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UNIVERSITY OF KANSAS PUBLICATIONS
MUSEUM OF NATURAL HISTORY
Volume 13, No. 2, pp. 19-72, pls. 1-8, 3 figs.
August 16, 1960
A Distributional Study of the Amphibians
of the Isthmus of Tehuantepec, Mexico
BY
WILLIAM E. DUELLMAN
UNIVERSITY OF KANSAS
LAWRENCE
1960
UNIVERSITY OF KANSAS PUBLICATIONS
MUSEUM OF NATURAL HISTORY
Volume 13, No. 2, pp. 19-72, pls. 1-8, 3 figs.
August 16, 1960
A Distributional Study of the Amphibians
of the Isthmus of Tehuantepec, Mexico
BY
WILLIAM E. DUELLMAN
UNIVERSITY OF KANSAS
LAWRENCE
1960
UNIVERSITY OF KANSAS PUBLICATIONS, MUSEUM OF NATURAL HISTORY
Editors: E. Raymond Hall, Chairman, Henry S. Fitch,
Robert W. Wilson
Volume 13, No. 2, pp. 19-72, pls. 1-8, 3 figs.
Published August 16, 1960
UNIVERSITY OF KANSAS
Lawrence, Kansas
PRINTED IN
THE STATE PRINTING PLANT
TOPEKA, KANSAS
1960
28-3859
A Distributional Study of the Amphibians of the Isthmus of Tehuantepec,
Mexico
BY
WILLIAM E. DUELLMAN
CONTENTS
PAGE
INTRODUCTION 21
Acknowledgments 23
Field Studies in the Isthmus of Tehuantepec 23
Sources of Material 24
DESCRIPTION OF THE ISTHMUS OF TEHUANTEPEC 25
Physiography 25
Climate 28
Vegetation 29
The Sierra de los Tuxtlas 32
GAZETTEER 33
THE AMPHIBIAN FAUNA OF THE LOWLANDS 37
Composition of the Fauna 37
Ecology of the Fauna 38
Distribution of the Fauna 42
THE AMPHIBIAN FAUNA OF THE FOOTHILLS AND ADJACENT HIGHLANDS 44
ESTABLISHMENT OF PRESENT PATTERNS OF DISTRIBUTION 45
ACCOUNTS OF SPECIES 49
SUMMARY 68
LITERATURE CITED 69
INTRODUCTION
Few regions in Middle America are so important zoogeographically as is
the Isthmus of Tehuantepec, that neck of land connecting North America
with Central America, separating the Pacific Ocean from the Gulf of
Mexico by a distance of only about 220 kilometers (airline), and
forming a low break between the highlands of Mexico and those of
Central America. Before World War II the isthmus could be reached
readily only by railroad or by ocean vessel to Salina Cruz or
Coatzacoalcos. With the advent of roads, principally the Trans-isthmian
Highway, vast areas of the interior of the isthmus became accessible to
biologists. Nevertheless, long before roads were built in the isthmian
region collectors and biologists visited it, especially the town of
Tehuantepec, from which collections date back to the 1870's. Therefore,
it is rather surprising that no attempt has been made to present a
faunal list of the amphibians or reptiles of the isthmus. Ruthven
(1912) summarized his collections from the vicinity of Cuatotolapam,
Veracruz, and Hartweg and Oliver (1940) presented an annotated list of
the species collected by them in the vicinity of Tehuantepec. In recent
years there have been only a few papers reporting species from the
isthmus (Fugler and Webb, 1957; Langebartel and Smith, 1959). The
zoogeographic significance of the Isthmus of Tehuantepec is exemplified
by the works of Burt (1931), Duellman (1958), Gloyd (1940), Oliver
(1948), and Stuart (1941), who in their discussions of evolution and
dispersal of various genera of reptiles, pointed out that the Isthmus
of Tehuantepec was a region of zoogeographic importance.
Originally I intended to study the entire herpetofauna of the isthmus.
But I have not had opportunity to study all of the reptiles, and I have
not had the inclination to solve certain taxonomic problems concerning
them. The amphibians that I collected, together with all other known
specimens in museums, have been studied. Therefore, the present report
is concerned only with the amphibians. Only the amphibians of the
lowlands of the isthmus have been sampled adequately. Although I have
commented on the highland species in the discussion of distribution,
they are not included in the systematic section, which deals solely
with the 36 species definitely known to occur in the lowlands of the
isthmus.
Among the species of amphibians that I would expect to occur in the
isthmus, the only one not yet found there is _Hyla phaeota_. Sufficient
specimens of most of the species are available to show their variation
in the isthmus. Consequently, the systematics of these amphibians is on
a fairly substantial basis. Probably certain species in the isthmian
region will be found to be conspecific with others to the south, for
example _Hyla ebraccata_ with _Hyla leucophyllata_ and _Hyla
robertmertensi_ with _Hyla underwoodi_. Nevertheless, such taxonomic
changes will not affect the distributional picture presented here. Our
greatest lack of knowledge concerning the amphibians is about their
life histories, as may be illustrated by the following questions, all
of which now are without definite answers. Where do many of the small
frogs conceal themselves during the dry season? What amount of, if any,
interspecific competition exists among several species of tree frogs,
all of which breed in the same ponds? What factors in the environment
permit certain amphibians, but not others, to live in the humid
rainforests, as well as in the arid tropical scrub forest? The answers
to these questions and many others must await additional field studies.
The purpose of this paper is to make known the species of amphibians
living in the Isthmus of Tehuantepec, to describe the environments in
which they live, and to discuss their distribution in the isthmus. With
respect to the distribution of animals in the Isthmus of Tehuantepec I
will attempt to explain the present patterns of distribution with
special reference to climatic fluctuation in the Pleistocene.
_Acknowledgments_
My extensive field work in the Isthmus of Tehuantepec was made possible
by grants from the Penrose Fund of the American Philosophical Society
(1956) and the Bache Fund of the National Academy of Sciences (1958).
Furthermore, my field work received the hearty support of the Museum of
Zoology at the University of Michigan; for their cooperation I am
indebted to Norman Hartweg, T. H. Hubbell, and Henry van der Schalie.
