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Observations on the
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JON C. BARLOW

LIFE SCIENCES CONTRIBUTIONS
ROYAL ONTARIO MUSEUM

NUMBER 75

Observations on the Biology
of Rodents in Uruguay

Publication date: 30 December 1969
Suggested citation: Life Sci. Contr., R. Ont, Mus,

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LAMBDA TI
Fey ry fh
bob bd i li il
\ A z r

JON C. BARLOW is Curator of the Department of Ornithology, Royal Ontario
Museum, and Associate Professor of Zoology, University of Toronto.

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Contents

Acknowledgments, J
Abstract, 3
Introduction, 4
Methods, 5

Species Accounts, 9
Oryzomys delticola Thomas, 10
Oryzomys flavescens (Waterhouse), 14
Thomasomys oenax Thomas, 16
Akodon azarae azarae (Fischer), 18
Akodon obscurus obscurus (Waterhouse), 22
Oxymycterus rufus nasutus (Waterhouse), 24
Scapteromys tumidus (Waterhouse), 27
Calomys laucha laucha (Desmarest), 34
Reithrodon physodes physodes Olfers, 35
Holochilus brasiliensis vulpinus (Brants), 39
Holochilus magnus Hershkovitz, 41
Mus musculus ssp. Linnaeus, 42
Cavia pamparum Thomas, 43
Hydrochoerus hydrochaeris uruguayensis (C. Ameghino

and Rovereto), 44

Myocastor coypus bonariensis (Commerson), 47
Ctenomys torquatus Lichtenstein, 48

Concluding Remarks, 55
Literature Cited, 57

Digitized by the Internet Archive
in 2011 with funding from
University of Toronto

http://www.archive.org/details/observationsonbi0Obar!

Acknowledgments

Many people have contributed in a variety of ways to the preparation of this
report. Dr. Richard G. Van Gelder (Chairman and Associate Curator of
Mammalogy) and Dr. Sydney Anderson (Associate Curator of Mammalogy )
of the American Museum of Natural History most kindly made possible my
participation in the expedition to Uruguay whence came the field data and
also made available to me the facilities of the Department of Mammalogy in
the course of the laboratory research. Comments and suggestions were made
in the field by these men and by Dr. Karl F. Koopman (Associate Curator of
Mammalogy), also of the American Museum.

In Uruguay I also received help from John K. Bouseman, Thomas J. McIn-
tyre, Merlin D. Tuttle, Sr. Armando J. Tocchéton (Sad Paulo, Brazil), and
especially from Sr. Alfredo Langguth and Sr. Alfredo Ximénez, mammalo-
gists from the Museo de Historia Natural of Montevideo (all of whom parti-
cipated in the field work). Later Sr. Langguth sent me additional data per-
taining to the ecology of certain rodents. Other personnel from the Museum
in Montevideo generously providing assistance were Sr. Juan Cuello, Sr.
Miguel Klappenbach, Sr. Martin Mowszowicz, Sr. Braulio Orejas and Sr.
Juan Uraga. I am indebted to Don Diego Legrand and especially to Sr.
Richardo Praderi for identification of the plants that I collected. Dr.
Fernando Mané-Garzoén identified certain ectoparasites. Sr. Eduardo Wilson
assisted in the field in Durazno as did officers and members of the Centro
de Estudias de Ciencias Naturales of Montevideo.

The present report is based on part of a doctoral dissertation submitted
to the Department of Zoology and the Faculty of the Graduate School of the
University of Kansas. I am most grateful to Dr. Richard F. Johnston, my
graduate advisor at the University of Kansas who guided the doctoral re-
search and to Professors William M. Bass and William A. Clemens (the
latter now of the University of California at Berkeley) who critically read
the original manuscript.

John E. Ubelaker identified nematodes and Professor George W. Byers
of the Snow Entomological Museum of the University of Kansas identified
insect remains from mammal stomachs. Others at Kansas who gave much
appreciated help include: Mrs. Elsa Bardack, J. F. Downhower, Mrs. John
Glinka, B. L. Keller, T. E. Lawlor and A. L. Metcalf.

For critical reading of various versions and parts of the present manuscript
I am grateful to Sydney Anderson (American Museum), Philip Hershkovitz
(Field Museum), Karl F. Koopman (American Museum), Randolph L.
Peterson (Royal Ontario Museum), Dennis M. Power (Royal Ontario
Museum) and J. R. Tamsitt (Royal Ontario Museum).

I thank also Mrs. C. Goodwin, Mrs. S. Poray, and Leighton Warren,
all of the RoM, for help in production of the plates, and Mrs. S. Dugdale
and Miss E. Woo who typed the final drafts of the manuscript. I am grate-
ful to my wife, Judith, for help in typing earlier parts of this report and
more especially for moral support and patience throughout the course of
study.

For financial assistance, I am indebted to the United States Army Medical
Research and Development Command which provided support for the
expedition under the auspices of Grant No. DA-MD-49-193-63-G82 to
the Department of Mammalogy of the American Museum. In the spring of
1964, the University of Kansas awarded me a travel grant enabling me to
visit the American Museum to examine specimens during a critical part of
the study.

Observations on the Biology
of Rodents in Uruguay

Abstract

An expedition from the American Museum of Natural History
visited the South American Republic of Uruguay between
December 1962 and the end of May 1963, to collect mam-
mals and their ectoparasites. More than 630 specimens com-
prising 16 species of rodents—Oryzomys delticola, O. flaves-
cens, Thomasomys oenax, Akodon azarae, A. obscurus, Oxy-
mycterus nasutus, Scapteromys tumidus, Calomys laucha,
Reithrodon physodes, Holochilus brasiliensis, H. magnus,
Mus musculus, Cavia pamparum, Hydrochoerus hydrochaeris,
Myocastor coypus and Ctenomys torquatus—were collected
in woodland, grassland and limnic habitats.

In the present report data on distribution, abundance, habi-
tat preferences, food habits, breeding and moult are presented
for most species of rodents collected. Oryzomys delticola was
common and favoured woodland and ecotonal situations
throughout the country. The rare Thomasomys oenax was
found only in the dense subtropical forest of deep canyons in
the north. Oryzomys flavescens, Akodon sp., Oxymycterus,
Scapteromys, and Holochilus brasiliensis were fairly common
principally in and near limnic habitats which predominated
especially in the southern half of the country. Holochilus mag-
nus, Myocastor coypus and Hydrochoerus hydrochaeris were
less common than the above but also were found in limnic
situations. The fossorial hystrichomorph Crenomys torquatus
and the mice Calomys laucha and Reithrodon physodes pre-
ferred grassland. The former species was common and the
latter pair uncommon. The only Mus musculus taken came
from a granary and beneath corn shocks.

Akodon sp., Oxymycterus and Scapteromys fed largely on
invertebrates. All other native species mentioned in this report
were predominantly herbivorous.

Evidence of current breeding activities were found in the
cricetines and Cavia. All demonstrated apparent reproductive
peaks in early summer (December) and mid-autumn (late
April to mid-May). The only other species obtained in good
numbers, Ctenomys torquatus, was not found to be in breed-
ing condition in the course of our field work, but is known to
give birth to young in early spring.

Ctenomys torquatus demonstrated evidence of a post-breed-
ing autumnal moult. In cricetines and Cavia pamparum moult
was more or less continuous with peaks in mid-summer and
late autumn.

Introduction

Much remains to be learned about the ecology and distribution of mammals
in the countries of South America. The mammalian fauna of Uruguay has
proved no exception in this regard. The present report deals with aspects of
the biology of 16 species of rodents represented by more than 630 speci-
mens collected in Uruguay primarily between early December 1962 and the
end of May 1963 by members of an expedition from the Department of
Mammalogy of the American Museum of Natural History.

The first collecting and recording of significant observations of animals in
Uruguay are attributable to the British naturalist, William Toller. Toller
mentions the capybara (Hydrochoerus hydrochaeris) in the account of his
visit in 1715 to the southeastern coast, in what is today the Department of
Rocha and the southern coast at the mouth of the Rio Santa Lucia near
Montevideo (Vaz Ferreira, 1955). Devincenzi (1935) enumerates studies
mentioning rodents that occur in Uruguay spanning the 130 years beginning
with Felix de Azara in 1802 and terminating with Nils Gyldenstoipe in
1932. Of special importance among those mentioned by Devincenzi are the
relatively detailed ecological accounts of Waterhouse (1839) who cites the
field notes of Charles Darwin taken in 1832, Aplin (1894), and Sanborn
(1929). More recently Praderi and Luzardo (1958), Ximénez (1965),
Langguth (1963) and Massoia and Fornes (1964) have contributed signi-
ficant observations. Cabrera and Yepes (1960) provide the only compilation
of ecological and natural history information for the more common kinds of
South American rodents. Professor Raul Vaz Ferreira, Sr. Alfredo Lang-
guth and Sr. Alfredo Ximénez, as well as other resident biologists, have
made a number of important trips to various parts of Uruguay since about
1957. Prior to our own expedition the most important field work was that
of Sanborn who, under the auspices of the Captain Marshall Field Brazilian
Expedition, spent four months in Uruguay between October 1926 and
February 1927. Covering the southern half of the country, he obtained
345 mammals comprising 30 species (Sanborn, 1929) including 12 species
of native rodents.

Geography. Uruguay, the smallest country in South America, comprises
131,200 square kilometres and lies between latitudes 30°04’ and 34°55’ S
and longitudes 52°50’ and 57°29’ W. It is bounded on the north by the
State of Rio Grande do Sul, Brazil, on the west by the Provinces of Corri-
entes and Entre Rios, Argentina, on the southeast by the estuarine Rio de
la Plata, and on the south and east by the Atlantic Ocean.

A gently rolling coastal plain characterizes the southern and eastern parts
of the country. Low grassy hills and hogback ridges are found in the central
portion. Deeply dissected terminal parts of the Brazilian Plateau and out-
lying mesa-like hills predominate in the north. However, the highest point
(501 m) in the country is a ridge named Sierra de Animas located near the
southern coast in the Department of Maldonado.

Climate. The climate is basically temperate (James, 1942:374), but is
characterized by dramatic daily or even hourly weather fluctuations which

4

occur in any season. Temperatures are milder in the south and southeast
because coastal and adjacent inland areas are subject to the moderating in-
fluence of the typically cooler maritime air masses. Recorded temperature
extremes are —5°C and 44°C (Bergeiro, 1945). Average annual precipi-
tation ranges between 1,193 mm in the north and 949 mm in the southwest.
Frigid winds, termed pamperos, emanating from Patagonia may flow over
the country from the southwest at any season, and they are usually respon-
sible for the rapid and dramatic weather fluctuations mentioned above. In
general the climate in conjunction with edaphic factors produces a protracted
growing season extending from late August or early September to late May
or early June.

Vegetation. The vegetation of Uruguay is transitional between the sub-
tropical woodland of southern Brazil and the open thorn woodland of
eastern mesopotamic Argentina. Historically, tall grass savannah predomin-
ated in this region. Although 15% of the total area now comprising Uruguay
was forested, woodland now represents only 2% of the area. The few large
tracts remaining are restricted to riparian situations, the steep rocky slopes
of larger hills unfit for grazing, and the deep moist canyons of the north.
Great coastal marshes (banados) have developed inland to the beaches at
the mouths of rivers and streams flowing into La Plata and the Atlantic
Ocean.

Methods

The expedition was concerned not only with taking mammals but with the
preservation of their ectoparasites. Therefore, immediately upon collection,
mammals were placed in plastic bags containing pieces of cotton saturated
with chloroform, thus preventing the loss of the ectoparasites before they
could be systematically removed and preserved in spirits. Although bio-
logical data recorded at the time of preparation varied with the mammalian
species, the general procedure was to note breeding condition, to save male
or female reproductive tracts, to save selectively stomachs for content ana-
lysis and, for many small rodents, to make sketches of the pattern of melanin
deposition on the inner surface of the skin.

General ecological, ethological and natural history data were recorded
in the field. More extensive studies, including excavation of burrows, were
undertaken when time permitted. Selective collections of the dominant plants
were made at nine stations in order to characterize various habitats.

In the laboratory, moult was studied by examining study skins under a
binocular dissecting microscope and observations were compared and
correlated with internal melanin patterns previously delineated. In larger
forms, the density, length and general condition of the fur were compared
in summer and autumn collected specimens to determine the status of the
moult.

When possible, breeding condition in males based on the stage of sperma-
togenesis was verified by microscopic examination of testicular biopsies 1
mm in diameter, stained with methylene blue, prepared by the squash

J

technique. Stomach contents were filtered through a fine silk screen and the
residue was separated into plant and animal components.

When sample sizes permitted, age categories based on toothwear, suture
closure, and overall body size were determined for various species of
rodents. Comparisons of pelage were also used as an aid in aging.

Collecting Stations. An annotated list of our principal collecting stations,
arranged in chronological order and followed by an alphabetical listing of
localities visited briefly is herein presented. Each numbered collecting station
is shown on the map (Fig. 1).

1. Department of Canelones; Arroyo Tropa Vieja. 6-7, 17 December
1962, 5 March 1963. Located 36 km east of Montevideo, the Arroyo Tropa
Vieja, a small marshy stream, dissects a zone of stabilized dunes planted with
exotic pines before flowing into the Rio de la Plata. The general region,
being under development or developed as a resort area, is dotted with
cabanas.

2. Department of Lavalleja, Rio Cebollati, 9 km S Piraraja. 13-15
December 1962. This was our first camp and was located on an estancia of
several thousand hectares owned by a colourful gaucho named Cipitria. The
Rio Cebollati here was bordered by a narrow band of riparian thorn wood-
land. The adjacent upland was heavily grazed.

KILOMETERS
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Fig. 1 Uruguay, showing major collecting stations visited by the expedition.
The numbers, in chronological order, refer to the accompanying text.

6

3. Department of Rio Negro; Arroyo Negro, 15 km S Paysandu. 26
December 1962-7 January 1963. The first place visited by all members of
the expedition was on the Estancia Santa Elisa, a ranch of about 10,000
hectares in western Uruguay. Several thousand head of cattle and some
sheep were grazed there. Gently rolling overgrazed pasture and low open
thorn forest up to 4 km in width, intergrading with a narrow band of dense
riparian woodland along the Arroyo Negro, were the principal habitats.

4. Department of Artigas, 6 km NW Belén. 7-21 January 1963. The
Estancia Santa Elisa de Yacuy, owned by Sr. Isodor Vejo, included 4,000
hectares of hilly rangeland dotted with granite outcrops. A narrow strip of
mature riparian forest grew along the Uruguay River, bordered in places by
stands of bunch grass occasionally up to 2 m in height.

5. Department of Tacuarembo, 40 km NW Tacuarembo, 22 January-
17 February 1963. Toward the end of January we moved to the Estancia
E] Infiernillo, some 8,000 hectares of forested canyons and grass covered
hills. Our camp was situated in the deeply dissected canyon of the Arroyo
Tres Cruces in an extensive tract of subtropical forest abundant with ferns
and epiphytes. Except during several days of torrential rain that caused
extensive flooding, the rocky bed of the stream contained only a series of
disjunct pools. In places the slopes of the canyon, rising sharply from the
level floor, were crowned by barren vertical granite cliffs. Tree growth
typically extended about two-thirds the distance up each slope.

6. Department of Cerro Largo, 6 km SE Melo. 19 February-6 March
1963. Here we camped 1 km south of the headquarters of the Estancia Las
Marias in a narrow rocky and wooded dell situated in a hilly, sandy-soiled
pasture. Several small marshes were present in low-lying drainage areas
between the hills. Large limestone boulders, exposed at random, were pitted
with small solution caves that served as refuges for a variety of animals.

7. Department of Cerro Largo, 20 km Melo. 6-21 March 1963. The
Rio Tacuari flows from northern Cerro Largo south and then east into the
Laguna Merin. We camped within the thorn woodland along the river at the
base of grass-covered rocky hills referred to as the Sierra de Vaz. The
Estancia Laboramus encompassed this area. The river contained stretches of
relatively quiet water, interconnected by narrower rocky, brush-filled rapids.

