78

intermedius intermedins — . 18, 19, 70, 71, 78

intermedius omiltemanus 20, 22, 23, 71, 72, 74, 75, 76, 78, 79

lannomi 71, 77

lepidus 22, 26, 29, 33, 39, 43, 59, 61, 63, 64, 70, 71, 74, 77

THE NATURAL HISTORY OF MEXICAN RATTLESNAKES 87

PAGE

lepidus klauberi 22, 24, 25, 26, 32, 64, 70, 71, 72, 74, 75, 76, 78

lepidus lepidus _... 22, 24, 25, 26, 70, 71, 72, 74, 75, 76

lepidus maculosus 7, 22, 25, 26, 27, 71

lepidus morulus 22, 24, 25, 26, 27, 28, 29, 70, 71, 72, 74, 76, 79

mitchelli 29, 31

mitchelli mitchelli 17, 29, 30, 71, 74, 76

mitchelli muertensis 31, 71, 74, 76

mitchelli ptjrrhus 10, 30, 31, 62, 63, 71

molossus 6, 8, 31, 32, 39, 48, 71, 77

molosstis estehanensis 32

molossus molossus 32, 33, 70, 71, 72

molossus nigrescens 6, 26, 27, 32, 33, 34, 69, 71, 73, 74, 76, 77

polystictus __ 10, 34, 35, 36, 37, 71, 72, 76, 77

pricei 33, 38, 41, 63, 64, 70, 71, 74

pricei pricei 26, 27, 38, 39, 40, 42, 64, 70, 71, 74, 76

pricei miquihuanus 40, 41, 42, 43, 70, 71, 72, 76

pusillus 7, 43, 44, 45, 46, 70, 71, 74, 75, 76, 78

ruber 46, 47, 61

ruber lorenzoensis 47

ruber lucasensis 8, 16, 30, 47, 48, 71, 72

ruber ruber 47, 48, 62, 63, 71, 72

scutidatus 4, 48, 71, 77

scutulatus salvini 49, 50, 51, 71

scutulatus scutulatus 48, 49, 50, 71, 72, 73, 76

stejnegeri 7, 50, 52, 71, 76, 77, 79

tigris 16, 50, 52, 53, 71, 72

tortugensis 53, 71, 76

transversus 53, 54, 55, 56, 70, 71

triseriatus 1, 22, 56, 71, 74, 77

triseriatus aquilus 56, 58, 59, 60, 61, 71, 72, 74, 76, 78, 79

triseriatus triseriatus 43, 53, 55, 56, 57, 58, 59, 61, 69

71, 72, 73, 74, 76, 78

viridis ._ 61, 62

viridis caliginus 61

viridis helleri 61, 62, 63, 71

willardi 10, 39, 63, 64, 65, 66, 67, 70, 71, 74, 75

willardi amabilis 64, 65, 71, 74

willardi meridionalis _ 26, 39, 64, 65, 71

willardi silus 17, 66, 67, 68, 71, 74, 76

willardi willardi 63, 64, 65, 66, 71, 74, 75

Crotaphytus 10

Dipsosaurus 10

Gerrhonotus 56

Htjla 56

Lampropeltis triangulum arcifera 37

Lichanura trivirgata roseofusca 37

Nerodia 56

Phyllodactylus 56

Sceloporus 22, 26, 42, 56, 72, 73, 74, 78

aeneus 56

jarrovi 63

poinsetti 39, 74

undulatus 63

Sistrurus ._ 69, 70, 77

ravus 51, 57, 68, 69, 70, 72, 74, 76, 77, 78, 79

Streptosaurus mearnsi 31

88 SPECIAL PUBLICATION-MUSEUM OF NATURAL HISTORY

PAGE

Thamnophis 56

Uma - 10

Uta 10, 56

stansbu riana 3 1

Xantusia 56

Date Due

AUG 4 4 1982

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3 2044 062 358 262

States of Mexico

1. Aguascalientes

2. Baja California del Norte

3. Baja California del Sur

4. Campeche

5. Chiapas

6. Chihuahua

7. Coahuila

8. Colima

9. Distrito Federal

10. Durango

11. Guanajuato

12. Guerrero

13. Hidalgo

14. Jalisco

15. Mexico

16. Michoacan

17. Morelos

18. Nayarit

19. Nuevo Leon

20. Oaxaca

21. Puebla

22. Queretaro

23. Quintana Roo

24. San Luis Potosi

25. Sinaloa

26. Sonora

27. Tabasco

28. Tamaulipas

29. Tlaxcala

30. Veracruz

31. Yucatan

32. Zacatecas