In the course of my studies I received helpful suggestions from Norman
Hartweg, L. C. Stuart, and Charles F. Walker, to whom I am grateful.
For permission to examine specimens in their care I thank Doris M.
Cochran, Hobart M. Smith, and Richard G. Zweifel. I am deeply indebted
to Thomas MacDougall for many suggestions and for aid in preparing the
gazetteer. I am most grateful for the efforts of my field companions,
Richard E. Etheridge, Jerome B. Tulecke, John Wellman, and especially
my wife, Ann S. Duellman, who spent many long days and nights gathering
much of the data on which this report is based. Our work in the isthmus
was furthered by the generous help and hospitality of many residents,
especially the late Wilbur Barker of Tehuantepec, Fortunado Delgado of
Rancho Las Hojitas near Acayucan, Cesar Farjas of Donaji, and Juan
Mayol of San Andres Tuxtla. Profesor Jordi Julia Z. of the Laboratorio
de Entomologia, Comision del Papaloapan, Ciudad Aleman, Veracruz,
helped make possible my field work in 1959; for this he has my sincere
thanks. In conclusion I express my gratitude to Ing. Juan Lozano
Franco, Secretaria de Agricultura y Ganaderia, for providing me with
the necessary permits.
_Field Studies in the Isthmus of Tehuantepec_
I first visited the Isthmus of Tehuantepec and collected on the Pacific
lowlands of the isthmus in July, 1955. At that time heavy rains and
impassable roads restricted travelling. In February and March of 1956
my wife and I concentrated our efforts in the central region between
the Rio Jaltepec and Matias Romero, but also made several trips across
the isthmus to gather ecological data in the dry season. In July of the
same year, accompanied by Richard E. Etheridge, we again crossed the
isthmus several times in order to gather ecological data in the wet
season, and studied especially hylid frogs, most of which had not been
seen in the dry season. Accompanied by Jerome B. Tulecke and John
Wellman, I collected again in the isthmus in July, 1958, between Salina
Cruz and Tehuantepec, and between Coatzacoalcos and Cosoleacaque. In
March and April, 1959, I collected at Ciudad Aleman. Nearly 1200
specimens of 30 species of amphibians were thus collected in the
Isthmus of Tehuantepec; all specimens are now in the Museum of Zoology
at the University of Michigan. Of other species known from the isthmus,
I have had field experience with all but one (_Bolitoglossa
veracrucis_) in other parts of Mexico.
_Sources of Material_
There are in museum collections nearly 3000 specimens of amphibians
with reliable data from the Isthmus of Tehuantepec. Among the first
herpetological specimens collected in the isthmian region are those
assembled by Francis Sumichrast in the 1870's from the vicinity of
Santa Efigenia and Tapanatepec, Oaxaca. These specimens were sent to
the United States National Museum and the Museum National d'Histoire
Naturelle in Paris; many served as the types of new species: _Bufo
canaliferus_ Cope, _Eleutherodactylus rugulosus_ Cope, _Syrrhophus
leprus_ Cope, and _Hylella sumichrasti_ Brocchi. In 1911 Alexander G.
Ruthven collected in the savanna country near Cuatotolapam, Veracruz;
the report on his collections (1912) is the first dealing with the
herpetofauna of a part of the isthmus. His specimens are in the
collection of the University of Michigan Museum of Zoology. Norman
Hartweg and James A. Oliver collected for the University of Michigan
Museum of Zoology in the vicinity of Tehuantepec, Oaxaca, during the
summer of 1936. The results of their work were published as an
annotated list of species occurring on the Pacific slopes of the
isthmus (1940). Hobart M. Smith collected in the vicinity of
Tehuantepec in January, 1940; his specimens are in the United States
National Museum. Specimens collected by Smith served as the types of
_Eleutherodactylus avocalis_ Taylor and Smith and _Diaglena reticulata_
Taylor. Walter W. Dalquest collected vertebrates for the University of
Kansas in southern Veracruz in the winters of 1947 and 1948; he spent
about six months on the Gulf lowlands of the isthmus, principally in
the vicinity of Jesus Carranza. For the past two decades Thomas
MacDougall, a resident of New York City, has spent his winters
collecting biological specimens in southern Mexico. He makes his
headquarters at Tehuantepec, but his compulsion to see the "back
country" has taken him to many remote parts of southern Oaxaca. His
earlier collections are in the American Museum of Natural History; the
later ones are in the University of Illinois Museum of Natural History.
Minor collections include those made by Matthew W. Stirling at San
Lorenzo, Veracruz, February-April, 1946 (United States National
Museum), by Fred G. Thompson on a trip across the isthmus in December,
1955 (University of Michigan Museum of Zoology), by the University of
Kansas Museum of Natural History field party under the direction of
Rollin H. Baker at Tolosita, Oaxaca, and by David A. Langebartel and
associates from southern Oaxaca in June, 1958 (University of Illinois
Museum of Natural History).
In the collections of the United States National Museum are several
species of amphibians sent to the museum from Tehuantepec by Francis
Sumichrast. These include _Bolitoglossa platydactyla_ (USNM 30305,
30344-6, 30528), _Bolitoglossa rufescens_ (10042), _Chiropterotriton
chiropterus_ (30347), _Lineatriton lineola_ (30353), _Parvimolge
townsendi_ (30352), _Pseudoeurycea cephalica_ (30350), _Thorius
pennatulus_ (30348-9), _Hyla miotympanum_ (30302-3), and _Hyla picta_
(30304). Because of the poor condition of the specimens, determinations
of those listed as _Bolitoglossa rufescens_ and _Pseudoeurycea
cephalica_ are uncertain. With the exception of the _Bolitoglossa
rufescens_, which is stated to have come from Santa Efigenia, all of
these specimens are catalogued as having come from Tehuantepec. None of
these species has since been recorded from the Pacific slopes of the
isthmus; however, they all occur in the vicinity of Orizaba, Veracruz.
Probably Sumichrast carried the specimens with him from Orizaba, his
home before moving to Santa Efigenia, and shipped them from Tehuantepec
to the United States National Museum. These species definitely should
not be considered as inhabitants of the Pacific slopes of the Isthmus
of Tehuantepec.