8. Department of Treinta y Tres, 16 km SSW Boca del Rio Tacuari. 26
March-6 April 1963. We camped on an estancia along the coast of the
Laguna Merin, the large lake separating northeastern Uruguay from south-
ernmost Brazil. Beginning several hundred metres south of camp, dense
woodland of willow (Salix humboldtiana) and sarandi (Cephalanthus sp.)
extended for several kilometres along the shore. Extensive marsh, at this
time mostly dry, and flat grassland predominated inland.

9. Department of Treinta y Tres, Rio Olimar Chico, 25 km WSW
Treinta y Tres. 7-19 April 1963. At this locality the Rfo Olimar Chico was
bordered by a narrow strip of riparian thorn woodland and surrounded by
hilly, closely grazed pasture.

10. Department of Durazno, Rio Negro, 15 km NNW San Jorgé. 6-13
April 1963. In the company of members of the Centro de Estudias de Cien-
cias Naturales, an organization of local amateur and professional naturalists

7

from Montevideo, a very profitable side trip was made by two members of
the party, Jon C. Barlow and Thomas J. McIntyre. We camped on a wooded
hilltop on the shore of the reservoir of the Rio Negro and most trapping was
done in nearby open thorn woodland.

11. Department of Lavelleja, 12 km WSW Zapican. 20-29 April 1963.
At this locality Sr. Juan Fernandez graciously hosted most of the party at
his estancia, Bella Vista. The ranch, located on a divide known as the
Cuchilla de Retamosa, comprised several thousand hectares of hilly prairie.
Cattle and sheep were grazed with somewhat varying intensity on different
pastures, and eucalyptus groves were scattered here and there.

12. Department of Rocha, 22 km SE Lascano. 29 April-3 May 1963.
El Sauce, an estancia of 18,000 hectares, was the site of our last prolonged
encampment. Thousands of cattle and a few sheep were grazed on the rolling
grassland dotted with eucalyptus plantings. Except for a very few palms, no
native trees grew in the general vicinity of camp. Trapping was done along
a stream, principally where a stone fence shielded lush marsh vegetation
from livestock.

13. Department of Maldonado, 17 km NE Aigud. 13-17 May 1963.
This region was characterized by rocky hills covered with low thorn forest
rising about 150 m above the surrounding farmland. One hill, Salamanca,
contained a large cave and was the focal point of our collecting activity.

14. Department of Maldonado, Barra del Arroyo Maldonado, 9 km
ENE Punta del Este. 15-17 May 1963. This place was approximately the
locality at which Charles Darwin worked in 1832. Three of us lived in a
small cabin with a fenced-in yard immediately behind which grew a narrow
strip of marsh vegetation dominated by pampas grass (Cortaderia selloana)
and caraguata (Eryngium paniculatum). The marsh was adjacent to the
Arroyo Maldonado which flowed southwestward into nearby La Plata.
Stabilized dunes extended inland for several kilometres.

15. Department of Montevideo, Rio Santa Lucia, I km SE Santiago
Vdsquez. 25-26 April 1963, 24-26 May 1963. At this locality we trapped in
an extensive salt marsh at the edge of the national zoological park, Lecoq.

16. Department of Soriano, 3 km E Cardona, 22-25 May 1963. Merlin
D. Tuttle collected at this place, in southwestern Uruguay, on the estancia
of Sr. Julio Gerber. A marshy stream with reeds, cattails, pampas grass and
water hyacinths in abundance was the centre of his activity.

No camps were established at the localities listed below:

Department of Artigas, 60 km W Artigas. 25 January 1963.

Department of Canelones, Bariado de Tropa Vieja. 8 December 1962,
15 January 1963.

Department of Cerro Largo, 10 km NW Paso del Dragon. 26 March
1963:

Department of Rocha, 10 km E Velazques. 30 April 1963.

Department of Tacuarembo, 10 km N Tacuarembo. 22 January 1963.

Department of Tacuaremb6é, 15 km NW Tacuarembo. 15 February
1963.

Department of Trienta y Tres, 10 km W Boca del Rio Tacuari. 26
March 1963.

Species Accounts

The sequence of families and genera used in this report is from Simpson
(1945). Specific and subspecific names are from Hershkovitz (1966) for
Scapteromys, from Hershkovitz (1962) for Calomys, from Hershkovitz
(1959 b) for Reithrodon, from Hershkovitz (1959 a) for Wiedomys, from
Talice and Momighiano (1954) for Ctenomys, and from Cabrera (1957,
1961) for all other names. For the sake of convenience within genera,
species are arranged in alphabetical order. Common names of mammals in
Spanish are from Devincenzi (1935) and Cabrera and Yepes (1960).
Scientific names of plants are after Herter (1930).

Check-list. The following list includes the 21 species comprising 17 genera,
and 8 families of the order Rotentia known to have occurred in Uruguay
within the last 150 years. The present status of each species is indicated as
species introduced into the country (x); species obtained by our expedi-
tion (*), and species believed to be extirpated (+). Species not marked
are thought still to occur in Uruguay but were not obtained by the expedi-
tion and their status is poorly known.
ORDER RODENTIA—rodents
Family Cricetidae—native rats and mice
* Oryzomys delticola Thomas
* Oryzomys flavescens (Waterhouse)
Thomasomys oenax Thomas
Akodon azarae azarae (Fischer)
Akodon obscurus obscurus (Waterhouse)
Oxymycterus rufus nasutus (Waterhouse )
Scapteromys tumidus (Waterhouse )
Calomys laucha laucha (Desmarest)
Wiedomys pyrrhorhinos Hershkovitz
* Reithrodon physodes physodes Olfers
* Holochilus brasiliensis vulpinus (Brants)
* Holochilus magnus Hershkovitz
Family Muridae—old world rats and mice
* Rattus rattus ssp. (Linnaeus)
x Rattus norvegicus (Berkenhout)
x* Mus musculus ssp. Linnaeus
Family Erethizontidae—porcupines
Coendu spinosus ssp. (F. Cuvier)
Family Caviidae—guinea pigs
* Cavia pamparum Thomas
Family Hydrochoeridae—capybaras
* Hydrochoerus hydrochaeris uruguayensis (C. Ameghino and
Rovereto )
Family Chinchillidae—vizcachas
xf Lagostomus maximus (Desmarest)
Family Capromyidae—nutrias
* Myocastor coypus bonariensis (Commerson)
Family Ctenomyidae—tuco-tucos
* Ctenomys torquatus Lichtenstein

Se HE ES EE

Plan of the Accounts. The accounts of the species include data on distribu-
tion and abundance, ecology, behaviour, food habits, breeding and moult.
The emphasis in these accounts is upon new information. Most specimens
mentioned in the text are housed in the American Museum of Natural His-
tory (AMNH). However, a few are in the collection of the Sociedad de
Taguato (ST) of Montevideo. The metric system is used for all measure-
ments. In sections on breeding CR equals crown-rump length of embryos.
Data are taken from unpublished field notes of R. G. Van Gelder, S. Ander-
son, K. Koopman, A. Langguth, A. Ximénez, A. Tocchéton, M. D. Tuttle,
T. J. McIntyre and J. K. Bouseman as well as from my own notes. The
absence of a bibliographic reference, when the observations of these men are
cited in a species account, indicates that the information was obtained from
the unpublished data.

Oryzomys delticola Thomas
Arboreal rice rat

Specimens: Total 62; Artigas, 6 km NNW Belén, 9; Cerro Largo, 6 km SE
Melo, , 9, ? sex; 20 km Melo, 9; Durazno, Rio Negro, 15 km NNW
San Jorgé, 15 oo’, 17 99; Lavalleja, 12 km WSW Zapican, o, 9;
Maldonado, 17 km NE Aigua, —%; Tacuarembo, 40 km NW Tacuarembo,
4 oo, 5 99, 3 ? sex; Treinta y Tres, Rio Olimar Chico, 25 km WSW
Treinta y Tres, 5 oo’, 5 99.

Ecology. Oryzomys delticola is the common woodland mouse of Uruguay. It
is found in mesic subtropical forest with a closed canopy and a scanty under-
story as in Tacuaremb6o and in more xeric, open thorn woods with dense to
moderate understory as in Durazno along the Rio Negro. Conspicuous vege-
tation at the latter locality includes espinillo (Acacia caven), Xilosoma war-
burgi, coronilla (Scutia buxifolia), arrayan (Blepharocalyx tweediei), Bac-
charis sp., quiebra-arados (Heima salicifolia) and zarzaparrilla (Smilax
campestris). We trapped this mouse along fallen logs, around outcrops,
among tangles of vines near streams, and in trees up to 2 m above the
ground. It has also been taken in tall grass bordering woodland (Praderi
and Luzardo, 1958:30) and in houses (Langguth, 1963:13). In Tacuarem-
bo at Infiernillo it was taken in the same line with Thomasomys oenax and
in Durazno with Akodon azarae.

Shelter. Oryzomys delticola does not make runways on the ground but
rather follows natural avenues and runways of other animals. Praderi and
Luzardo (1958:31) found a nest of this species in a wooden box lodged 2
m up in the crotch of a thorn tree. In Durazno I saw one of these mice
leaving a burrow beneath a large rock on a wooded slope.

Behaviour. Since most of our specimens were taken at night O. delticola is
judged to be primarily nocturnal. However three, one a young male (AM
NH 205956), were trapped at mid-day and a female (AMNH 205928)
was shot in mid-afternoon as she foraged in a garbage pit at our campsite
near Melo.

10

In Durazno I observed several individuals by lantern light shortly after
dark as they climbed about in trees and over large rocks in search of food.
They are agile and are regarded by Langguth (1963) to be better adapted
for climbing than the congeneric O. flavescens because they have longer
tails than that species.

Parasites and Predators. Two mice (AMNH 305970; AMNH 205924)
each had a large cuterebrid larva beneath the epidermis of their left flanks.
Other ectoparasites preserved have been identified, and results will be pub-
lished elsewhere.

Post-cranial elements of a skeleton thought to be Oryzomys delticola
were found in the stomach of an Ihering owl (Aegolius theringi harrisi)
netted in Tacuarembo (Barlow and Cuello, 1964:517). Langguth (1963:
13) reported remains of this mouse in pellets of the barn owl (Tyto alba
tnidara). A partially digested O. delticola (AMNH 205927) was regurgitat-
ed by a green snake (Philodryas aestivus) captured near Melo.

Food Habits. Oryzomys delticola, although primarily herbivorous, includes
more animal material in its diet than does O. flavescens and has been readily
trapped when pieces of fish were used for bait. Four stomachs contained
about 10% insect remains, including a larva (Pyralidae [Lepidoptera]), one
nymph (Aphidae [Homoptera]) and fragments of Coleoptera and Hemip-
tera not identifiable to family. The stomach is single-chambered and not
noticeably muscularized or glandularized.

Breeding. Information concerning reproductive status was available for 52
of the 62 specimens obtained. None of the six adults taken in January,
March and May were in breeding condition, but this sample is too small to
be truly indicative of reproductive activity during those months. Two of four
adult females obtained in March were pregnant and two of four adult males
taken in this month had testes of breeding size, i.e. 7 mm or greater in
length, as determined by histological examination.

Populations sampled in early April at two widely separated localities in
the country demonstrated asynchrony in their respective breeding schedules.
Two of three adult females obtained along the Rio Olimar Chico in Treinta
y Tres were pregnant, in contrast to none of 13 females trapped along the
reservoir of the Rio Negro in Durazno, although one individual from the
latter locality had a post-partum uterus containing four placental scars, two
in each horn. However, the capture of several juveniles in Durazno suggested
that reproductive activities had only recently ceased. Two of five adult males
from Treinta y Tres had scrotal testes of breeding size, in comparison to
three of nine adult males in Durazno with scrotal testes of breeding size. The
five pregnant females taken during our work contained an average of 3.4
(2-4) embryos. Fourteen embryos were in the right uterine horn and three
were in the left. In this species the lack of the relatively large samples which
are available for May for most of the other commonly collected cricetines
results in an incomplete picture of reproduction for summer and autumn.
Thus on the basis of the data presented here it is only possible to say that
Oryzomys delticola breeds in late summer and early autumn.

Ti

— moult

CL] —no moult
1 —juvenile
2 —subadult
3 — adult

4 —old adult

20

15

je)

NUMBER OF MICE
oO

1234 1234 1234 i234 1234
January February March April May

Fig. 2 Moult in Oryzomys delticola by age classes between January and
May 1963. Analysis based on 50 skins.

Moult. Four age categories based on the wear of the molar occlusal surfaces,
and on total (head and body + tail) length were recognized in this species:

Class 1, juvenile—Molars unworn; total length less than 160 mm; pelage
short, grayish on dorsum, light gray on venter.

Class 2, subadult—Molars moderately worn but with crown pattern not
obscured; total length between 161 and 180 mm; pelage of varying length
contingent upon season and wear, light grayish brown on dorsum, grayish
white on venter.

Class 3, adult—Molars heavily worn, cusp pattern partly obliterated,
especially that of the third molars; total length between 181 and 228 mm;
pelage grayish brown or reddish brown on dorsum, whitish on venter.

Class 4, old adult—Molars very heavily worn; on all, cusp pattern partly
or nearly completely obliterated; total length between 229 and 258 mm;
pelage, as in Class 3.

Classes 2 to 4 obtained in summer (January to mid-March) generally
had worn, short pelage conspicuously paler than that of autumn-caught in-
dividuals. Individuals in moult were obtained in both seasons (Fig. 2). Data
from the two widely separated populations sampled in early April demon-
strate that moult and reproductive activity may occur concurrently in both
sexes. Specimens from Treinta y Tres included pregnant females in the
moult, and moulting males with enlarged, scrotal testes were trapped in
Durazno.

No juveniles were moulting. A male (AMNH 205949) having subadult
characteristics was in the post-juvenile moult (Fig. 3). The soft, grayish
juvenile pelage was being replaced by coarser reddish-brown hair. New
growth began ventrally, then continued up the sides to the back, and pro-
ceeded from a mid-dorsal centre anteriorly and posteriorly.

The autumnal moult in adult individuals resembles that of other Urugu-
ayan cricetines examined. A growth centre first develops on the mid-belly,
and hair replacement proceeds anteriorly and posteriorly from it. Then the
moult spreads up the sides to the mid-dorsum, resulting in the formation of
the characteristic “saddle” configuration. From this secondary centre, moult
spreads anteriorly and posteriorly. New growth seems to lag somewhat on the
head and two variations are seen. Moult occurs first on the sides of the head
and the new growth spreads upward to meet on the mid-line of the muzzle,

12

Fig. 3 Post-juvenile moult in a sub-
adult ¢ of Oryzomys delticola
(AMNH 205949) as seen in the ex-
terior view of the dorsum (arrows
show direction of hair growth).

Fig. 4 Autumnal moult in an adult
@ Oryzomys delticola (AMNH
205938), as demonstrated in a dorsal
exterior view, showing two growth
centres (arrows show direction of
hair growth).

dorsum

«

venter

Fig. 5 Autumnal moult in an older adult @ Oryzomys delticola (AMNH
205961) showing dorsal exterior view (1; arrows show direction of hair
growth) with the corresponding dorsal interior pattern of melanin deposition
(2). Also shown is the ventral interior pattern of melanin deposition (3).

13

whence moult grows posteriorly over the crown and converges between the
ears with the main growth wave of the dorsum. Or moult may begin indepen-
dently on the muzzle from a separate centre and later join tracts from the
sides of the head passing along the neck from the principal moult of the
thorax. From the centre on the muzzle, hair replacement then proceeds
caudally in the way already described.