DESCRIPTION OF THE ISTHMUS OF TEHUANTEPEC
The Isthmus of Tehuantepec is a strip of land forming a low pass, which
separates the mountain masses of Mexico proper from those of Central
America, and at the same time provides a continuum of lowlands from the
Gulf of Mexico to the Pacific Ocean. This topography combines with the
climatic conditions to create extremely diverse environments, the
distribution of which can be adequately understood only after an
acquaintance with the topography and climate of the region.
_Physiography_
In east-central Oaxaca the mountain masses comprising the Sierra Madre
Oriental and the Sierra del Sur terminate in a series of ranges--Sierra
de Juarez, Sierra de los Mijes, and Sierra de Choapam. From lofty
peaks, such as Cerro de Zempoaltepetl (3400 meters), the highlands
diminish eastward to succeedingly lower ridges, until in the middle of
the Isthmus of Tehuantepec the continental divide is about 250 meters
above sea level. Eastward from this low divide the land rises to form
the Sierra Madre de Chiapas, which is continuous with the highland
masses of Guatemala.
For the purposes of this description, the lowlands of the isthmus may
be divided into three parts--the Gulf Coastal Plain, the central
ridges, and the Pacific Coastal Plain, which in the isthmus is called
the Plains of Tehuantepec (Figs. 1 and 2).
The Gulf Coastal Plain is broad and fairly level near the coast, but
rolling in the interior. The plain, throughout most of its length in
the isthmus, is at least 75 kilometers wide. The majority of the region
in the isthmus is drained by the Rio Coatzacoalcos, which flows in a
northerly course to the Gulf of Mexico. The western part is drained by
the Rio San Juan, the principal tributary of the Rio Papaloapan. Behind
the coastal dunes are frequent, and sometimes large, lagoons.
Immediately inland from Coatzacoalcos and along the lower stretches of
the Rio Papaloapan are extensive marshes. Essentially the entire
coastal plain, with the exception of the coastal dunes, consists of
rich alluvial deposits.
[Illustration: FIG. 1. Map of the Isthmus of Tehuantepec based on the
American Geographical Society's "Map of Hispanic America on the Scale
of 1:1,000,000."
The localities shown are numbered in the gazetteer; the numerical
sequence of localities is an arrangement whereby north takes precedence
over south and west over east. 1. Alvarado. 2. Lerdo de Tejada. 3.
Tlacotalpan. 4. Tula. 5. Tecolapan. 6. Amatitlan. 7. Cosamaloapan. 8.
Chacaltianguis. 9. Novillero. 10. Ciudad Aleman. 11. Papaloapan. 12.
Tuxtepec. 13. Cuatotolapam. 14. Hueyapan. 15. Berta. 16. Coatzacoalcos.
17. Ayentes. 18. Rio de las Playas. 19. Cosoleacaque. 20. Minatitlan.
21. Acayucan. 22. Aquilera. 23. San Lorenzo. 24. Naranja. 25. Suchil.
26. Jesus Carranza. 27. La Oaxaquena. 28. Ubero. 29. Donaji. 30.
Tolosita. 31. El Modelo. 32. Sarabia, 33. Guichicovi. 34. La Princesa.
35. Santa Maria Chimalapa. 36. Matias Romero. 37. Santo Domingo Petapa.
38. El Barrio. 39. Palmar. 40. Chivela. 41. Santiago Chivela. 42.
Nizanda. 43. Agua Caliente. 44. Portillo Los Nanches. 45. Ixtepec. 46.
La Ventosa. 47. Zanatepec. 48. Union Hidalgo. 49. Tres Cruces. 50.
Juchitan. 51. Escurano. 52. Salazar. 53. Santa Efigenia. 54.
Tequisistlan. 55. Cerro de Quiengola. 56. San Pablo. 57. Mixtequilla.
58. Tapanatepec. 59. Zarzamora. 60. Limon. 61. Tehuantepec. 62.
Bisilana. 63. Santa Lucia. 64. Cerro de Arenal. 65. Cerro de San Pedro.
66. La Concepcion. 67. Tenango. 68. San Antonio. 69. Huilotepec. 70.
Salina Cruz.]
[Illustration: FIG. 2. Topographic profile of the Isthmus of
Tehuantepec showing major localities along the Trans-isthmian Highway
and major types of vegetation. Vertical exaggeration approximately 165
times.]
The central ridges extend from the Rio Jaltepec southward to within 40
kilometers of the Pacific coast. It is in this area that the continuity
of the high ridges and volcanic peaks, which extend nearly the entire
length of the Americas, is interrupted at a point almost directly in
line with the shortest distance between the two oceans. The northern
part of this central region consists of hills dissected by tributaries
of the Rio Coatzacoalcos; the principal ones from north to south
are--Rio Jaltepec, Rio Tortuguero, Rio Sarabia, and Rio Malatengo. The
plains of Chivela are south of these rivers and lie at an elevation of
about 200 meters; at the southern edge of these plains a range of hills
rises to 250 to 400 meters above sea level. These hills drop abruptly
to the Plains of Tehuantepec. In the northern and central parts of this
central region the rocks are granitic; the hills to the south of the
Plains of Chivela are limestone.
The Pacific Coastal Plain or Plains of Tehuantepec have a maximum width
of about 30 kilometers. From the base of the hills at an elevation of
about 75 meters the plains slope gradually to the sea. To the west of
the Rio Tehuantepec and to the east of the Plains of Tehuantepec at the
base of the Sierra Madre de Chiapas, the coastal plain becomes much
narrower; in these places the continuity of the plain is frequently
interrupted by low north-south ridges extending outward from the
mountains or by isolated hills. The soil is poor, varying from volcanic
rock to gravel and sand.
_Climate_
The prevailing winds are from the north across the Gulf of Mexico.
These moisture-laden winds precipitate most of their moisture north of
the central ridges. This results in high rainfall on the northern
slopes and Gulf Coastal Plain and relatively little rainfall on the
southern slopes and the Pacific Coastal Plain. Precipitation is cyclic;
there is a marked wet and a dry season throughout the region, but this
is most noticeable on the Pacific lowlands (Fig. 3). At Salina Cruz on
the Pacific Ocean the average annual rainfall is 1040 mm. (Contreras,
1942); of this amount, only 15 mm. falls from November through April.
On the Gulf Coastal Plain (Minatitlan station) the average annual
rainfall is 3085 mm. In this region the driest months are February
through May, during which time 236 mm. of rain falls. At Salina Cruz
the wettest month is June; at Minatitlan it is September. There is
little variation in temperature throughout the isthmus; the average
annual temperature at Salina Cruz is 26.6 deg. C.; that at Minatitlan is
26.2 deg. C. During the winter when masses of air from the arctic move
southward into the Great Plains of the United States, cool winds blow
across the isthmus. These are usually accompanied by overcast sky and
sometimes a slight amount of precipitation. These "nortes" may cause a
drop in temperature of about six to eight degrees in a few hours.
[Illustration: FIG. 3. Climatographs for Minatitlan, Veracruz, and
Salina Cruz, Oaxaca, based on data given by Contreras (1942). Plotted
points are for mean monthly temperatures and rainfall; months are
indicated by numbers.]
_Vegetation_
The topography and climate combine to produce drastically different
types of climax vegetation on the northern and southern lowlands of the
isthmus. The picture is somewhat complicated by the savannas on the
Gulf Coastal Plain, which, as will be shown later, are dependent upon
edaphic features more than climatic conditions. The following brief
account of the vegetation in the Isthmus of Tehuantepec is based on
data provided by Williams (1939) and Goldman (1951), supplemented by
personal observations. The purpose of this description is not to
analyze the flora of the isthmus, but to give the reader a picture of
this aspect of the biota of the major environments with which I shall
be concerned in the ensuing discourse on the amphibians of the region.
The three divisions of the isthmus recognized in the account of the
physiography serve equally well in describing the vegetation. Those
divisions are as follows:
Gulf Lowlands
On the lowlands north of the continental divide and extending to the
Gulf of Mexico are three major types of vegetation--tropical
rainforest, arid tropical scrub forest, and savanna. Aside from these,
there are marshes and lagoons near the coast.
On the coastal dunes there are thickets of sea grape, patches of
_Cenchrus_, and clumps or scattered _Opuntia_. The lagoons are bordered
by mangrove thickets made up primarily of _Lonchocarpus hondurensis_.
In the marshes along the lower Rio Coatzacoalcos and Rio Papaloapan the
tall tough grass, _Gynerium sagittatum_, is common.
According to Beard (1953: 291) the development of savanna vegetation is
dependent upon soil, topography, and drainage. Level regions having
permeable soil horizons lying on top of an impermeable horizon provide
poor drainage. In most savanna regions in the Americas the grasslands
become waterlogged or even partly flooded during the rainy season and
desiccated in the dry season. Many ecologists and phytogeographers have
postulated that savannas are either man made or are examples of a fire
climax. Beard (_op. cit._: 203) provided multitudinous evidence that
the association of savanna vegetation and certain types of edaphic and
topographic conditions was so strongly marked that grassland is the
natural vegetation in these areas.
Savannas are scattered through southern Veracruz eastward to British
Honduras. These usually are grasslands having scattered trees or clumps
of trees around depressions, which may contain water throughout the
year (Pl. 1, fig. 1). According to Williams (_op. cit._), the most
common trees in the savannas in southern Veracruz are _Ceiba
pentandra_, _Chlorophora tinctoria_, and _Byrsonima crassifolia_.
Lying in a rain shadow cast by the Tuxtlas and on sandy and
well-drained soils is a dense xerophytic forest. The crown of this
deciduous forest usually is little more than ten to twelve meters above
the ground (Pl. 1, fig. 2). Conspicuous trees in this scrub forest are
_Acacia cornigera_, _Bauhinia latifolia_, _Calliandra bijuga_, _Cassia
laevigata_, _Guazuma ulmifolia_, and various species of _Bursera_.
The most extensive type of vegetation on the Gulf Coastal Plain is a
tall evergreen forest resembling tropical rainforest. Although this
forest is made up of many species of trees that are characteristic of
true rainforest, the forest on the Gulf Coastal Plain cannot be
classified as true rainforest, neither by the climatic conditions, nor
the structure of the forest. The seasonal variation in rainfall
probably is the chief factor in hindering the development of a
rainforest climax vegetation. Usually a minimum of 65 mm. of rainfall
each month is considered essential for the development of true
rainforest. At Minatitlan the average rainfall for March (39 mm.) and
April (36 mm.) is far below this minimum. Structurally, this forest has
a crown about 30-35 meters above the ground but individual trees rising
five meters or more above the crown (Pl. 2, figs. 1-2). There is no
clear stratification within the forest; in many parts of it there are
dense growths of bushes, small trees, and palms. The forest on the Gulf
Coastal Plain, therefore, most properly might be referred to as a
quasi-rainforest, a term that has been applied to other such forests in
tropical America.
Among the abundant and dominant trees in this forest are _Swietenia
macrophylla_, _Calophyllum brasiliense_, _Achras zapota_, _Ceiba
pentandra_, _Castilla elastica_, _Cedrela mexicana_, _Tabebuia
Donnell-Smithi_, _Calocarpum mammosum_, _Bombax ellipticum_, and a
variety of _Ficus_. Epiphytes and Ilianas are abundant.
Central Ridges
The vegetation of the central ridges of the isthmus is, for the most
part, transitional between the tall rainforest of the Gulf Coastal
Plain and the low xerophytic scrub forest of the semi-arid Pacific
Coastal Plain. On the northern slopes of the ridges the rainforest is
more poorly developed than on the plains to the north. Many of the same
species of trees are present, including _Ceiba pentandra_, _Cedrela
mexicana_, _Swietenia macrophylla_, and _Ficus_ sp.; nevertheless,
these seldom are as large as members of the same species in the forest
on the plains. Other species present on the forested slopes include
_Tabebuia Donnell-Smithi_, _Zanthoxylum melanostictum_, _Pithecolobium
arboreum_, and a species of _Pterocarpus_. The structure of this forest
differs from that on the Gulf Coastal Plain in that there is no
continuous upper canopy and there is a dense undergrowth (Pl. 3, fig.
1). This type of forest extends from Mogone southward to about Matias
Romero.