Moult patterns of old adults are more complicated than those of younger
mice. One female (AMNH 205938) had a centre between the ears from
which hair replacement proceeded anteriorly to the tip of the snout and also
posteriorly to a zone in the middle of the back, where the moult converged
with another moulting front passing anteriorly from another growth centre
located anterior to the rump (Fig. 4). A breakdown in the simpler pattern
of development followed in subadult mice is characteristic of older adults.
Additional growth centres develop, and hair replacement more often pro-
gresses to the tip of the muzzle from a growth centre between the ears,
rather than from the tip of the muzzle posteriorly to convergence between
the ears. A large pregnant female (AMNH 205961), taken on 26 April in
Lavalleja, near Zapican, had three growth centres on the dorsum from
which moult was progressing both anteriorly and posteriorly (Fig. 5). The
melanin on the interior of this skin was deposited in a highly irregular
pattern.

Oryzomys flavescens (Waterhouse)
Pampas rice rat, colilargo chico

Specimens: Total 32; Artigas, 6 km NNW Belén, <, ? sex; Cerro Largo,
20 km SE Melo, 6; Lavalleyja, 12 km WSW Zapican, 2 oo, 2 99;
Maldonado, Barra del Arroyo Maldonado, 9 km ENE Punta del Este,
©, ?sex; Montevideo, Rio Santa Lucia, 1 km SE Santiago Vasquez, <’,
© Rocha, 22)kmn SE Lascano, 8 oo [ll Cos Soriano, 3) kur Ee Candona
2; Treinta y Tres, 16 km SSW Boca del Rio Tacuari, <.

Ecology. We found that Oryzomys flavescens generally frequents stands of
tall grass in marshes and along rivers and streams in Uruguay, but at one
locality in Lavalleja we obtained specimens around corn shocks at some
distance from water. In Maldonado, along the Arroyo Maldonado, in a trap-
line overlapping several distinct vegetational zones within a marshy area,
O. flavescens was taken most frequently in pure stands of pampas grass
(Cortaderia selloana) and junco (Juncus acutus), although a pregnant female
(AMNH 206013) was caught in a trap actually floating in shallow water at
the base of some rushes. Scapteromys tumidus and Akodon obscurus were
taken in the same trapline, but most often in parts of the habitat that were
on the average wetter and with a somewhat different vegetational aspect
(mixed Eryngium sp. and C. selloana). In general, Oxymcterus rufus was the
most frequent associate of O. flavescens in wet tall grass situations. O.
flavescens apparently does not make runways of its own but rather follows

14

EE

natural pathways through the grass or utilizes runways of other small rod-
ents, especially Cavia pamparum.

Food Habits. The single-chambered stomach of O. flavescens possesses
neither hypertrophied muscularization of the cardiac region nor conspicuous
glandularization of the fundic region. Each of 10 stomachs examined con-
tained green plant material. Five of these also contained some invertebrate
remains. Parts of a beetle (Chrysomelidae) were found in the stomach of
one individual (AMNH 206013) trapped 3 May 1963, near Lascano,
Department of Rocha.

Breeding. Of the total of 14 adult females taken in January, March, April
and May, 12 showed evidence of breeding. The only female obtained in
January was lactating. April and May apparently are months of peak repro-
ductive activity as eight of ten females in some phase of the breeding cycle
were pregnant. These females averaged 5.1 (3-7) embryos apiece with 25
implantations in the right uterine horn and 16 in the left.

In males examination of histological preparations of testicular biopsies
demonstrated that testes measuring at least 7x5 mm contained quantities of
mature sperm. Seven adults with scrotal testes of that size or larger were
trapped between March and May. Two adult males obtained by Langguth
(pers. comm.) on 18 September 1963, at Tropa Vieja, Department of
Canelones, had testes measuring 5x3 mm and 6.5x4 mm respectively, that
would have been by the above criterion less than reproductively active size.

Moult. Individuals demonstrating various stages of moult are known from
March, April, May and September. Both breeding and non-breeding adults
collected in April and May were moulting, and there was no relationship
between status of the moult and breeding condition (the two were inde-
pendent: X?=.202[ns], df=1).

Three adults collected in January (AMNH 205787) and March (AMNH
205988, 206017) were in the short summer pelage. Age groups were estab-
lished with criteria similar to those used for O. delticola as the pattern of
moult development closely resembles that species. Three of five individuals
collected in April and nine of 20 taken in May were moulting.

Too few specimens are available to diagnose in detail the pattern of
growth demonstrated in the post-juvenile moult. However, a young male
(AMNH 206012) measuring 187 mm in total length was in heavy moult
dorsally and ventrally. Growth on the mid-venter in this animal was pro-
gressing in a caudal direction. Latero-ventrally the line of growth passed up
over the flanks and converged on the latero-dorsum. From this newly formed
centre hairs grew both posteriorly and anteriorly to the tip of the muzzle.
The incoming hair was a brighter yellowish-brown than the old pelage.

Moult in adults develops as in most other species examined, with ventral
growth centres appearing first from which moult spreads up over the flanks
and toward the mid-dorsum; initial convergence of these lateral growth
fronts forms a “‘saddle” pattern. Growth proceeds both anteriorly and pos-
teriorly from this initial site on the dorsum. The epidermis is a golden colour
when the black tips of the guard hairs first break through the surface of the
skin. The golden colour gives way to a salmon pink when the hairs are about

1)

half grown and to black just prior to the completion of growth. Salmon and
black colouration on the exterior are represented on the interior of the skin
by bluish-black areas of melanin. Examination of specimens suggests that
moult lags on the head and rump. Moreover, all vestiges of hair replacement
elsewhere disappear significantly in advance of completion of hair growth on
the head and rump. No irregular moult patterns such as found in the very
old individuals of Scapteromys tumidus were seen.

Thomasomys oenax Thomas
Red-nosed tree mouse, raton del sur de Brazil

Specimens: Total 4; Tacuarembo, 40 km NW Tacuaremb6é, 3 oo’, 9.

This rare arboreal mouse was known only from four specimens taken in
the State of Rio Grande do Sul, Brazil, until Vaz Ferreira (1960:67)
reported its occurrence in Uruguay. Another specimen is now known from
Curitiba, State of Parana, Brazil (Avila-Pires, 1960:2). Pellets of the barn
owl, Tyto alba, found in southern Tacuaremb6, contained several skulls of
this mouse (Sr. A. Langguth, pers. comm.). We obtained four additional
specimens of Thomasomys oenax in one month of trapping in dense sub-
tropical wocdland in the canyon of the Arroyo Tres Cruces, 40 km N of
Tacuaremb6, Department of Tacuarembé.

General Description. Thomasomys oenax resembles the larger oryzomines
in body form, except that the tail is much longer. In colour the dorsum is
pale gray with a yellowish cast. The venter is buffy white. The tip of the
muzzle and the lightly haired ears are bright ochraceous and there is another
ochraceous patch on the rump. The long tail is haired and uniformly buffy.
The largest adult (AMNH 206021) obtained by us measured: total length,
315 mm; length of tail, 186 mm; length of hind foot with claw, 30.5 mm;
ear from notch, 22 mm; weight, 61.3 gm. Three individuals were considered
to be young adults and the other (AMNH 206020) a subadult on the basis
of tooth wear.

Range. Thomasomys oenax occurs from Curitiba, Parana, Brazil, south into
Uruguay to southern Tacuaremb6 and thus is a resident of the zone of tran-
sition between extensive subtropical forest and more temperate savannah.
In Uruguay the species appears to be restricted to patches of dense sub-
tropical woodland characteristic of the Brazilian Plateau region in north
central parts of the country. It has not been found in adjacent parts of
Artigas, Salto and Cerro Largo, which differ significantly from Tacuarembo
in topography and character of vegetation.

Ecology. The four mice were caught in 4400 trap nights in mesic sub-
tropical woodland. Conspicuous vegetation included canel6n (Rapanea
ferruginea), arrayan (Blepharocalyx tweediei), Cupania verna'is, Octea acuti-
folia, Maytenus ilicifolia, blanquillo (Sebastiana klotzschiana) and espina
amarilla (Berberis laurina). These trees, festooned with bromeliads (Tilland-
sia aoranthos), as well as ferns (Polypodium phyllitides and Aneimia tomen-
tosa) and lichen (Usnea hieronimii), constitute dominants of a type of sub-

16

tropical woodland characteristic of Rio Grande do Sul in southern Brazil.
Such forest is less common in Uruguay. The general aspect was of low
(7 m) to medium-sized (14 m) forest with a nearly continuous canopy.
Understory was sparse and consisted primarily of ferns and saplings of the
dominant trees. The forest growth spread two thirds of the way up the rocky
canyon walls before appreciably thinning.

The first specimen (AMNH 206018), obtained 27 January, was trapped
among large rocks within the forest on the north facing slope of the canyon
and was the only mouse caught in a line of 50 Museum Special traps. The
second specimen (AMNH 206019), taken 8 February, was trapped 20 m
from the first in a trap placed at a height of 2 m in a tree on a lateral branch
10 cm in diameter. Here all trees were 7 m or less in height. The third in-
dividual (AMNH 206020), obtained 10 February, was trapped in the
crotch of a tree 142 m above the ground and within 4 m of the site of
capture of the first specimen. The fourth mouse (AMNH 206021), taken
11 February, was trapped in the same tree and on the same branch as the
second individual. The third and fourth were adult males in breeding condi-
tion. Suitable habitat that stretched along 112 km in the canyon was sampled
intensively for nearly a month but additional specimens were not secured.

The only other cricetine obtained within the same habitat, both on the
ground and in trees, was Oryzomys delticola. Two Akodon azarae were
trapped at the forest edge in bunch grass, and a Calomys laucha was ob-
tained in the upland rocky pasture, above the canyon.

Food Habits. Stomachs of three of the mice contained about 6 cc of greenish
plant material, bait and purplish berries from the arbol malo (Lithraea
molleoides [Anacardiaceae]). This species is widely distributed in Uruguay.
In the study area fruiting trees were abundant on the lower and middle parts
of the north facing slope of the canyon.

Breeding. The young female (AMNH 206020) was not pregnant. The testes
of the smallest male (AMNH 206018) were ant-eaten and could not be
measured. The two largest males had testes measuring 10 x 6 mm that
contained abundant mature sperm in the epididymides and the seminiferous
tubules.

Moult. No indication of moult was found on the specimens. The type speci-
men, in the British Museum (Nat. Hist.), was also secured in summer and
was not in the moult (Karl Koopman, pers. comm. ).

Remarks, Ximénez (1965:135) has reported two specimens of Wiedomys
pyrrhorhinos from the Arroyo Tres Cruces in Tacuarembéo. Since this is
the same locality at which our examples of Thomasomys oenax were col-
lected I questioned the original identification of our specimens. Sydney
Anderson (pers. comm.) reports that our specimens are definitely T. oenax
and differ from the description of W. pyrrhorhinos of Hershkovitz (1959a)
in (1) larger size, (2) underparts not well defined from colour of sides, (3)
basal part of ventral hairs dark grey, (4) supra-orbital region parallel-sided
and square, not ridged, and (5) mesolophostyle functional, rather than low
and obsolescent or absent. In addition the Uruguayan specimens differ from

17

the holotype of W. pyrrhorhinos (housed in the American Museum) in
characters 1 through 3. I briefly examined the type of Thomasomys oenax
in the British Museum (Nat. Hist.) and did not find it to differ conspicu-
ously in any characters included in my notes on our Uruguayan specimens
of this species.

Akodon azarae azarae (Fischer)
Field mouse, raton de las arenas

Specimens: Total 70; Artigas, 60 km W Artigas, 2 oo, 2; 6 km NNW
Belén, 4 oo’; Cerro Largo, 6 km SE Melo, 6, 2 99; 20 km SE Melo,
o&; Durazno, Rio Negro, 15 km NNW San Jorgé, 2 oo’, 4 99; Laval-
leja, Rio Cebollati, 9 km S Piraraja, 9; 12 km WSW Zapican, 9 9,
9 9; Maldonado, 17 km NE Ajigua, 2 oo’, 9; Rio Negro, Arroyo
Negro, [5 km S*Paysandu, G4; Rocha; 22 kmp-SE Lascano, 13° 447.9
©; Soriano, 3 km E ‘Cardona; 2 Go’, 2 09: Tacuarembo, 40° kin NW
Tacuarembo, 9, ? sex.

Ecology. Akodon azarae occurs in a variety of habitats in Uruguay. We took
it in open thorn woodland dominated by Acacia sp. and with a dense under-
story of Baccharis sp. and Eupatorium sp.; in tall grass (Scirpus giganteus)
adjacent to riparian woodland; at the edge of a brush pile in dense riparian
woodland; and in upland stands of tall grass (Andropogon sp.). A. azarae
also has been taken in dry places in marshes. In Rocha, a stone wall passing
through a stream-fed marsh afforded a higher and drier refuge for this
mouse, and individuals apparently were living within crevices in the wall.
Reithrodon physodes also was obtained along this wall. In edge situations
Oryzomys delticola was commonly trapped with this Akodon. Akodon
azarae was also trapped with Akodon obscurus, Oxymycterus rufus and
Oryzomys flavescens in marshy areas with a variety of conspicuous plants.
An adult male (AMNH 206061) was trapped at the edge of a drainage strip
of short green grass between harvested grain fields, several kilometres from
open woodland in Rio Negro. Although no burrows or runways definitely
attributable to Akodon azarae were found, it is certain that these mice were
utilizing crevices within a stone wall in Rocha. Faeces were found on rocks
immediately in front of holes in the wall. In Durazno I found a subadult
individual in a shallow excavation beneath a board.

Fornes and Massoia (1965:106) found A. azarae equally active at night
and in the day at Miramar, Buenos Aires Province, Argentina. In Uruguay
we found A. azarae to be mainly nocturnal. A. obscurus seems to be pri-
marily diurnal, and where these species are ecologically sympatric, compe-
tition may be reduced by differences in time of activity.

Parasites. Nine of 65 individuals trapped between December and May dem-
onstrated active myiasis or scars from infestation by cuterebrid larvae. Only
Oryzomys delticola demonstrated a similar incidence of parasitism.

Food Habits. The stomach is simple and unspecialized. Eleven stomachs
contained 20% plant material, 10% Anthony’s bait, and 70% remains of
invertebrates.

18

Invertebrate material, on the basis of the per cent of stomachs containing
the item that had been eaten, was identified as follows: Coleoptera (Scara-
baeidae, Carabidae, Chrysomelidae) 90.9%; Orthoptera (Gryllidae, Gryl-
lotalpidae) 45.4%; Hymenoptera (Formicidae, Ichneumonidae) 45.4%;
Lepidoptera (Noctuidae) 27.2%; Isoptera (Termitidae) 9.9%; Hemiptera
(Tingidae) 9.9%; Diptera (Sciaridae) 9.9%; Oligochaeta 9.9%.

Breeding. Females 150-159 mm in length had post-partum uteri and were
lactating. Some males of similar size had scrotal testes 7 mm in length con-
taining few mature sperm. Few data are available for the period from June
through August; however, the breeding season is obviously protracted, per-
haps year round. Pearson (1967:55) reports a breeding season extending
from October to April in A. azarae collected near Buenos Aires, Argentina.
All available data for 1963 from Uruguay show a reproductive period ex-
tending from October through May (Table 1). Actually the breeding season
is at least a month longer, as demonstrated by a female (ST 40), taken 27
September 1957 in Canelones, that contained five embryos (CR=17 mm).
Males with scrotal testes measuring in excess of 8 mm in length were taken
in January, February, March, April, May, October and December. Twelve
females taken in February, April, May, September and October, either preg-
nant or with placental scars, averaged four (3-7) embryos per pregnancy.
Thirty-five embryos were in the right uterine horn and 25 were in the left
one. Five other females taken in April and May were lactating.

Moult. As there was no difference in pattern, colour or texture, it was not
possible to distinguish the post-juvenile from adult moults. However, by
comparison with other Uruguayan cricetines, moults in subadults and young
adults that emanated from only one or two growth centres were considered
close to the post-juvenile developmental pattern. A typical sequence of
moult development is depicted in Fig. 6. In subadults the autumn moult
begins from a pair of latero-ventral centres, extending, from a position just
cephalad to the posterior limbs, forward in a band only a few millimetres
wide to a position just cephalad to the anterior limbs. Growth proceeds onto
the belly first and then upward from the lateral centre toward the dorsal
mid-line. Growth on the dorsum and venter seems to proceed at about the
same rate in the early and middle stages of development, but the ventral
moult is completed first. On the venter the wave of moult spreads simul-
taneously anteriorly, posteriorly and toward the ventral mid-line. The “sad-
dle-back” condition is developed just anterior to the rump on the dorsum,
more rarely in the scapular region. No examples were found with the “sad-
dle” developed in the middle of the back. Two waves of moult, one on each
side of the mid-dorsum, may pass forward laterally in advance of the wave
progressing anteriorly along the mid-dorsum; or the mid-dorsal growth waves
may progress more rapidly than the lateral area. Moult on the head, al-
though varying in pattern, is simply a continuation of the dorsal body moult
spreading forward to the tip of the muzzle. Older adults, 190 mm or longer,
demonstrate several additional growth centres, including one that develops
on the posterior part of the mid-dorsum prior to the arrival in this area of
the waves of moult advancing toward the mid-line from the sides. An addi-
tional growth centre on the tip of the muzzle spreads posteriorly.

19

20

Fig. 6 Diagrammatic sequence (1-10) of autumnal
moult in Akodon azarae. The large figure in each
stage represents the dorsal exterior view (arrows indi-
cate direction of hair growth). The accompanying
smaller figures at stages 1-4, 7, 8 and 10 represent
interior patterns of melanin deposition on dorsum
(upper figure) and venter (iower figure).

Table 1
Reproductive status of Akodon azarae*

o's Total Specimens 99

Adult Adult
Month Locality Juv. Norte Breed: Juv. Nem br Breed. Totals
**December Lavallaja (Rio 1
Cebollati)
Rio Negro 1
1 2
January Artigas 4
Tacuaremb6 1
4 1 5)
February Cerro Largo (6 km 1 2
SE Melo) 1 2 3
March Cerro Largo (20 km 1
SE Melo) 1 1
April Durazno 1 I 1 1
Lavalleja 1 1 7 4 if
1 Z d 5 2 Deh?
May Maldonado (17 km 1 1
NE Aigua)
Rocha 4 9 2 3 6
Soriano 2 2
At 12 2 4 a. 30
August San José 1 2
1 Z 3
September Canelones 1
1 1
~ October Colonia 2 1
Z 1 3
November Canelones 1
1 1
(a=

*Includes specimens taken by A. Langguth between August and No-
vember 1963.
*#1962

ee

The youngest individuals, 150 mm and less in length, were not in the
moult. Since lactating females, only 159 mm long and considered to be
adults, were in the moult, it is suggested that the post-juvenile moult occurs
in individuals between 150-158 mm in total length.

Specimens were available for all months except June, July and September.
At least some individuals collected between August and December showed a
light moult generally confined to the dorsal surface of the head. Specimens
collected between January and May had more conspicuous moult over the
body. This was especially true in April and May, when the autumnal moult
was underway.

Akodon obscurus obscurus (Waterhouse)
Marsh akodon, rat6n obscuro

Specimens: Total 74; Artigas, 6 km NNW Belén, 9; Canelones, Arroyo
Tropa Vieja, 4 o'', 3 99; Cerro Largo, 6 km SE Melo, 3 99; 20 km
SE Melo, —&; Maldonado, 17 km NE Ajigua, <6; Barra del Arroyo
Maldonado, 9 km ENE Punta del Este, 5 #6’, 5 29; Montevideo, Rio
Santa Lucia, 1 km SE Santiago Vasquez, 3 oo’, 4 99; Rocha, 22 km
SE Lascano, 20 oo’, 13 99; Tacuarembo, 40 km NW Tacuarembo, ?
sex; Treinta y Tres, 16 km SSW Boca del Rio Tacuari, 6 , 3 99, ?
Sex:

Ecology. Akodon obscurus is restricted to limnic habitats. It was taken in
inundated places having plentiful vegetation where traps could only have
been reached by swimming mice. Only Scapteromys tumidus was also found
in these wet places. In drier parts of marshes, A. obscurus was associated
with Oryzomys flavescens, A. azarae and Oxymycterus rufus. This akodon
is apparently diurnal as most individuals were trapped in the day.

Food Habits. Stomach contents show Akodon obscurus to be primarily en-
tomophagous. One of seven stomachs examined contained only plant ma-
terial. The remaining six on the basis of the per cent of stomachs containing
the item included arthropod material as follows: Coleoptera (Scarabaeidae,
Tenebrionidae) 83.3%; Orthoptera (Gryllidae) 66.6% ; Homoptera (Cica-
dellidae) 16.6%; Lepidoptera 16.6%; Diptera 16.6%; Arachnida 16.6%.

Breeding. Males obtained in December, April and May had subscrotal
testes measuring 6 x 3 mm or less and were not in breeding condition. Ap-
parently, the breeding season is protracted, as pregnant females are known
from October and February and lactating females have been trapped in
December, April and May. A female (ST 35), obtained 22 October 1957
in the Department of San José, contained four embryos (CR=17 mm); a
female (AMNH 206100), trapped 23 February 1962 in Cerro Largo, had
three embryos (CR=6 mm); and a lactating female (AMNH 206145),
caught 8 May 1963 in Rocha, showed five placental scars. So many of our
specimens lack data that it has not been possible to demonstrate the ratio
of breeding to total monthly sample in this species. It is noteworthy that
27.3% (9 of 33) individuals taken in May were juveniles.

Ze

Fig. 7 Post-juvenile moult in a subadult to young adult ¢ Akodon obscurus
(AMNH 206125). Shown are dorsal exterior (1) and ventral (2) views
(arrows show direction of hair growth) and corresponding patterns of dorsal
interior (3) and ventral (4) melanin deposition.

Moult. Progression of the moult is similar to Akodon azarae. Age categories
were determined as with other cricetines. A number of subadults taken in
early May were in heavy post-juvenile moult (Fig. 7). A young adult male
(AMNH 206125) taken 4 May 1963 in Rocha, showed melanin deposition
over the entire interior of the skin except for a single patch. Evidence of
moult was found on some animals obtained in the months of February,
April, May, June, September, October and December.

Remarks. Massoia (1963:240) has recently reported Akodon kempi
Thomas from Uruguay. Sydney Anderson (pers. comm.) notes that neither
Thomas (1917) nor Massoia (1963) compare kempi with obscurus and
that the characters that distinguish kempi from A. azarae are those that
separate obscurus, including specimens from the type locality (Maldonado,
Department of Maldonado) collected by us, from our specimens of A.
azarae. Anderson considers kempi conspecific with obscurus. I examined
the types of A. obscurus and A. kempi in the British Museum (Nat. Hist. )
and found the type of the latter to be a subadult specimen of obscurus.

23

Oxymycterus rufus nasutus (Waterhouse)
Burrowing mouse, ratén rojo

Specimens: Total 42; Canelones, Arroyo Tropa Vieja, 4 7’, 4 99; Cerro
Largo, 6 km SE Melo, &, 3 99; 20 km SE Melo, 3 6’, 3 99; Maldonado,
Barra del Arroyo Maldonado, 9 km ENE Punta del Este, 3 ('o’, 4 99;
Rocha, 22 km SE Lascano, 8 oo’, 8 99; Soriano, 3 km E Cardona, ¢.

Ecology. Oxymycterus rufus occurs in wet meadows with stands of bunch
grass (Paspalum sp.); tall grass adjacent to streams and rivers; and drier
parts of marshy areas among clumps of pampas grass (Cortaderia selloana)
outside the frequently inundated zone. Although taken in the same trapline
with the hydrophilic Scapteromys tumidus and Akodon obscurus, O. rufus
was most often taken in the slightly drier places frequented by Oryzomys
flavescens, with which it was commonly associated. Massoia and Fornes
(1964:294) have noted that Oxymycterus sp. swims only from necessity
and that in water the pelage of this species quickly becomes thoroughly
soaked.

O. rufus constructs no runways of its own but uses those of Cavia pam-
parum, the broad trails of Hydrochoerus hydrochaeris in stands of tall bunch
grass, or natural pathways through such vegetation.

Odour. An example caught in a live trap smelled strongly of an odour dis-
tinctly reminiscent of acrolein (acrylic aldehyde), a chemical used commer-
cially to warn of leakage of toxic odourless gases from mechanical systems
(Brewster, 1953:173). This acrid, penetrating scent may serve a similar
warning function in these rodents, possibly serving to discourage potential
predators.

Endoparasites. Seven of 12 stomachs from individuals captured in March,
April and May were infested with between six and more than 30 nematodes
of the genus Protospirura (P. muricola, P. muris; J. Ubelaker, pers. comm.).
The stomach of an eighth individual, a female (AMNH 206192), con-
tained a single Capillaria hepatica.

Food Habits. These red mice are primarily entomophagous, although many
other kinds of invertebrates are eaten. Glandular and structural modifications
of the stomach include a thickening of the fundus. This and other characters
presumably associated with the entomophagous habit are shared with North
American grasshopper mice (Onychomys; Horner, Taylor and Padykula,
1964: 523). The contents of 12 stomachs analyzed in detail comprised in-
vertebrate material, on the basis of the per cent of stomachs containing the
item, as follows: Coleoptera (Tenebrionidae, Scarabaeidae, Lampyridae,
Chrysomelidae, Carabidae, Pselaphidae, Dytiscidae) 100.0%; Hymenop-
tera (Formicidae) 83.3%; Diptera (Sciaridae, Bibionidae, Chironomidae,
Tipalidae) 83.3%; Hemiptera (Tingidae, Pentatomidae) 41.6%; Orthop-
tera (Gryllidae, Acrididae, Gryllacrididae) 25.0%; Lepidoptera (Noctui-
dae, unidentified families) 25.5%; Homoptera (Cicadellidae, Fulgoridae)
16.6%; Oligochaeta (earthworms) 33.3%; Chilopoda (centipedes)
25.0% ; Gastropoda (slugs) 16.6%; Aranea 16.6%; plants (unidentified
fragments) 16.6%.

24

In many marshy places Scapteromys tumidus and Oxymcterus rufus are
the principal insectivorous rodents. The two differ in micro-habitat prefer-
ences and time of activity; Oxymcterus rufus is primarily diurnal, although
specimens have been trapped at dusk and in the early evening. Consequently
competition with the nocturnal Scapteromys tumidus would probably be
minimal.

Breeding. Available data indicate that the breeding season of this mouse is
protracted (Table 2) in Uruguay. Females were taken only in February,
March, May, September and December but samples for each of these
months included pregnant or lactating individuals. Likewise, males with
scrotal testes of breeding size (9 x 6 mm and larger, as demonstrated
by histologic analysis of testicular biopsies) were found in the above
months. Seven gravid females averaged 2.1 (1-4) embryos ranging in
length from 4 to 29 mm in length. Twelve embryos were in the left
uterine horn and three were in the right. Massoia and Fornes (1964:294)
reported a female with six embryos. Oxymycterus rufus possesses eight
nipples, and litters larger than those indicated by the embryo counts
might be expected.

Table 2
Reproductive status of Oxymycterus rufus*

oj: Total Specimens ¢o¢9

Adult Adult
Month Locality Juv. Non-br. Breed. Juv. Non-br. Breed. Totals
December Canelones 1 1 2 2 1
1 it Z Z 1 a
February Canelones I
Cerro Largo 1 2 1
(6 km SE Melo)
1 2 1 3)
March Cerro Largo 1 y) 2
(20 km SE Melo)
1 2 2 l 6
May Maldonado 2. 1 1 3
Rocha 1 4 3 1 1 6
Soriano 1
l 6 5 2 4 GG) 4.24
September Canelones 2 3
2 4 3 9

*Includes data of A. Langguth (pers. comm. ).

yt)

Fig. 8 A typical autumnal moult pattern found among specimens of Oxymyc-
terus rufus (9, AMNH 206191). The diagrams represent (1) lateral, (2) dor-
sal and (3) ventral views of the melanin deposition on the interior surface of
the skin.

Moult. Because age categories within groups of individuals in adult pelage
were difficult to establish in this species, moult patterns were not traced in
detail. In some specimens narrow lateral bands of new growth from which
hair replacement was progressing toward the mid-dorsum and toward the
mid-venter were noted. But most of these also had additional growth
centres on the dorsum and venter (Fig. 8). On the mid-dorsum an obvious
“‘saddle-back” condition develops and growth proceeds both anteriorly and
posteriorly from this secondary growth centre (Fig. 9). Hair replacement is
completed on the head and rump last. On the head hair grows posteriorly
from the tip of the muzzle and converges with growth progressing anteriorly
from the mid-dorsum. Moult apparently may occur at any season, as indi-
viduals showing some hair replacement have been taken in January, Febru-
ary, March, May, September, October, November, and December. Moult
into adult winter pelage seemingly begins in late March and continues
through May.

Fig. 9

“Saddle-back” stage of
moult development in
Oxymycterus rufus
(4, AMNH 206182)
as demonstrated by
dorsal (1) and ventral
(2) views of the
melanin deposition on
the interior surface

of the skin.

26

Scapteromys tumidus (Waterhouse)
Swamp rat, rata de banado

Specimens: Total 123; Canelones, Arroyo Tropa Vieja, 15 oo’, 18 99;
Cerro Largo, 6 km SE Melo, 2 oo; Maldonado, Barra del Arroyo Mal-
donado, 9 km ENE Punta del Este, 7 oc’, 9 99; Montevideo, Rio
Santa Lucia, 1 km SE Santiago Vasquez, 2 oc’, 5 99; Rocha, 22 km
SE Lascano, 2 6‘; Soriano, 3 km E Cardona, 21 60, 24 99; Treinta
y Tres, 16 km SSW Boca del Rio Tacuari, 8 oo’, 10 99.

Prior to 1950 Scapteromys tumidus was represented from Uruguay by
only a few specimens. These included the type from Maldonado (Water-
house, 1839:15), a specimen obtained by Aplin (1894:308) and at least
one taken in Soriano in 1924 by W. E. Budin (Hershkovitz, 1966:98).
Sanborn (1929:158) trapped extensively in marshy places and along water-
ways, but did not obtain this species.

Ecology. Often Scapteromys tumidus was found in marshy or low places
where the dominant plants were Eryngium sp. and Cortaderia selloana and
the ground between the plants was soggy. In many coastal areas regular
inundation occurred. In Argentina in the delta of the Parana and at Punta
Lara (Buenos Aires Province) Massoia and Fornes (1964:291) found
Scapteromys tumidus in frequently flooded places dominated by stands of
Cortaderia sp. and the false cortaderia (Scirpus giganteus). Along the Laguna
Merin in Treinta y Tres, Eryngium sp. grew in isolated patches from 2 m
to more than 15 m in diameter and Scapteromys tumidus, Akodon obscurus
and Cavia pamparum were common there. At this place Scapteromys tumi-
dus was also taken in a swampy area within woodland dominated by Salix
humboldtiana. Cavia pamparum, Akodon obscurus, Oxymycterus rufus and
Oryzomys flavescens were the commonest associates of Scapteromys tumidus
in moderately wet places. In the wettest areas, as in parts of the salt marsh
along the Rio Santa Lucia in the Department of Montevideo, only Akodon
obscurus occurred with Scapteromys. In Soriano, in a wet meadow with
bunch grass 1 m tall and an understory of Bermuda grass (Cynodon
dactylon), Scapteromys tumidus was taken with Oxymycterus rufus and
Akodon azarae. Scapteromys tumidus was less abundant, or absent, in those
wet meadows and small marshes in which a variety of grasses and com-
posites (such as Baccharis sp.) predominated, as at Sierra de Vaz, Cerro
Largo, and in Rocha, near Lascano.