In the vicinity of Matias Romero open pine-oak forest (_Pinus caribaea_
and _Quercus_ sp.) is found on some ridges as low as 250 meters above
sea level.
On the Plains of Chivela in the southern part of the central region
the vegetation takes on a semi-arid appearance, especially in a savanna
on the plains. Clumps of small trees and bushes, consisting of _Croton
nivea_, _Cordia cana_, _Jacquinia aurantiaca_, _Calycophyllum
candidissimum_, and _Cassia emarginata_, are scattered on a grassy
plain, from which rise widely-spaced palms of an unknown species (Pl.
3, fig. 2).
Pacific Coastal Plain
The vegetation of the Pacific lowlands definitely is semi-arid in
character. Most of the trees are deciduous, thorny, and short. During
the dry season the landscape presents a barren appearance, but shortly
after the first summer rains dense green foliage appears (Pl. 4, figs.
1 and 2). Between Juchitan and La Ventosa few trees are more than two
meters high (Pl. 5, fig. 1). In many areas the trees and bushes form an
almost impenetrable tangle, whereas on especially rocky soils or on
slopes those plants are more widely spaced. Abundant and widespread
species of trees on the Plains of Tehuantepec include _Acacia
cymbispina_, _Prosopis chilensis_, _Caesalpinia coriaria_, _Caesalpinia
eriostachys_, _Celtis iguanaea_, _Cordia brevispicata_, _Jatropha
aconitifolia_, and _Crescentia alata_.
Montane Vegetation
In order to illustrate the interruption of subtropical and temperate
types of vegetation by the lowlands of the Isthmus of Tehuantepec, it
is necessary to digress for a moment from the isthmus and consider the
types of vegetation present on the adjacent highlands. On the higher
peaks, such as Cerro de Zempoaltepetl, above about 2500 meters is fir
forest (_Abies religiosa_); lower on the slopes are extensive pine
forests, which on some slopes are mixed with oak or replaced entirely
by oaks. Subtropical cloud forest, characterized by relatively cool
temperatures and high humidity, is found at elevations usually between
1000 and 1800 meters on the windward slopes of the Sierra Madre
Oriental in Veracruz and northern Oaxaca and on the northern and
southern slopes of the Chiapan-Guatemalan Highlands. None of these
forest types is continuous across the Isthmus of Tehuantepec.
_The Sierra de los Tuxtlas_
Although actually located in the region of the Isthmus of Tehuantepec,
the Sierra de los Tuxtlas, because of its isolated position, need not
be considered in great detail in analyzing the distribution of animals
inhabiting the lowlands of the isthmus. Nevertheless because some
species living in the highlands adjacent to the isthmus also live in
the Tuxtlas, this range is briefly described here. The Sierra de los
Tuxtlas is a range of volcanos lying near the Gulf Coast in southern
Veracruz between the mouths of the Rio Papaloapan and the Rio
Coatzacoalcos. Volcan San Martin, the highest peak, rises above 1800
meters. This range of volcanos is surrounded by lowlands, which
immediately to the south and west are covered with savanna and in
places by scrub forest. The luxuriant nature of the vegetation on these
volcanos indicates that this range receives much more rainfall than the
surrounding lowlands. Especially on the northern slopes, tropical
rainforest is well developed; this is replaced at about 1200 meters by
cloud forest. The southern and western slopes are drier, for the lower
slopes are covered with a scrubby, but evergreen, forest.
Detailed comments on the herpetofauna of the Tuxtlas have been omitted
purposefully, for the reptiles and amphibians of the region currently
are being studied by Douglas Robinson.
GAZETTEER
The following localities are those referred to in the text. The name of
the locality (listed alphabetically by states) is followed by latitude,
longitude, elevation, general description (town, ranch, etc.), and
general type of habitat. Unless otherwise noted, distances are
straight-line (airline) distances in kilometers. The localities have
been plotted from the American Geographical Society's "Map of Hispanic
America on the Scale of 1:1,000,000" (Millionth Map). Numbers in
brackets identify the position of a locality on the accompanying map
(Fig. 1).
_Oaxaca_
Agua Caliente.--Lat. 16 deg. 38'; long. 94 deg. 48'; elev. 140 m. A
hot spring, 6.9 km. north of La Ventosa on the
Trans-isthmian Highway; arid scrub forest [43].
Arenal, Cerro de.--Lat. 16 deg. 18'; long. 95 deg. 32'; elev. 925 m.
(crest). A ridge northeast of Tenango; scrub forest on
slopes and pine-oak forest on top [64].
Barrio, El.--Lat. 16 deg. 38'; long. 95 deg. 07'; elev. 314 m. A
village about 10 kilometers southwest of Matias Romero;
transition between scrub forest and broadleaf hardwood
forest [38].
Bisilana.--Lat. 16 deg. 20'; long. 95 deg. 13'; elev. 35 m. A place
name for a former ranch at the edge of Tehuantepec; open
arid scrub forest [62].
Chivela.--Lat. 16 deg. 20'; long. 95 deg. 01'; elev. 195 m. A
village on the Trans-isthmian Railroad, 26 kilometers by
rail south of Matias Romero and on the western edge of the
semi-arid Plains of Chivela [40].
Concepcion.--Lat. 16 deg. 17'; long. 95 deg. 29'; elev. 1200 m. A
ranch on the slopes of Cerro Arenal, east-northeast of
Tenango; dry pine-oak forest [66].
Coyol.--Exact position unknown; according to Smith and
Taylor (1950: 10), Coyol is "between San Antonio and Las
Cruces."
Donaji.--Lat. 17 deg. 13'; long. 95 deg. 02'; elev. 90 m. A village
at Km. 155 on the Trans-isthmian Highway; rainforest [29].
Escurano.--Lat. 16 deg. 25'; long. 95 deg. 27'; elev. 500 m. A ranch
about 25 kilometers west-northwest of Tehuantepec; arid
scrub forest [51].
Guichicovi, San Juan.--Lat. 16 deg. 58'; long. 95 deg. 06'; elev.
250 m. A village on the north slopes of the isthmus, 12
kilometers north-northwest of Matias Romero; cleared
hardwood forest and coffee plantations [33].