Swamp rats seem not to make runways of their own, but rather follow
any open route through the vegetation or frequently use the abundant run-
ways of Cavia pamparum. Near the Laguna Merin, swamp rats moved
between clumps of taller vegetation in closely grazed pasture without run-
ways. Swamp rats dig to obtain food, but there is no evidence that they
excavate burrows. We searched at various collecting sites in Uruguay where
Scapteromys tumidus was common and found no burrows attributable to
these swamp rats. Massoia and Fornes (1964:293) found beneath matted
vegetation shallow depressions containing nestling young which might have
been excavated.

27

Vocalization. Vocalizations are often heard when animals are in distress.
We heard high pitched squeals when these rats were first caught in traps.
Massoia and Fornes (1964:293) reported a variety of vocalizations, includ-
ing a sharp cry, phonetically chi-chi, emitted by nestling young, a sonorous
adult call, ro-ro-ro, not unlike one of the songs of the wren, Troglodytes
aedon, and a sharp, clicking cry of excitement, crec-crec-crec.

Time of Activity. Although most individuals were trapped at dusk or in the
night, 30% of all specimens were obtained between 7:00 a.m. and 5:00
p.m. Diurnal activity may be correlated with high population levels or in-
creased nutritional needs. The majority of animals taken in the daytime were
juveniles, subadults, or pregnant females in contrast to night catches com-
prised predominantly of adult males. Massoia (1961:124) found Scapter-
omys to be nocturnal and crepuscular in Argentina. When traps were
checked by us on an hourly basis, from 6:00 p.m. to 1:00 p.m., 90% of
the individuals were found to be trapped between 6:00 p.m. and 10:00 p.m.
A few rats were caught at dawn.

Parasites. Only one individual (AMNH 206314), a male with scrotal testes,
demonstrated possible cuterebrid infection. A circular area of scar tissue,
with a growth of new hair, 1 mm in length, was located on the right side
of the throat. Cysts of unidentified cestodes, often several per individual,
were noted in the livers of about 40% of the swamp rats so examined. A
single nematode worm (Pharyngodon sp.) was found in the stomach of one
female (AMNH 206261 ).

Niche. Scapteromys tumidus is of moderate to large size. It is semiaquatic,
occupying marshy places and frequently inundated areas, where it eats
mostly invertebrates and some plant material. Scapteromys tumidus is as
aquatic as the primarily herbivorous species of Holochilus often found in
similar habitats. Oxymycterus rufus, which is primarily insectivorous, is like-
wise found in wet meadows and marshes. However, it lacks the water-
proofed pelage of Scapteromys tumidus and swims only when necessary
(Massoia and Fornes, 1964:294).

Excellent swimming ability and agility in climbing contribute to the
success of Scapteromys tumidus in frequently inundated areas. The predom-
inately insectivorous mouse Akodon obscurus, a smaller ecological counter-
part of S. tumidus, also occurs in frequently inundated places and swims
well. A. obscurus is more distinctly diurnal than S. tumidus, and this differ-
ence in activity periodism may reduce competition.

Food Habits. Scapteromys tumidus, like Oxymycterus rufus, has a largely
insectivorous diet. Stomach contents of 11 individuals obtained in April and
May were 85% invertebrate. The other 15% included Anthony’s bait used
in trapping and plant material—mostly the skins of an unidentified fruit and
the seeds of Raphanus sp. (Cruciferae). In Argentina, the species demon-
strates a preference for oligochaetes (Massoia and Fornes, 1964:295). Only
two of 18 stomachs of Uruguayan specimens contained parts of oligo-
chaetes; a third stomach contained two leeches. In all stomachs analyzed
Coleoptera, especially scarabiid beetles, predominated. In captivity a
Scapteromys tumidus readily accepted slices of carrot and pieces of cheese.

oe)

The contents of the stomachs of the 11 individuals mentioned above, on
the basis of the per cent of stomachs containing the item, included the
following: Coleoptera (Scarabaeidae [Melolonthinae], Chrysomelidae, Car-
abidae, Elateridae) 100.0%; Hymenoptera (Formicidae) 45.5% ; Orthop-
tera (Gryllidae, Acrididae 27.2% ; Lepidoptera (Noctuidae, Pyralididae, un-
identified caterpillars) 27.2% ; Diptera (Sciaridae, Tipulidae [Limoniinae])
27.2% ; Hemiptera (Coreidae) 18.1%; Isoptera (Termitidae) 9.1%; Ara-
nea, Oligochaeta, Hirudinea, Gastropoda (snail) 27.2%; plants, Cruci-
ferae (Raphanus sp.), unidentified berries 27.2%.

Although resembling Oxymycterus rufus in dietary habits, Scapteromys
tumidus lacks the distinctively bilobular stomach with a glandularized fundic
region possessed by the former species. However, the stomach of S. tumidus
is characterized by some muscularization in the cardiac region and there is
some thickening of the fundic region that may be glandular tissue.

Breeding. Males with scrotal testes at least 12 mm long had abundant
mature sperm both in the seminiferous tubules and in the epididymides.
Males with testes of at least minimum breeding size have been found in
every month of the year except January, June and July, months for which
no males were available (Table 3). In April and May, 97% of all adult
males had testes of breeding size. Pregnant females were taken in December
1962, April and May 1963 by the expedition, and in October and Novem-
ber 1963 by Sr. Alfredo Langguth (pers. comm.).

Most specimens in breeding condition could be satisfactorily placed in
one of the adult age categories (see the following section on moult). One
individual, a female (AMNH 206251) with teeth worn nearly to the roots,
was somewhat smaller than normal adult size.

Of the embryos observed, 25 were implanted in the left uterine horn and
19 in the right. The average number of embryos and/or young was four
(3-5). This seems low in view of the fact that females possess four pairs
of nipples. In a non-pregrant female (AMNH 206261 ) with an open vagina,
obtained on 26 April, the nipples were pink and slightly swollen and the
lacteal tissue had hypertrophied; however, no milk could be expressed.

Moult. For convenience, I recognize four age-classes of Scapteromys tumi-
dus based, as with Oryzomys delticola, on the wear of the molars, especially
M®*, and on the combined body and tail length. The classes are:

Class 1, juvenile—M2? not erupted and other molars unworn; pelage
significantly darker on both dorsum and venter than in older age-classes;
total length, less than 225 mm.

Class 2, subadult—M? erupted, all molars slightly worn; pelage as in
adults; total lengths from 226 mm through 299 mm.

Class 3, adult—M? moderately worn, other molars with cusp pattern so
worn as to be partly obscured; the adult pelage is described elsewhere;
total lengths from 300 mm through 325 mm.

Class 4, old adult—Molars heavily worn, cusp pattern completely obliter-
ated, the occlusal surface often being smoothly concave; pelage as in adults,
but somewhat more grizzled; total length, 326 mm and greater.

29

Table 3
Reproductive status of Scapteromys tumidus

oc’ Total Specimens 99

Adult Adult
Month Locality Juv. Non-br. Breed. Juv. Non-br. Breed. Totals
December Canelones 3 2 5 4 5 8
1962 3 2 5 4 5 8 D2)
February* Cerro Largo 2
(6 km SE Melo)
Canelones i
2 1 3
March Canelones 3 1
Treinta y Tres 2
3 1 2 6
April Montevideo 1 4
Treinta y Tres 2 5 8 1
1 2 5 12 1 Di
May Maldonado ff 1 i
Montevideo 1
Rocha 1 1
Soriano 1 it 8 9 14 2
1 19 10 Da 5 65
August San José 1 3
1 3 4
September Canelones 3 2 if
3 D4 a 12
October Canelones 1
Colonia 1 Z 1 1
1 3 1 1 6
November Canelones yp, 1
Montevideo 1 1
3 1 1 5
149

* All remaining data for 1963.

30

20

— —
oe) o1

NUMBER OF MICE
Ol

[J — moult
[_]—no moult
1 —juvenile
2 —subadult
3 —adult

4 —old adult

1234. 1234 11234 1234 1234 +« «2421234 ~°«241234 + «241234 «1234

December February March April May August September October November

Fig. 10 Moult in Scapteromys tumidus by age classes between December 1962
and November 1963. Analysis based on 111 skins. Data from A. Langguth.

Aging was sometimes difficult in the case of certain gravid females as
some individuals smaller than Class 4 showed excessive wear of the molars.

Most swamp rats collected between December 1962 and May 1963 (less
January 1963 for which no specimens are available) showed some evidence
of moult. Some Scapteromys collected in August and September of 1963
were also in the moult in contrast to small samples taken in October and
November of that year (A. Langguth, pers. comm.). Fig. 10 summarizes
data on moult available for the period December 1962 through November
1963. Specimens showing moult collected in January, June, October and
November were examined in the collection of Museo de Historia Natural in
Montevideo. Thus it would appear that moult is a year-round phenomenon.
Peaks of moulting activity were in autumn (April and May) and again in
late spring (December).

The autumn moult may require as much as a month for completion, as
12 of 20 adults (60.0% ) obtained in early April around the Laguna Merin
were in early stages, whereas 21 of 52 (40.4% ) of the individuals collected
in May were in the middle stages of moult or beyond, and 47 of 52
(93.7% ) at this time were in some stage of moult.

Pregnancy and moult, both physiologically taxing, might tend to occur at
different times. However, in the sample collected in December (summer),
three pregnant females were moulting. A simple X? analysis of these data
indicates that moult and pregnancy are independent (X? = 2.82 [ns],
df = 1). Furthermore, in the autumn, all females whether pregnant or not
moult into winter plumage.

None of the smaller young animals were moulting. Post-juvenile moult
was noted in a number of somewhat older individuals. Growth of new hair
begins at a pair of ventro-lateral centres that overlap the junction between
the grayish-white venter and the blackish hair of the upper parts. Initially
these areas comprise single narrow bands on each side of the body between
the hind limb and forelimb, but growth soon spreads posteriorly onto the
flanks, anteriorly beyond the forelimbs, and downward onto the whitish
throat. Growth then proceeds from these areas onto the belly, passing
ventro-medially to converge mid-ventrally. Shortly thereafter, waves of

a1

Fig. 11 The sequence of post-juvenile moult in Scapteromys tumidus as
shown in dorsal and lateral exterior views (1, 2), dorsal and ventral views (3)
and dorsal views (4, 5).

moult that have been spreading up the sides onto the back fuse mid-dorsally
forming a “saddle” stage. At this time growth has spread anteriorly on the
belly and soon covers the entire throat region. Also the postero-ventral
growth appears to be proceeding more slowly. When moult on the venter is
complete, the grayish-white juvenile hairs have been replaced by the
yellowish-white hairs characteristic of adults.

Apparently, dorsal moult lags behind that on the ventral surface. After
the formation of the “saddle” stage, growth of new hair proceeds both
anteriorly and posteriorly from the “saddle,” but more rapidly toward the
head. At the same time the lateral zones of growth have progressed anter-
iorly and then upward toward the mid-line, fusing on the tip of the muzzle.
From this place, moult spreads posteriorly to a zone between the ears, where
it converges with the wave from the dorsal surface of the thorax. The post-
juvenile moult, in its terminal stages, shows moult in a narrow arc above
the tail and in a patch on the crown. The sequence of moult just described
is shown in Fig. 11.

Adults undergo moult resembling the post-juvenile pattern, except that
additional growth centres may be present (Fig. 12). The lateral growth
centres may fuse anteriorly on the dorsal mid-line in the shoulder region
and shortly thereafter fuse posteriorly entrapping briefly a zone of old fur.
This contrasts with the “saddle” usually developed in the post-juvenile moult.

Moult in old adults follows no apparent pattern. In fact, moult in the
oldest individuals may be initiated at about the same time at a number of
scattered sites (Fig. 13).

a2

Fig. 12 Autumnal moult in an adult Scapteromys tumidus (¢?, AMNH
206321) as demonstrated by the internal pattern of melanin deposition on the
dorsal (1) and ventral (2) surfaces.

Fig. 13. Autumnal moult in an old adult Scapteromys tumidus ( 6, AMNH
206267) as demonstrated by the internal pattern of melanin deposition on the
dorsal (1) and ventral (2) surfaces.

a3

Calomys laucha laucha (Desmarest)
Vesper mouse, laucha

Specimens: Total 15; Lavalleja, 12 km WSW Zapican, 7 oo’, 6 99;
Tacuarembo, 40 km NW Tacuarembo, o; Treinta y Tres, 16 km SSW
Boca del Rio Tacuari, 9.

Ecology. Sanborn (1929:157) obtained three of these mice in four months
of collecting in Uruguay. The laucha experiences periodic dramatic popu-
lation increases (ratadas; Hershkovitz, 1962:45); thus our meagre sample
may reflect a low ebb in the population cycle. The laucha is usually found
in open grassland, rocky hillsides and edge situations but is also taken
occasionally in the vicinity of marshes and swamps. Dwellings and out-
buildings are likewise frequented (Hershkovitz, 1962:41). We often saw
Calomys laucha at night in overgrazed rocky pasture. Reithrodon physodes
was the typical associate in such places.

Calomys laucha does not make runways and rarely utilizes those of other
species. Nests have been found under boards and rocks, in crevices in the
ground and even in trees, often high above the ground (Massoia, 1961:
120). These mice have even sought refuge in dried skulls of livestock
plentiful in the rangeland (Vaz Ferreira, 1960:67).

Calomys laucha climbs well and on the ground often hops on its hind
legs in the manner of Dipodomys (Massoia, 1961:126). A female (AMNH
206345), caught at night by hand in overgrazed pasture, alternated between
hopping in a zig-zag course and crouching motionlessly to avoid capture.

Fig. 14

Advanced autumnal moult
in Calomys laucha

(6, AMNE 206338).
Diagram 1 shows exterior
growth of hair with the
arrows indicating direction.
The smaller diagrams show
corresponding interior
melanin deposition dorsally
(2) and ventrally (3).

34

Food Habits. The laucha is predominantly herbivorous, but the stomach of
one individual (AMNH 206336) contained unidentified beetle fragments.

Breeding. A female (AMNH 206345), taken 28 March 1963 in Treinta y
Tres, contained four embryos (CR = 10 mm). Six females taken 28 April
in Lavalleja had four to eight embryos (average six) measuring CR = 1-10
mm. The total of seven females had 24 embryos in the right uterine horn
and 18 in the left. Testicular biopsies demonstrated that scrotal testes meas-
uring 6 x 4 mm contained abundant mature sperm. Males in breeding con-
dition were obtained on 5 February and 28 April 1963. These data suggest
that in Uruguay in summer and autumn, the breeding season of Calomys
laucha is protracted. The gestation period has been calculated at 25 days
(Hershkovitz, 1962:148).

Moult. Hershkovitz (1962:147) suggested that moult in this species is
seasonal. Examples taken in early February and late March were not in the
moult. The mouse obtained in February was in worn summer pelage. The
12 mice obtained in late April were in approximately the same advanced
stage of autumn moult (Fig. 14).

Although extensive delineation of moult progression was difficult, the
moult basically resembled the pattern of other Uruguayan cricetines. The
April specimens demonstrated extensive moult on the dorsum. Moult spreads
from lateral growth centres toward the dorsal and ventral midlines. After
convergence at the mid-dorsum, hair replacement apparently proceeds
rapidly posteriorly and also anteriorly to a place between the ears, where
forward moving moult converges with growth sweeping posteriorly and also
anteriorly from the tip of the muzzle.