Huilotepec.--Lat. 16 deg. 14'; long. 95 deg. 09'; elev. 30 m. A
small village on the Rio Tehuantepec, 13 kilometers
south-southeast of Tehuantepec; open arid scrub forest [69].
Ixtepec.--Lat. 16 deg. 34'; long. 95 deg. 06'; elev. 60 m. A town
and railroad junction on the northwestern edge of the Plains
of Tehuantepec; arid scrub forest [45].
Juchitan.--Lat. 16 deg. 26'; long. 95 deg. 02'; elev. 15 m. A town
on the Plains of Tehuantepec, 22 kilometers by road
east-northeast of Tehuantepec; arid scrub forest [50].
Limon.--Lat. 16 deg. 20'; long. 95 deg. 29'; elev. 600 m. A former
agrarian colony and now a small ranch about 27 kilometers
west of Tehuantepec; arid scrub forest [60].
Matias Romero.--Lat. 16 deg. 53'; long. 95 deg. 02'; elev. 200 m. A
town on the Trans-isthmian Highway and railroad in the hills
near the crest of the isthmus; broadleaf hardwood forest and
open pine-oak forest [36].
Mixtequilla.--Lat. 16 deg. 24'; long. 95 deg. 18'; elev. 40 m. A
village on the Rio Tehuantepec, northwest of Tehuantepec;
dense scrub forest [57].
Modelo, El.--Lat. 17 deg. 07'; long. 94 deg. 43'; elev. 200 m. An
old rubber plantation on the Rio Chalchijapa, a tributary to
the Rio Coatzacoalcos; rainforest [31].
Nanches, Portillo Los.--Lat. 16 deg. 35'; long. 95 deg. 37'; elev.
500 m. A place name, about 4 kilometers southeast of
Totolapilla; scrub forest [44].
Nizanda.--Lat. 16 deg. 42'; long. 95 deg. 02'; elev. 150 m. A
village on the Trans-isthmian Railroad between Chivela and
Ixtepec; dense scrub forest [42].
Nueva Raza.--Exact location unknown; according to Thomas
MacDougall, this locality is in the lowlands of northern
Oaxaca; rainforest.
Palmar.--Lat. 16 deg. 43'; long. 94 deg. 40'; elev. 300 m. A small
ranch on the west base of Cerro Atravesado; scrub forest
[39].
Papaloapan.--Lat. 18 deg. 11'; long. 96 deg. 06'; elev. 25 m. A
small village on the Rio Papaloapan in northern Oaxaca; low
evergreen forest and savanna [11].
Princesa, La.--Lat. 16 deg. 56'; long. 95 deg. 02'; elev. 150 m. A
ranch on the northern slopes of the isthmus, 6 kilometers by
road north of Matias Romero; poorly developed rainforest
[34].
Quiengola, Cerro de.--Lat. 16 deg. 24'; long. 95 deg. 22'; elev. 900
m. (crest). A hill 15 kilometers west-northwest of
Tehuantepec; dense scrub forest on slopes and scattered
pines on top [55].
Salazar.--Lat. 16 deg. 25'; long. 95 deg. 20'; elev. 45 m. A ranch
on the Rio Tehuantepec, northwest of Tehuantepec; dense
scrub forest [52].
Salina Cruz.--Lat. 16 deg. 10'; long. 95 deg. 12'; sea level. A port
on the Golfo de Tehuantepec; open arid scrub forest [70].
Collections were made in the vicinity of the town and in the
open scrub forest 2.4 kilometers north at an elevation of 20
meters.
San Antonio.--Lat. 16 deg. 15'; long. 95 deg. 22'; elev. 40 m. A
ranch about 25 kilometers west-southwest of Tehuantepec;
arid scrub forest [68].
San Pablo.--Lat. 16 deg. 24'; long. 95 deg. 18'; elev. 40 m. A ranch
on the Rio Tehuantepec, northwest of Tehuantepec; dense
scrub forest [56]. Cerro San Pablo probably is the hill
north of this ranch; this is shown on some maps as Cerro de
los Amates.
San Pedro, Cerro de.--Lat. 16 deg. 18'; long. 95 deg. 28'; elev.
about 1100 m. (crest). A ridge about 24 kilometers west of
Tehuantepec and east of Cerro Arenal; scrub forest on slopes
and pine-oak forest on top [65].
Santa Efigenia.--Lat. 16 deg. 25'; long. 94 deg. 13'; elev. 500 m. A
ranch on the southern slopes of the Sierra Madre de Chiapas,
8 kilometers north-northwest of Tapanatepec; scrub forest.
Former home of Francis Sumichrast [53].
Santa Lucia.--Lat. 16 deg. 18'; long. 95 deg. 28'; elev. 800 m. A
place name for a former ranch on the east slopes of Cerro
Arenal; scrub forest [63].
Santa Maria Chimalapa.--Lat. 16 deg. 55'; long. 94 deg. 42'; elev.
296 m. A village on the Rio de los Milagros, a tributary to
the Rio Coatzacoalcos; rainforest [35].
Santiago Chivela.--Lat. 16 deg. 42'; long. 94 deg. 53'; elev. 200 m.
A village on the Trans-isthmian Highway, 13.4 kilometers by
road south of Matias Romero; dry, grassy plains and
scattered clumps of scrubby trees and palms [41].
Collections were made in the vicinity of the village and at
a rocky stream, 11 kilometers south on the Trans-isthmian
Highway at an elevation of 230 m.
Santo Domingo (Petapa).--Lat. 16 deg. 50'; long. 95 deg. 08'; elev.
225 m. A village about 13 kilometers west-southwest of
Matias Romero; semi-arid scrub forest [37].
Sarabia.--Lat. 17 deg. 04'; long. 95 deg. 02'; elev. 100 m. A
village 25 kilometers north of Matias Romero on the
Trans-isthmian Highway; rainforest [32]. Collections were
made in the vicinity of the village and in the rainforest
along the Rio Sarabia, 5 kilometers north of the village at
an elevation of 80 meters.
Tapanatepec.--Lat. 16 deg. 32'; long. 94 deg. 12'; elev. 90 m. A
town on the Pan-American Highway on the lower slopes of the
Sierra Madre de Chiapas; dense scrub forest [58].
Tehuantepec.--Lat. 16 deg. 20'; long. 95 deg. 14'; elev. 35 m. A
large town on the Plains of Tehuantepec; scrub forest [61].
Collections were made in the vicinity of the town and in the
dense scrub forest 8.6 kilometers west at an elevation of 85
meters and 14 kilometers west at an elevation of 120 meters.
Tenango.--Lat. 16 deg. 16'; long. 95 deg. 30'; elev. 1100 m. A town
in the mountains about 40 kilometers west-southwest of
Tehuantepec; scrub forest [67].
Tequisistlan.--Lat. 16 deg. 24'; long. 95 deg. 37'; elev. 190 m. A
village in the valley of the Rio Tequisistlan, a tributary
to the Rio Tehuantepec; dense scrub forest [54]. Most
collections were made about one kilometer north of the
village where the Pan-American Highway crosses the Rio
Tequisistlan.
Tolosita.--Lat. 17 deg. 12'; long. 95 deg. 03'; elev. 80 m. A
village on the Rio Tortuguero near the Trans-isthmian
Highway; rainforest [30].
Tres Cruces.--Lat. 16 deg. 26'; long. 95 deg. 51'; elev. 750 m. A
ranch near the Pan-American Highway, 70 kilometers by road
west-northwest of Tehuantepec; dense scrub forest [49].
Tuxtepec--Lat. 18 deg. 06'; long. 96 deg. 05'; elev. 80 m. A town on
the Rio Papaloapan in northern Oaxaca; low evergreen forest
[12].
Ubero.--Lat. 17 deg. 18'; long. 95 deg. 00'; elev. 80 m. A lumber
camp and railroad station, 8.5 kilometers south of the Rio
Jaltepec on the Trans-isthmian Highway; rainforest [28].
Union Hidalgo.--Lat. 16 deg. 27'; long. 94 deg. 48'; elev. 7 m. A
village on the railroad, 20 kilometers east-northeast of
Juchitan; open scrub forest [48].
Ventosa, La.--Lat. 16 deg. 30'; long. 94 deg. 51'; elev. 25 m. A
village at the junction of the Pan-American and
Trans-isthmian highways; open scrub forest [46].
Zanatepec.--Lat. 16 deg. 28'; long. 94 deg. 22'; elev. 80 m. A
village on the Pan-American Highway at the eastern edge of
the Plains of Tehuantepec; dense scrub forest [47]. Most
collections were made in the scrub forest 5 to 8 kilometers
west-northwest of the village.
Zarzamora.--Lat. 16 deg. 21'; long. 95 deg. 48'; elev. 800 m. A
ranch between La Reforma (16 kilometers west of
Tequisistlan) and Santa Maria Ecatepec; scrub forest with
oaks on higher ridges [59].
_Veracruz_
Acayucan.--Lat. 17 deg. 57'; long. 94 deg. 55'; elev. 160 m. A large
town on the Trans-isthmian Highway; rainforest [21].
Collections were made in the vicinity of the town, but
principally at Rancho Las Hojitas, 7 kilometers northwest of
town at an elevation of 150 meters.
Alvarado.--Lat. 18 deg. 47'; long. 95 deg. 47'; sea level. A fishing
village at the mouth of the Rio Papaloapan; coastal dunes
and marshes [1]. Most collections were made 1-3 kilometers
southeast of the village in marshes on the leeward side of
the coastal dunes.
Amatitlan.--Lat. 18 deg. 26'; long. 95 deg. 45'; elev. 4 m. A
village on the bank of the Rio Papaloapan; savanna and sugar
plantations [6].
Aquilera.--Lat. 17 deg. 48'; long. 95 deg. 01'; elev. 150 m. A
village 21 kilometers southwest of Acayucan on the
Trans-isthmian Highway; rainforest [22].
Ayentes.--Lat. 18 deg. 10'; long. 94 deg. 26'; elev. 2 m. A railroad
station on the east bank of the Rio Coatzacoalcos, across
the river from the city of Coatzacoalcos; scrub forest and
marshes [17].
Berta.--Lat. 18 deg. 07'; long. 94 deg. 27'; elev. 5 m. A ranch just
south of Coatzacoalcos; scrub and low evergreen forest [15].
Chacaltianguis.--Lat. 18 deg. 18'; long. 95 deg. 52'; elev. 5 m. A
village on the Rio Papaloapan; savanna [8].
Ciudad Aleman.--Lat. 18 deg. 13'; long. 96 deg. 07'; elev. 30 m. A
new government town, headquarters of the Comision del
Papaloapan; scrub and low evergreen forest [10].
Coatzacoalcos (formerly Puerto Mexico).--Lat. 18 deg. 10'; long.
94 deg. 27'; elev. 2 m. A seaport at the mouth of the Rio
Coatzacoalcos; scrub on coastal dunes; marshes and low
evergreen forest inland [16]. Most collections are from the
forest-savanna ecotone, 8 kilometers southwest of town.
Cosamaloapan.--Lat. 18 deg. 22'; long. 95 deg. 50'; elev. 4 m. An
agricultural town on the Rio Papaloapan; savanna and sugar
plantations [7].
Cosoleacaque.--Lat. 17 deg. 59'; long. 94 deg. 38'; elev. 55 m. A
village 8 kilometers by road west of Minatitlan; savanna
[19].
Cuatotolapam.--Lat. 18 deg. 08'; long. 95 deg. 16'; elev. 13 m. A
village on the Trans-isthmian Railroad; savanna and low
evergreen forest along streams [13].
Hueyapan.--Lat. 18 deg. 08'; long. 19 deg. 09'; elev. 85 m. A town
32 kilometers by road northwest of Acayucan; savanna and low
evergreen forest [14]. Collections were made in the vicinity
of the town and from forest 10 kilometers southeast of town
at an elevation of 135 meters.
Jesus Carranza (formerly Santa Lucrecia).--Lat. 17 deg. 27';
long. 95 deg. 02'; elev. 80 m. A town and railroad junction in
the middle of the isthmus; rainforest [26]. Most of
Dalquest's specimens came from varying distances from Jesus
Carranza along the Rio Coatzacoalcos and its tributaries.