Reithrodon physodes physodes Olfers
Coney rat, rata conejo

Specimens: Total 16; Artigas, 6 km NNW Belén, 9; Cerro Largo, 10 km
NW Paso del Dragon, —%; Rio Negro, Arroyo Negro, 15 km S Paysandu,
4 99; Rocha, 22 km SE Lascano, 2 oo’, 9; Soriano, 3 km E Cardona,
o'; Tacuarembo, c. 10 km N Tacuarembo, <’, 9; 40 km NW Tacuar-
embo, 2 99; Treinta y Tres, Rio Olimar Chico, 25 km WSW Treinta
VoAPTCS. 2 oicn

The coney rat, Reithrodon physodes, is one of three sigmodont rodents
occurring in Uruguay and the only cricetine restricted to grassland. Only
16 individuals were obtained by us in six months, most by shooting at night.
Sanborn (1929:158) found coney rats widespread but uncommon. This
mouse is an ecological equivalent of the hares (Lepus) of the temperate
grasslands of North America, but resembles both rabbits (Sy/vilagus) and the
pika (Ochotona princeps) in various aspects of behaviour.

Ecology. Reithrodon physodes is usually found in overgrazed pasture, among
rocky outcrops or on well-drained slopes with scanty vegetation. Dominant
plants in typical Reithrodon habitat are crab grass (Digitaria sanguinalis);
Bermuda grass (Cynodon dactylon); macachin (Oxalis sp.); chirca (Eupa-
torium buniifolium); and Baccharis melastomaefolia. In Rocha coney rats

DD

lived in a marshy area in a stone wall built on slightly higher ground. This
stone wall extended through a frequently inundated part of the marsh and
provided a permanently dry refuge. Reithrodon physodes has also been
taken in wheatfields and along the sandy beaches of the coast (Sanborn,
1929:158). Coney rats dig burrows in soil varying from sandy to hard-
baked clay, or utilize with or without modification abandoned burrows of
Dasypus hybridus and other fossorial mammals or natural crevices and holes
among rocks. Open grassland or sheltered sites beneath rocks may be
excavated. In Artigas this mouse has been found living in cracks and crevices
in the sun-baked mud of a road bed (Praderi and Luzardo, 1958:30).

The nearness of burrows to each other suggests that the coney rat may be
variously gregarious or solitary. A colony was found in an overgrazed
pasture on the western slope of a low hill on the Estancia Santa Elisa in
Rio Negro. The burrow entrances averaged 5 cm in diameter. From each a
passageway descended vertically an average of 25 cm, at which depth the
tunnels became level and followed a course parallel to the surface. Active
systems were marked by fresh cuttings and droppings near the entrances.
Conspicuous plants in the vicinity of the burrows were chirca, crabgrass
and Oxalis sp. The crabgrass had been grazed to the root tops and the
chirca growth was sparse. No burrows were traced by us at this site, but
we did examine a single burrow 200 m north where a Reithrodon previously
had been trapped. This burrow was 1 m in length and 18 cm in depth, and
terminated blindly in an unlined chamber measuring 14 x 12 cm. The
single tunnel, 5 cm in width, turned once near the vertical part of the
passage. This may have been an escape burrow as no cuttings, nesting
material, or faeces were found.

In Treinta y Tres on a rocky slope above the Rio Olimar Chico several
extensive burrow systems were excavated and the immediate vicinity of each
studied. Individual mice not only had dug tunnels but had incorporated
natural crevices under rocks and abandoned armadillo burrows into their
own burrow systems. All burrows were less than 2 m in length and had two
entrances. They were occasionally branched and tortuous, with the direction
of the passageways being modified by the position of buried rocks. The
burrow entrances were from 3 to 5 cm in diameter. The tunnels were 5 to
7 cm in diameter and were 10 to 25 cm beneath the surface. Some burrows
contained central, oval chambers up to 30 cm in diameter. In one such
chamber we found a nest platform composed of a fine dry grass, the stems
of a coarser grass cut into pieces 4 cm in length, and down of Eupatorium
buniifolium. There were no faeces in the burrows but plant debris, possibly
refuse from feeding, was often found concentrated in one or two places near
the entrances. Defecation sites were just outside of the entrances of the
burrows or within a radius of 9 m from them. No distinct runways led to
any of these burrows.

Predators. Few data are available about the predators of this species. A
skull was found in an owl pellet in Artigas. Occasional burrows showed
evidence of partial excavation, possibly by a fox.

36

Behaviour. The coney rat and the laucha were the most difficult of the
Uruguayan cricetines to trap. At one site in Rio Negro no mice were caught
in traps placed in pairs at the entrances of 36 burrows. Most Reithrodon
taken were shot at night. Curiosity was often the undoing of these mice
when hunted. On several occasions individuals fled into burrows when fright-
ened, only to reappear at the entrances within a few seconds. Sometimes
these animals crawled completely out of the burrows and sat up on their
haunches while illuminated in the beam of a flashlight. One mouse was run
down and caught by hand after a short chase, during which the animal
utilized zig-zag tactics in an effort to avoid capture. However, Hershkovitz
(1955:639) indicates that sigmodont rodents may be found abroad both
day and night. In our experience Reithrodon physodes was strictly nocturn-
al; however, this may simply have reflected a low level of population coinci-
dent with our work in the country.

Food Habits. Reithrodon physodes, a pastoral rodent, has sigmodont denti-
tion specialized for an herbivorous diet. The single-chambered stomach is
somewhat sacculated in the fundic region. An enlarged caecum is present
at the junction of the large and small intestines.

Grasses, other plants with tuberous rhizomes, and roots are eaten.
Corms of Oxalis sp. and rhizomes and roots of Digitaria sp. appear to be
favoured. Countless discarded outer coverings from Oxalis corms were
found in the immediate vicinity of burrow entrances, at feeding shelters, and
at specific places within burrows.

Coney rats either take food into the main burrow or feed in specific
shelters away from the main burrow. In Treinta y Tres a feeding shelter was
found that comprised a large flat rock beneath an overhanging embankment.
The surface of the rock was littered with faeces and plant fragments. No
food stores were found within the burrows.

Breeding. Only two pregnant females were collected. One (AMNH 206350)
taken in January in Rio Negro contained four embryos (CR =20 mm) and
the other (AMNH 206353) taken in May had three (CR= 23 mm). A
lactating female (AMNH 206349) was shot in January also in Rio Negro
and another with four nestling young (CR=80 mm) was taken 7 October
1963 in Canelones (A. Langguth, pers. comm. ).

Males with enlarged scrotal testes measuring at least 8 x 5 mm containing
abundant mature sperm and thus thought to be capable of breeding were
collected in January, March, April and May. In general, sigmodont rodents
breed throughout the year (Hershkovitz, 1955:639) and the temporal dis-
tribution of the breeding data for Reithrodon in Uruguay suggests that the
season is long.

Moult. One of three adults trapped by Sr. Alfredo Langguth at Tropa Vieja,
Canelones, 24 October 1963, showed extensive moult. Two other individuals
obtained there on that date were not moulting. Only one of 10 mice taken
between January and March, a subadult female (AMNH 206348; Fig. 15)
showed any significant moult. Five adult Reithrodon physodes obtained in
May were in various stages of autumn moult. Among these individuals the
heaviest moult was found in a pregnant female (AMNH 206353; Fig. 16).

oe

Fig. 15 Post-juvenile moult in a
subadult ¢@ Reithrodon physodes
\ (AMNH 206348) as shown in a

dorsal exterior view (arrows show
direction of hair growth).

Fig. 16 Autumnal moult in an adult @ Reithrodon physodes (AMNH
206353) as in a dorsal exterior view (1; arrows show direction of hair growth)
and the corresponding interior dorsal pattern (2) of melanin deposition. Also
shown is the interior ventral pattern (3) of melanin deposition.

In the subadult individual mentioned above the ventral pattern of moult
progression was not determinable. The skin of the entire dorsum posterior
to the head was darkly pigmented. In this zone growth spread from the sides
of the thorax toward the dorsal mid-line and fused; whence growth pro-
ceeded both toward the head and toward the rump, with the “saddle” de-
veloping just anterior to the rump. A characteristic of post-juvenile moult in
this species was widespread melanin deposition in the epidermis and dermis,
in contrast with melanin concentration in distinctive patterns typical of the
adults examined.

38

Holochilus brasiliensis vulpinus (Brants)
Brazilian marsh rat, rata de banado

Specimens: Total 21, Canelones, Bafiado de Tropa Vieja, o, 9; Lavalleja,
12 km WSW Zapican, 9; Soriano, 3 km E Cardona, 8 oo’, 7 99;
Tacuarembo, Rio Negro, Isla Sanchez Chica, c. 16 km NNW San Jorgé,

ZG, 2.

Ecology. This widely distributed marsh rat often has been identified as a
principal species involved in irregular population eruptions (ratadas) in
Brazil (Hershkovitz, 1955), Guyana (Twigg, 1962), and Argentina
(Llanos, 1944). No ratadas have been recorded in Uruguay. However,
irregular population fluctuations may occur. Sanborn (1929:158) consid-
ered this species common in southern Uruguay, whereas in the same general
area a few years later Devincenzi (1935:71) regarded it as rare. During our
visits to some localities in the west, especially in Soriano, this rat was found
to be locally abundant. Holochilus brasiliensis has a broader ecological tol-
erance than H. magnus and often occurs in the most open limnic situations.

In extreme southern Tacuaremb6 the waters of the reservoir of the Rio
Negro have created islands by isolating tops of higher hills. On one such
island, Sanchez Chica, with an area of approximately 50 hectares, three rats
were obtained from 45 nests found in seven trees. Conspicuous vegetation
on the island was crab grass (Digitaria sanguinalis), with scattered clumps
of taller grasses (Andropogon sp., Paspalum sp.) and open thorn woodland
dominated by Scutia buxifolia, Acacia caven, and Xilosoma warburgi.
A small flock of sheep had heavily grazed the shorter grasses.

In Soriano Holochilus brasiliensis was obtained in the vicinity of a small
stream. Reeds and Eryngium sp. were found on the banks, and Eichhornia
sp. covered part of the surface of the water. Narrow passages through the
shorter emergent plants gave evidence of Holochilus brasiliensis activity.

Holochilus brasiliensis constructs ovoid nests of leaves, sticks and grasses,
averaging 22 x 27 cm in diameter. Nests examined on Sanchez Chica Island
were 2 to 3 m above the ground in terminal forks of branches and among
tangles of vines growing in the trees Scutia buxifolia and Xilosoma war-
burgi. The unlined nests were made of sticks and leaves of these trees.
Nests lined with fine plant material distinct from an outer shell of coarser
plant parts have been described (Twigg, 1962:371).

On Sanchez Chica as many as 11 nests, often only a metre apart, were
found in a single tree. Three of 20 nests contained one rat apiece. Marsh
rats have been reported to drop to the ground when the nest is disturbed
(Twigg, 1962:371) but in my experience agilely climbed higher in the
trees. Since most of the trees with nests were more than 20 m from the
water’s edge, and cover was lacking between them and the water, it is under-
standable why the rats remained in the trees when disturbed.

Food Habits. Holochilus brasiliensis possesses high-crowned molars with a
complex occlusal surface that facilitates grinding of plant materials, and
the finely macerated plant material steeps in an enormous caecum. The cae-
cum from an adult male (AMNH 206385) measured 67.8 mm in length.

39

Although we found that the diet of Holochilus brasiliensis consists primarily
of the more tender parts of aquatic, riparian and domestic plants as rice and
cane, Twigg (1962:369) reported finding a small percentage of remains of
snails and beetles in stomachs.

Breeding. Two of five adult females taken in Soriano in late May were
breeding. One female (AMNH 206374) was lactating. The other (AMNH
206369) contained four embryos measuring CR=10 mm. Three of five
males obtained in late May in Soriano had scrotal testes measuring 15 x 10
mm or larger. The testes of the other two adult males from this place
averaged 9 x 6 mm. A pregnant female (AMNH 206391) with two de-
veloping embryos measuring CR=14 mm and resorbing two embryos was
shot 10 April 1963 in extreme southern Tacuarembo on Sanchez Chica
Island. A male (AMNH 206390) with scrotal testes measuring 17 x 11 mm
was shot 10 April at the same locality. The presence of a significant propor-
tion of non-breeding adults among the May collected sample suggests a
reduction in breeding activity associated with the onset of winter.

Moult. Only three of 14 individuals preserved as study skins available for
April and May were not moulting. A juvenile (AMNH 206367) taken
April 26 showed no moult. Two apparent subadults less than 310 mm in
length demonstrated formation of a “saddle-back” condition. Typically,
hair grew both toward the mid-line and anteriorly and posteriorly from the
mid-dorsum. On the tip of the muzzle, zones progressing laterally turned
toward the mid-line and growth then progressed posteriorly. Young adults
between 350 mm and 400 mm in total length had parallel latero-dorsal
growth centres from which hair replacement was progressing upward to-
ward a zone of convergence on the mid-dorsum. Older (larger) adults
showed several parallel dorsal and lateral growth centres (Fig. 17).

Fig. 17

Autumnal moult in

an adult Holochilus
brasiliensis ( @, AMNH
206390). Shown is a
dorsal exterior view (1)
in which arrows indicate
the direction of hair growth
in the region of active
moult. Also shown are the
corresponding views of
melanin deposition on the
interior dorsal (2) and
ventral (3) surfaces.

40

Holochilus magnus Hershkovitz
Hershkovitz’s marsh rat, rata de banado grande

Specimens: Total 11; Canelones, Bafiado de Tropa Vieja, 2 do’, 92;
Soriano, 3 km E Cardona, 2 607, 4 99; Treinta y Tres, Rio Olimar
Chico, 25 km WSW Treinta y Tres, <’, 9.

This large cricetine, taken by C. C. Sanborn in Uruguay in 1926-27, was
undetected among a series of specimens identified as Holochilus brasiliensis
until its status as a distinct species was recognized by Hershkovitz (1955).
The distribution of H. magnus still is poorly known. In Uruguay it 1s seem-
ingly less abundant than H. brasiliensis.

Ecology. Holochilus magnus was taken in Treinta y Tres along a stream 90
cm wide, 15 cm deep and with banks 180 cm high. Low thorn woodland with
a closed canopy grew to the edge of the stream. The understory consisted
of tall grasses and ferns with fronds 120 cm in length, many of which over-
hung the stream. In Soriano several rats were obtained where tall bunch
grass, reeds and Eryngium sp. grew profusely along a stream 1 m in width.
Scapteromys tumidus and H. brasiliensis were also taken at this locality, but
the latter was found in vegetation less dense that that preferred by H. mag-
nus. Holochilus magnus was also obtained deep within the extensive Banado
of Tropa Vieja in Canelones.

Parasites. A female nematode (Capillaria sp.) containing numerous eggs was
removed from the caecum of a male H. magnus (AMNH 206381) taken in
Soriano on 25 May 1963.

Food Habits. The dentition of Holochilus magnus is more primitive than
that of H. brasiliensis, and the molars are transitional between the “browsing-
crushing and specialized grazing types” (Hershkovitz, 1955:654). Any dif-
ferences in food preference existing between these two marsh rats are not
readily apparent.

Stomachs examined contained primarily macerated greenish plant mater-
ial. Hershkovitz (1955:654) has suggested that Holochilus magnus might
have been the mollusc-eating rat mentioned by Burmeister (1879:211-212).
The only invertebrate remains found by us were of a spider and a cricket
(Gryllidae), and invertebrates comprised less than 5% of the total contents
of three stomachs examined.

Breeding. A lactating female (AMNH 206392) with three fresh placental
scars was obtained 12 April 1963, in Treinta y Tres. Another lactating
female was taken in Soriano on 25 May 1963. An adult female, taken on
10 October 1963 in Canelones, was not pregnant (A. Langguth, pers.
comm.).

In males, testes measuring 19 x 12 mm contained abundant mature
sperm. Males in breeding condition were collected in October, November
(A. Langguth, pers. comm.) and in April and May. This species appears to
have a protracted breeding season in Uruguay, extending at least from mid-
spring to early autumn.

41

Fig. 18 Autumnal moult in an adult Holochilus magnus (9, AMNH
206392). Shown are dorsal (1) and ventral (2) exterior views (arrows indicate
direction of hair growth) and corresponding interior dorsal (3) and ventral (4)
patterns of melanin deposition.