Minatitlan.--Lat. 17 deg. 58'; long. 94 deg. 32'; elev. 15 m. An oil
refinery center on the Rio Coatzacoalcos; savanna [20].
Naranjo.--Lat. 17 deg. 35'; long. 95 deg. 07'; elev. 100 m. A
village on the Trans-isthmian Highway, 45 kilometers south
of Acayucan; rainforest and palm forest [24].
Novillero.--Lat. 18 deg. 16'; long. 95 deg. 59'; elev. 10 m. A
village on the Rio Papaloapan; scrub forest and grassland
[9].
Oaxaquena, La.--Lat. 17 deg. 26'; long. 94 deg. 53'; elev. 80 m. A
hacienda on the Rio Coatzacoalcos about 12 kilometers east
of Jesus Carranza; rainforest [27].
Playas, Rio de las.--Lat. 18 deg. 08'; long. 94 deg. 07'; elev. 3 m.
The river (sometimes known as the Rio Tonola) forming the
boundary between the states of Veracruz and Tabasco;
rainforest [18].
San Lorenzo.--Lat. 17 deg. 44'; long. 94 deg. 42'; elev. 25 m. A
village on the Rio Chiquito, about 30 kilometers southeast
of Acayucan; rainforest [23].
Suchil.--Lat. 17 deg. 31'; long. 95 deg. 03'; elev. 40 m. A village
on the Trans-isthmian Railroad, about 10 kilometers north of
Jesus Carranza; rainforest [25].
Tecolapan.--Lat. 18 deg. 24'; long. 95 deg. 18'; elev. 275 m. A
village on a small river of the same name in the western
foothills of Los Tuxtlas; rainforest [5].
Tejada, Lerdo de.--Lat. 18 deg. 37'; long. 95 deg. 31'; elev. 60 m.
An agricultural village, 35 kilometers by road
east-southeast of Alvarado; scrub forest, marshes, and sugar
plantations [2]. Collections were made in a marsh, 5
kilometers west-northwest of the village.
Tlacotalpan.--Lat. 18 deg. 37'; long. 95 deg. 42'; elev. 3 m. A town
at the confluence of the Rio San Juan and Rio Papaloapan;
marshes and sugar plantations [3].
Tula.--Lat. 18 deg. 36'; long. 95 deg. 22'; elev. 150 m. A village
near the western base of Los Tuxtlas; low evergreen forest
and marshes [4]. Collections were made in a marsh 3
kilometers northwest of the village.
THE AMPHIBIAN FAUNA OF THE LOWLANDS
In presenting an account of the amphibian fauna of the lowlands of the
Isthmus of Tehuantepec three items must be considered:
1. The composition of the fauna.
2. The ecology of the fauna.
3. The distribution of the fauna.
These items, together with similar data concerning the amphibians of
the adjacent highlands, will form the basis for the subsequent
discussion of the establishment of present patterns of distribution in
the isthmian region.
_Composition of the Fauna_
The amphibian fauna of the lowlands of the Isthmus of Tehuantepec
consists of 36 species definitely recorded from the area. These include
one genus and species of caecilian, one genus, including three species
of salamanders, and 14 genera and 32 species of anurans.
In comparison with the known amphibian fauna of the forested and
savanna portions of El Peten, Guatemala (Stuart, 1935 and 1958), we
find that there are more species recorded from the isthmus than from El
Peten. Stuart found only 20 species of amphibians in both forest and
savanna habitats in El Peten. Of the 36 species of amphibians known
from the isthmus, 28 occur on the Gulf lowlands and live in forest or
savanna habitats.
The geographic position of the isthmus with regard to major faunal
areas in Middle America, and the diversity of the environment are
important factors in understanding the presence of a large number of
species of amphibians in the isthmus. The large number of species
probably is a reflection of the diversity of the environment; this
diversity is the result of fluctuation of climate, and thus
environments, in the not too distant past. In no individual habitat,
such as rainforest, savanna, or scrub forest, does the number of
species approach the total for the region.
_Ecology of the Fauna_
In the preceding section on the description of the Isthmus of
Tehuantepec I have outlined the major environments in the region. With
respect to the distribution of amphibians we may recognize three major
environments in the isthmus--rainforest, semi-arid scrub forest, and
savanna. Each of these has varying combinations of physical and biotic
factors that are important in the ecology of amphibians. Because of the
importance of moisture, not only for the maintenance of life in these
animals, but in most species their dependence on water for breeding
purposes, this environmental factor is considered the most significant
in the ecological distribution of amphibians. A second factor is the
availability of necessary shelter, especially aestivation sites. These
factors will be compared in the three major environments in the region.
Moisture is present in the environment in the form of free water or
atmospheric moisture. With respect to the latter, it is well known that
dense shaded forests have a considerably higher relative humidity than
do open plains or areas with only scattered trees. Thus, the
rainforests of the isthmus are characterized by a much higher relative
humidity than are the savannas or semi-arid scrub forests. Although
with regard to rainfall there is a pronounced dry season in the regions
supporting rainforest, there still remains considerable atmospheric
moisture in this environment throughout the year. The dense foliage
provides shade and protection from desiccating effects of wind and
sunlight; furthermore the foliage contributes moisture by
transpiration. The deep alluvial soils mixed with large quantities of
organic matter (decaying leaves and rotting logs) maintain considerable
quantities of moisture.
Conversely, the savannas and scrub forests have little atmospheric
moisture during the dry season. In the former habitat there are few
trees to provide shade or moisture through transpiration; in the latter
most of the trees lose their leaves during the dry season. Thus, these
environments are desiccated by the dry winds and direct sunlight.
Furthermore, the soils in these environments become dry and caked.
There is little or no terrestrial matter to hold moisture.
Free water in these environments is present in a variety of forms at
different times of the year. During the dry season the more extensive
marshes in the savannas persist; many ponds and most of the streams in
the rainforest are permanent throughout the year. In the scrub forest
all except the largest streams become dry during the dry season, and no
ponds exist through the dry season. With the advent of the first heavy
summer rains the stream beds fill with water, marshes expand, and many
depressions become ponds (Pl. 5, fig. 2). At this time the amount of
free water in the scrub forests and savannas greatly increases, much
more so than that in the rainforests.