Moult. One juvenile (AMNH 206366) and two subadults (AMNH 206364,
AMNH 206365) were obtained in December and January and the pelage
was short and worn in all three. On the backs of the subadults were small
patches of moult.

Three of five adults caught in April and May were in moult, apparently
developing from several centres on the dorsum and on the venter. The in-
ternal pattern of pigment deposition closely resembled that of H. brasiliensis.
Parallel bands from which growth proceeded both upward toward the mid-
line and downward toward the sides of the body anteriorly and posteriorly
were characteristic (Fig. 18).

Mus musculus ssp. Linnaeus
House mouse, minero

Specimens: Total 9; Lavalleja, 12 km WSW Zapican, 9; Maldonado, 17
km NE Aigua, 3 oc’, 2°99, ? sex? Soriano, 3 kin E Cardona, G's; Rio
Negro, Arroyo Negro, 15 km S Paysandu, <.

House mice have long been common around estancia buildings and in
towns in Uruguay (Arechavaleta, 1887:419; Aplin, 1894:308; Sanborn,
1929:161). We caught one by hand in a granary in Rio Negro on the
Estancia Santa Elisa. Others were obtained under a shock of corn with
Calomys laucha at an estancia in Treinta y Tres and also near farm build-
ings in Maldonado.

42

Cavia pamparum Thomas
Guinea pig, aperea

Specimens: Total 36, Canelones, Bafiado de Tropa Vieja, 2 99, ? sex;
Cerro Largo, 6 km SE Melo, &; Lavalleja, 12 km WSW Zapican, 3
dc’, 4 99; Maldonado, 17 km NE Aigua, 3 6’, 3 99; Barra del Arroyo
Maldonado, 9 km ENE Punta del Este, 2 66%; Rocha, 22 km SE
Lascano, 2 ‘0’, 9; 10 km E Velazques, 9; Treinta y Tres, Boca del
Rio Tacuari, 9; 16 km SSW Boca del Rio Tacuari, 7 o’o’, 4 99, ? sex.

Sanborn (1929:159) considered Cavia pamparum the most common
rodent in Uruguay. Guinea pigs are often so abundant locally that they
severely deplete grasses and other plants.

Ecology. Cavia pamparum does not excavate burrows but constructs a maze
of interconnecting surface tunnels and runs, 8 to 12 cm in diameter. These
pass through even the densest clumps of Cortaderia selloana and Eryngium
sp. Earth, bared by intensive use, is exposed over much of the floor of a
typical runway, but piles of the characteristic bean-shaped faecal pellets
(averaging 25 mm in length) and plant cuttings are to be found at irregular
intervals. Scapteromys tumidus, Oxymcterus rufus, Oryzomys flavescens,
Akodon azarae, and Akodon obscurus make frequent use of guinea pig
runways.

Cavia pamparum may forage in the open away from tunnels but typically
remains close to shelter of some sort. If surprised in the open, individuals
may enter burrows, natural crevices, or even holes under buildings (Aplin,
1894:309).

Guinea pigs were not attracted by bait and hence were exceedingly diffi-
cult to trap. As a consequence, traps rarely killed these rodents, but a male
(AMNH 206420) weighing 367 gm was caught by the muzzle in a Museum
Special trap and apparently died from the blow when the trap was sprung
shut. Best trapping results were obtained with #0 and #1 steel traps placed
within runways.

Predators. While baiting a line of snap traps in a small marsh near Melo,
Thomas J. McIntyre witnessed a hunting sequence involving a grison
(Galictis cuja) and a guinea pig (Cavia pamparum) on a hill adjacent to his
position. As the guinea pig moved up the slope among closely cropped
clumps of bunch grass a grison suddenly emerged from the edge of a patch
of Eryngium sp. bordering the marsh proper. The grison, apparently search-
ing for scent by swinging its head in an arc close to the ground, described a
zig-zag course as it trotted up the hill. When the fresh trail of the guinea
pig was crossed the grison quickened its pace and began to move in the direc-
tion of the potential prey. At this juncture neither the grison nor the Cavia
had caught sight of one another. Then the grison, preoccupied with tracking,
actually collided with the guinea pig who then took alarm and fled. A chase
began as the guinea pig, with the grison a scant 15 cm behind, ran farther
up the hill before it turned down the slope in a looping path and raced to-
ward the marsh. The outcome of the pursuit is not known as both animals
entered the Eryngium sp. growth and were lost from view.

43

In addition to the grison, foxes (Dusicyon gymnocercus, Cerdocyon thous)
are important predators (Aplin, 1894:309). In Rocha on 10 May 1963,
the stomach of a Bothrops alternatus (snout-vent length 1115 mm, weight
1.5 kg) contained a large Cavia pamparum which had been swallowed head
first.

Cavia pamparum occasionally is hunted by gauchos for food. Sanborn
(1929:159) reports that the flesh is tasty when roasted.

Food Habits. Cavia pamparum is a grassland herbivore, whose niche is simi-
lar to that of many North American microtines. The diet of this species in-
cludes a variety of plants, as Digitaria sanguinalis, Cynodon dactylon, the
tender parts of the stems of the taller grasses Paspalum sp. and Cortaderia
selloana and the tender basal part of the leaves of Eryngium sp. At Tropa
Vieja in Canelones every plant in a patch of Eryngium sp. 50 m? was partly
eaten.

Breeding. Females with one or two embryos or with post-partum uteri were
collected in March, April and May. Juveniles were trapped only in Decem-
ber. A male (AMNH 206406) obtained on 27 February 1963 near Melo
had conspicuously vascularized scrotal testes measuring 15 x 12 mm that
contained abundant mature sperm in the seminiferous tubules. However,
males taken in April had scrotal testes as large as 20 x 13 mm that contained
few or no mature sperm. Two males (AMNH 206422, 206423) collected
in May had inactive testes measuring 8 x 5 mm and 24 x 14 mm. An adult
female obtained by Langguth (pers. comm.) in August of 1963 was not
pregnant. This guinea pig has a protracted breeding season, beginning per-
haps in early spring (September) and extending until late fall (April or
May). In late autumn the testes of males become inactive.

Moult. The loosely-held guard hairs are continuously lost and replaced and
moult seemingly is not well defined. However, a pregnant female (AMNH
206426) caught on 25 March 1963 had an oblong area of melanin on the
interior of the skin in the interscapular region and externally a fairly exten-
sive growth of new hair.

Hydrochoerus hydrochaeris uruguayensis (C. Ameghino and Rovereto)
Carpincho, capybara

Specimens: Total 15; Canelones, Arroyo Tropa Vieja, ? sex; Cerro Largo,
20 km SE Melo, 9, ? sex; Lavalleja, Rio Cebollati, 9 km S Piraraja,
3 oo’, 3 ? sex; 12 km W Zapican, ? sex; Rio Negro, 15 km S Paysandu,
? sex; Rocha, 22 km SE Lascano, 2 ? sex; Treinta y Tres, 16.km SSW
Boca del Rio Tacuari, 9, ? sex.

The present distribution and abundance of Hydrochoerus hydrochaeris
in Uruguay has been strongly affected by local hunting pressure. In relatively
populous areas near Montevideo, where marshes persist and hunting is
somewhat curtailed, carpinchos still commonly occur. Fresh piles of faeces
were found in the marsh along the Arroyo Tropa Vieja, in Canelones.
Neither the animals themselves nor sign were noted along the Arroyo Negro

44

in Rio Negro. In southwestern Artigas along the Rio Uruguay, only two
individuals were observed in 15 days, and sign were scarce. Praderi and
Luzardo (1958:311) saw several swimming in the Arroyo Tres Cruces
Grande in northern Artigas, but nonetheless considered Hydrochoerus
hydrochaeris to be uncommon in that region. Several stretched hides were
in the possession of gauchos at Infiernillo in Tacuaremb6, but we found no
sign there in the canyon of the Arroyo Tres Cruces. Along the Rio Tacuari,
in Cerro Largo at Sierra de Vaz, capybaras were fairly common, as many
as seven were seen together at one time, and fresh faeces were noted daily
on both banks of this stream. In Treinta y Tres around the Laguna Merin
and along the Rio Olimar Chico in western Treinta y Tres, capybaras were
fairly common and were seen both at night and in the daytime. Along the
Rio Cebollati in Lavalleja, the riparian woodland was laced with pathways
made by these animals, and several individuals were seen daily. In Durazno,
at the reservoir of the Rio Negro, I saw no live animals although one was
shot by hunters during our visit. In Rocha at the Estancia El Sauce capy-
baras were not uncommon in the more extensive tracts of marsh close to
camp.

Ecology. In most localities capybaras inhabited dense to open woodland
near swiftly flowing or sluggish waterways but were also found, as in Rocha,
in marshes where tall grasses provided cover and the surface of the water
was covered by waterlilies (Eichhornia sp.). In a fairly extensive woodland
along the Laguna Merin the dominant tall tree was the Humboldt willow
(Salix humboldtiana) and the principal component of the understory was
sarandi (Cephalanthus sp.) which grew profusely and well beyond the area
dominated by the willows. The zone of sarandi was abundantly penetrated
by intersecting tunnel-like runways made by capybaras, 30 cm or more in
width and more than 38 cm in height. These runways were used by many
other mammals, notably the foxes Dusicyon gymnocercus and Cerdocyon
thous, the wildcat Felis geoffroyi, the skunk Conepatus chinga and the
opossum Didelphis azarae.

Behaviour. Capybaras are primarily nocturnal in parts of Uruguay where
they are hunted, but where they have been little disturbed, as along the
Rio Cebollati in Lavalleja, they may be seen grazing, wallowing in mud or
dozing at various times of the day. Along the Rio Tacuari in Cerro Largo,
they were occasionally heard during the day. Here at the Laguna Merin they
emerged at dark from the woodland to graze in the open pastureland.

Capybaras are hunted by a lone man stalking the animals from a down-
wind position or by several men on horseback with dogs. Hunters try to
keep capybaras from reaching water, into which they readily dive and sub-
merge. All capybaras obtained by us were shot, either in the day or jack-
lighted at night. A female (AMNH 206452) shot in April 1963 at the
Laguna Merin, emitted yowls, whines and screams reminiscent of those of
a large dog in pain.

Capybaras behave variously when approached by man, depending appar-
ently upon whether or not the intruder has been scented. Hearing and

45

vision seem relatively weak. Those that flee immediately emit a characteristic
“woof.” If the capybara has not fled immediately, it first stands alertly and
tests the air with its nose, then begins to move slowly toward water. Sydney
Anderson approached to within 30 m of a capybara before it fled. The
female that I subsequently shot was initially approached to within 13 m
and showed only slight alarm. Merlin Tuttle shot the first of two young
capybaras, lying only 2 m apart and dozing in a mud wallow along the Rio
Cebollati, without causing the second individual to flee. Alarm leading to
flight is an individual response in most instances. One individual in a group
may detect danger and dive into the water, while the others show little
alarm; on other occasions all may be stimulated and take flight simultan-
eously.

Capybaras wallow in muddy areas along stream banks and in quiet back-
waters. A wallow was found among the exposed roots of willows in a clear-
ing in the woodland adjacent to the Laguna Merin. A wallow c. 40 m from
the main stream of the Rio Cebollati was approached from several directions
by well marked pathways, and the bank above the wallow was deeply dis-
sected at one place as a result of intensive use.

Food habits. Capybaras are strictly herbivorous. At Sierra de Vaz in Cerro
Largo, animals were seen foraging upon the shorter range grasses, especially
crabgrass (Digitaria sp.), Bermuda grass (Cynodon dactylon) and Oxalis sp.
Although long nocturnal treks for food have been mentioned (Aplin,
1894:30), we never knew this animal to forage more than 100 m from
water. Foraging sites were repeatedly used, and near the Laguna Merin and
at Sierra de Vaz the resident capybaras returned nightly to the same areas.

Capybaras have a long intestinal tract typical of herbivores. In the female
taken in Treinta y Tres, the small intestine was 13.5 m in length and about
2 cm in diameter. The caecum which was enormously distended with macer-
ated plant material was 80 cm in length and 22 cm across at its widest part.
The colon tapered from a diameter of nearly 20 cm to 2 cm in the first
metre and maintained this diameter for the remaining 3.8 m of its length.
The distinctive oblong faecal pellets were well formed when they had
reached the last metre of the colon. These pellets measuring about 25 x 15
mm are green when defecated, but in a day or two they fade to brownish
green and eventually to ash gray.

Although there is no direct evidence of territorial behaviour in capybaras,
deposition of faeces seems to be somewhat localized and occurs near but not
within frequented runways in tang!ed vegetation or on regularly travelled
paths. Specific foraging sites frequented are also near sites of extensive
faecal depositions.

Breeding. Two females and six juveniles were seen together in the riparian
growth along the Rio Cebollati on 13 December 1962. Herds of from seven
to 15 animals of two distinct size groups were seen at several localities
between December and May. Aplin (1894:312) saw a young individual not
more than 30 cm long in the company of an adult female on 8 May in
Soriano. The breeding season in Uruguay is protracted, perhaps extending
for about seven months from early spring to late summer.

46

Myocastor coypus bonariensis (Commerson)
Nutria

Specimens: Total 8; Rio Negro, Arroyo Negro, 15 km S Paysandu, ¥, 9;
Rocha, 22 km SE Lascano, ? sex; Treinta y Tres, Boca del Rio Tacuari,
2 ? sex; 16 km SSW Boca del Rio Tacuari, 4, 2 ? sex.

Although the nutria is protected by law in Uruguay it has long been sub-
jected to intensive poaching, especially since the turn of the century. Aplin
(1894:313) found it common along some of the larger water courses in
Soriano, but by the late 1920s and early 1930s it had become rare in the
more populous regions of the country (Sanborn 1929:160; Devincenzi,
1935:74-75). However, Sanborn (1929:160) indicated that nutrias were
still common around the Laguna Negra in Rocha at the time of his expedi-
tion. In 1963 estancia owners told us that at one time thousands of nutrias
were poached annually around the Laguna Negra, but more recently the
number had dropped to a few dozen yearly. It is indeed ironic that Myo-
castor coypus is in danger of extinction throughout much of its native range
in South America, yet flourishes to the point of becoming a pest where it
has been introduced in the United States, Europe and Asia (U.S.S.R.).

Our records come from widespread localities in the country. Two young
males were shot along the Arroyo Tres Cruces Grande in northern Artigas
on 29 September 1957 (Praderi and Luzardo, 1958). Two (AMNH
206454, AMNH 206455) were taken by us in Rio Negro along the Arroyo
Negro. Another (AMNH 206461) was obtained in far eastern Treinta y
Tres near the Laguna Merin. One was seen in a drainage ditch on 23 March
1963 in southern Cerro Largo. Sign and later a fresh skeleton were found in
Rocha on the Estancia El Sauce. Merlin D. Tuttle found fresh faeces in
Soriano near Cardona.

Ecology. In Uruguay the nutria inhabits large swamps and marshes dom-
inated by Scirpus sp. and Cortaderia sp., edges of rivers and streams, man-
made ponds and even drainage ditches with little surrounding vegetation.
Waterways having steep banks are preferred as burrow sites. If steep
banks are not available, nest platforms are built above ground among tall
marsh vegetation. Such platforms were found in Rocha along an unnamed
arroyo. A complex burrow system was excavated and traced in Flores by
Sierra de Soriano (1960:9). Three divisions were recognized. Tunnels and
passageways found among the rushes and the emergent water lilies growing
in the pond led to the main entrance of the subterranean burrow in the bank.
The tunnels and enlarged chambers of this part of the system extended
inward 7 m from the water’s edge and averaged 4.5 cm below the surface.
Two openings to the surface communicated with extensive runways in
vegetation which surrounded the pond. These runways widened at intervals
where platforms of vegetation were present. Surface runways of nutrias
frequently provide routes for other mammals as Oxmycterus rufus, Cavia
pamparum, Oryzomys sp. and Conepatus chinga.

47

Breeding. The subadult female from Rio Negro was not pregnant. The epi-
didymides of a male with testes measuring 32 x 20 mm, taken 26 March,
contained mature sperm.

Ctenomys torquatus Lichtenstein
Tuco-tuco

Specimens: Total 94; Cerro Largo, 6 km SE Melo, 8 6, 18 99; 20
km SE Melo, 8 oo”, 16 99; Lavalleja, 12 km WSW Zapican, 10 3,
24 29, ? sex; Maldonado, Barra del Arroyo Maldonado, 9 km ENE
Punta del Este, 9; Tacuaremb6, 15 km NW Tacuaremb6, 0’, 4 29; 40
km NW Tacuaremb6, 2 0’, 9.

Species of the hystricomorph genus Cftenomys abound in the southern half
of South America. These rodents remarkably resemble North American
pocket gophers (Geomys sp. and Thomomys sp.) both in morphology and
in fossorial habits. Differences in burrowing do exist, however; gophers
excavate with their teeth and the claws of their forefeet, pushing loosened
dirt out of the burrow with their head and chest. Tuco-tucos dig with their
forefeet, then sweep the dirt out of the tunnel with their hind feet. Ctenomys
torquatus can close its lips behind its teeth which should make possible
their use in digging, but neither Sanborn (1929:159) nor Pearson
(1959:44) observed use of the incisors in burrowing.

A single species, Ctenomys torquatus, occurs in Uruguay. The results of
the research of Dr. Rodolfo Talice and associates, who have studied this
animal in the field and laboratory for more than ten years, have not been
widely available in English and thus many of their data are incorporated
below.

Ecology. Tuco-tucos were found in cultivated ground in Tacuarembé, in
vacant lots within a residential district in the city of Montevideo, in stabilized
dunes along the Atlantic coast in Maldonado, and in over-grazed pasture in
many places. Vegetation typical of the latter habitat near Melo included:
Oxalis sp.; Setaria geniculata; Digitaria sanguinalis; Aspilia sp.; Discaria
longispina; and Eryngium sp. Digitaria and Oxalis sp. formed a thick turf
throughout which other plants were scattered. At Sierra de Vaz clumps of
the taller grass, Paspalum sp., were growing within the limits of a Ctenomys
torquatus colony.

Soil composition probably limits local distribution of this mammal.
Preference for sandy, rock-free soils has been noted by several authors
(Devincenzi, 1935:74; Sanborn, 1929:159; Talice and Momigliano,
1954:126). This predilection was best demonstrated in upland areas in
Tacuarembo, where tuco-tucos occurred in isolated pockets of sandy soil,
but were absent from adjacent rockier and more compact soils. Along the
banks of the Rio Tacuari at Sierra de Vaz, Ctenomvs torquatus lived only
in the adjacent rock-free alluvial soils.

4§

Tuco-tucos live in loosely organized colonies and burrow systems are
generally more than 120 cm apart. At the surface, these burrows were
characterized by the presence of a single mound of dirt (25 to 75 cm in
diameter) containing faecal material and some plant cuttings. Some nearby
burrow systems in the colony apparently had more than one mound
although we did not excavate them to verify this. Besides the entrance
associated with the mound of dirt, the three burrows examined by us had
an average of seven other surface openings, each marked by an “apron”
or cleared area averaging 8 x 7 cm. These entrances were either open or
plugged, depending upon the activities of the resident tuco-tuco. Long
unused entrances had plugs greater than 10 cm thick. In burrows in
stabilized dunes in Maldonado plugs more than 60 cm thick blocked the
entrances of some passages.

We explored in detail the burrows of a young adult male (AMNH
206466) (Fig. 19) and of an adult female (AMNH 206465) (Fig. 20).
These systems consisted of an essentially straight main tunnel with several
shorter lateral passages. Typically the lateral pasages either ended blindly,
their ends being packed with sandy soil containing faecal pellets and plant
cuttings (especially of Eryngium sp. and Digitaria sp.) or they opened to the
surface at feeding sites. Both systems had a nest chamber off the main
passage. In the burrow of the male this chamber was at a depth of 30 cm,
measured 18 x 15 cm, and contained a nest measuring 15 x 10 cm made
of pieces of grasses, mostly Digitaria sp., averaging 1.5 cm long. The nest
chamber of the female measuring 27 x 14 cm, was completely filled with
a nest composed of short grass cuttings.

The tunnels averaged 7 cm in diameter, but occasionally widened to 13
cm and nowhere exceeded 40 cm in depth. Ctenomys torquatus tends to
dig at a uniform depth after an initial descent from any entrance. In con-
trast, burrows of Thomomys bottae navus depicted by Miller (1957:435)
varied more in depth along their lengths and had occasional nearly vertical
tunnels 1 m deep which ended blindly. Typically Ctenomys torquatus
tunnels passed significantly beneath the depth of the root zone of plants in

plug

—
awn
»

nest chamber

a
“~~ surface mound

plug

Fig. 19 The burrow system occupied by a young adult ¢ Ctenomys tor-
quatus (AMNH 206466; 25 February 1963; Cerro Largo, 6 km SE Melo).
The nearest burrow system (that of a ¢@) was 2 m to the north. Measure-
ments refer to the depths of the floor of the passages beneath the surface.

49

SCALE : Imm = 4cm

nest chamber

WJ—plug SCALE: Imm=4cm

apron

Fig. 20 The burrow system occupied by an adult 9 Ctenomys torquatus
(AMNH 206465; 23 February 1963; Cerro Largo, 6 km SE Melo). The
nearest burrow system was 20 m northwest. Measurements refer to the depths
of the floor of the passages beneath the surface.

the habitat. Dense masses of roots and tubers penetrated to an average
depth of 10 cm with occasional roots as deep as 30 cm or within range of
the tunnels. The burrows excavated averaged about 412 m long. Dimensions
of the above mentioned burrows differed little from those examined by
Talice and Momigliano (1954:131) in the Departments of Montevideo and
Maldonado.

The microclimate of the burrows varies little. In summer, the temper-
ature ranges from 19°C to 21°C and the humidity from 78.8% to 81.8%
at a depth of 25 cm (Talice and Momigliano, 1954:129).

50

Eighty burrow systems were noted in an area of seven hectares near our
campsite outside of Melo. An area of three hectares at this same locality
yielded 26 Ctenomys torquatus. At Sierra de Vaz, 24 tuco-tucos were
trapped in an area 30 x 100 m. By comparison, Pearson (1959:35) re-
moved 28 Ctenomys peruanus from a census area 82 m square. The
number of tuco-tucos per unit area is, in part, a function of abundance of
preferred food plants. In one hectare in stabilized dunes at Maldonado only
four burrows were counted.

All but one tuco-tuco were caught in Macabee traps placed within the
entrances of the burrows. Most were caught by the hind parts, probably
as a result of attempts to sweep unfamiliar objects out of the burrow with
their hind feet, as described by Pearson (1959:15). In the afternoon they
were sometimes caught two minutes after a trap had been emplaced. One
individual succeeded in plugging its burrow behind the trap on seven
successive occasions before capture. Trapping success was high, and
virtually every active system eventually yielded a specimen.

Behaviour. Cfenomys torquatus is diurnal in surface activity. At all colonies
investigated maximum surface activity occurred between 1:00 p.m. and
4:00 p.m. A captive individual was active daily between 7:00 a.m. and
6:00 p.m., with a period of quiescence between 11:00 a.m. and 1:00 p.m.
In the field individuals were frequently seen foraging or sitting quietly at
burrow entrances for periods of up to 15 minutes during the hours of
maximum activity.

Tuco-tucos are purportedly awkward above ground (Pearson, 1954:44).
Sanborn (1929:159) reported that individuals found at a distance from a
burrow entrance became disoriented and dashed erratically about seeking
shelter when frightened. Tuco-tucos were never observed to move more
than two-thirds of the way out of a burrow entrance when foraging or
resting, a precaution tending to reduce danger from predators. When
released for brief periods, captive individuals were less agile and slower than,
for example, terrestrial cricetines but certainly did not seem exceptionally
awkward. Talice et al. (1959:457) noted that Ctenomys torquatus walked
normally for a quadruped, moving in a rapid and sinuous fashion, pausing
periodically to look about, but found it to be a poor swimmer.

By deliberate stalking, it was possible on several occasions to crawl from
a position downwind to within 3 m of a tuco-tuco foraging at the entrance
to its burrow. When we walked over the burrow systems tuco-tucos active
at the surface seemingly detected our presence downwind at a distance of
20 m and subsequently disappeared from sight.

We only distinguished three vocalizations. The repetitive syllabic call,
the basis of the vernacular name tuco-tuco, was uttered by animals from
within the entrances of burrows. Not all species of Ctenomys give this type
of call but among those that do, significant differences in tonal qualities
are recognizable. In Pert, Pearson (1959:30) found that only Ctenomys
peruanus of three species studied had a conspicuous bubbling and melodious
vocalization which reminded him of the song of the bobolink (Dolichonyx
oryzivorus).

SI

To me the primary vocalization of Ctenomys torquatus was not melodious
but rather guttural, gravelly and at times like the sound produced by rub-
bing together two pieces of coarse sand paper. Talice et al. (1959:457)
described the call as being composed of dry sounds, solemn, more or less
measured and difficult to locate in the field. The call consists of a repetition
of the syllable tloc. In two minutes of monitoring I counted 23 repetitions
in the first minute and 29 in the second. On quiet days calls were audible
up to a distance of about 30 m.

In captivity a series of pig-like grunts, uttered in rapid succession, was
noted in times of mild excitement. Talice et al. (1959:457) report similar
vocalizations in captive animals. Grunts, and squeals were uttered by indi-
viduals caught in Macabee traps whenever we pulled a caught individual
to the surface.

Food Habits. Ctenomys torquatus is strictly an herbivore and possesses hyp-
sodont, continuously growing molars with complex occlusal surfaces adapted
to crush and grind coarse vegetation. Pearson (1959:45) noted that
Peruvian species ate roots, stems, leaves, blossoms and fruits of a wide
variety of plants. C. peruanus ate some while at the surface, but C. opimus
cut food and retreated into the burrow. C. torquatus feeds primarily on the
surface and eats entire plants in the vicinity of an entrance. A semicircular
area or “apron” some 8 x 7 cm that typically cuts into the root zone to a
depth of 2 cm is formed. Some cuttings were found within the burrows,
but no distinct food stores were located, nor was there evidence of exten-
sive subterranean feeding. Talice and Momigliano (1954:131) indicate that
food items presented to captive animals were often hoarded.

One of our captive tuco-tucos, when offered rhizomes of Digitaria sp.,
sat on its haunches, held the stems alternately with one and then both fore-
paws and slowly pushed the plant parts into its mouth as it chewed.
Carrot slices were manipulated in a similar fashion.

In the wild, tuco-tucos eat tender parts of the leaves of Eryngium sp.,
entire plants of Digitaria sanguinalis, and corms of Oxalis sp. Talice and
Momigliano (1954:131) report consumption in the wild of the following
plants: Solidago chilensis, Androtrichum trigynum, Comphrena perennis,
Aster montevidensis, Oenothera indecora, Cynodon dactylon, Sporobolus
poiretu, Cyperus cayennensis, Anthemis mixta and Cenchrus tribuloides.

Breeding. Ctenomys torquatus is monoestrous. Pregnant females with two
or three embryos are found from September through January. The gestation
period in this species is approximately 105 days (Talice et al., 1959:459).
None collected by us between February and May was breeding. Histological
analysis of biopsies of males with testes averaging 8 x 4 mm indicated
breeding inactivity.

Moult. Tuco-tucos were trapped between early February and mid-May at
four localities in eastern and southern Uruguay, and most showed some
moult. In February and March moult in adults and in young of the year
was patchy and of slight extent and small growth centres developed separ-
ately on the head, behind the shoulders, and on the rump. Moult was less

a2

'

Fig. 21 Sequence of moult in late summer captured Ctenomys torquatus.
The accompanying small figures in stages 4 - 8 represent the internal deposition
of melanin on the dorsal (upper small figure) and ventral (lower small figure)
surfaces.

BW)

conspicuous on the belly. Based on the extent of moult in animals collected
in April, the hair replacement noted in February and March may repre-
sent the beginning of autumn moult, or simply may be regular hair replace-
ment necessitated by attrition due to abrasion by gritty soil. Fifteen per
cent of the Ctenomys torquatus obtained in February and March showed no
indication of moult.

Tuco-tucos obtained in April were in the moult, and although a great
deal of variation occurred, a basic pattern of hair replacement was discern-
ible (Fig. 21). The initial growth centre develops mid-dorsally in an inter-
scapular position and new growth proceeds anteriorly over the head. Poster-
iorly a narrow strip spreads along the mid-dorsal line and diagonally toward
the sides of the thorax. Secondary centres may develop on the rump and
mid-venter. Moult from the dorsum sweeps rapidly down the sides and
ento the venter, converging with the growth of new hair radiating from the
ventral growth centre. Growth is first completed on the head, then on the
belly and finally on the rump. Autumnal moult is not always completed in
April as the individual taken in May, a female (AMNH 206547) obtained
in Maldonado, showed extensive moult.

54

Concluding Remarks

Prior to the arrival of the first European settlers in the 17th century,
Uruguay was covered with a distinctive tall grass savannah fringed with
thorn woodland (Wilhelmy and Rohmeder, 1963). Three centuries of
intensive grazing by livestock on the great estancias and a general clearing
of the uplands for other agricultural purposes combine to give modern-day
Uruguay the appearance of disturbed tall-grass prairie. Upland woodland
has largely disappeared and has been replaced by symmetrical plantings
of eucalyptus which are notably poor in resident mammals. In general, the
distribution and abundance of virtually all wild mammals has been in-
fluenced profoundly by the deterioration of the habitat.

In Uruguay rodents occur in the three basic habitats, i.e. woodlands,
grasslands and limnic situations. Woodlands vary in plant composition,
dimensions, and degree of association with waterways. Four of the species
collected frequented woodland (Table 4) which varies from dense sub-
tropical forest with a continuous canopy to open parkland-like growth.
Only the arboreal mouse, Thomasomys oenax, is seemingly restricted to the
dense subtropical forest.

Table 4
Habitat preferences of some Uruguayan rodents based on
specimens and field observations obtained in 1962-63.

Species Woodland Grassland Limnic

Dense Open Situations
(with canopy)

Oryzomys delticola xX xX

O. flavescens

Thomasomys oenax xX

Akodon azarae xX

A. obscurus

Oxymycterus rufus

Scapteromys tumidus

Calomys laucha

Reithrodon physodes

Holochilus Xx

brasiliensis

H. magnus xX

Mus musculus X

Cavia pamparum xX xX
xX xX

xX

xe MK
aa mKm eM

Hydrochoerus xX
hydrochaeris
Myocaster coypus
Ctenomys torquatus xX

D2

Grasslands occupy the major portion of the land area and are inhabited
by 11 species, three of which also occur in open woodland. Only Calomys
laucha, Reithrodon physodes, Mus musculus and Ctenomys torquatus were
found only in grassland.

Limnic habitat types included marshes, wet meadows and watercourses.
Often where marshes and wet meadows are shielded from livestock by
fences or dense growths of spiny caraguata (Eryngium sp.) small mammals
are locally numerous. Ten species characterized these habitats and three
of them (Holochilus magnus, H. brasiliensis and Scapteromys tumidus)
were taken nowhere else.

Hershkovitz (1963:43) has classified all South American mammals on
the basis of their origin in a habitat type as reflected by their morphological
adaptations and on the basis of their current habitat preferences. In
Uruguay, among the species we collected, sylvan forms (species primarily
adapted to living in forests) included arboreal cricetines. Pastoral forms
(species primarily specialized for living in savannahs, scrublands, open
swamp lands and marshes) include the caviomorph Cavia pamparum, the
hystrichomorph Ctenomys torquatus, and some cricetines, e.g. Reithrodon
physodes.

56

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59

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