Chapter I: Part 1

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.

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.]

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.

_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.

Environments are vertically stratified in the rainforests. There is the deep alluvial soil, the ground litter of leaves and decaying logs, the low bushes and small trees, and finally the tall trees of the forest. Each of these provides certain types of shelter for amphibians. The moist soil and litter on the forest floor is an important microhabitat for fossorial and strictly terrestrial species. The dense foliage of the trees, tree holes, and bromeliads growing on the trees provide shelter for arboreal species. Arboreal and terrestrial bromeliads and the terrestrial elephant-ear plants (_Xanthosoma_) contain water in the axils of their leaves throughout the year and thus provide an important habitat for amphibians. The low, spiny, deciduous trees of the scrub forest and the grasses and scattered trees in the savannas provide little shelter. In the savannas there are depressions, some of which contain water throughout the year; these are often surrounded by trees providing refugia for amphibians during the dry season. In the scrub forest many species congregate along streams and in moist stream beds during the dry season.

Now that the important ecological factors of the major environments have been outlined, we may examine the local distribution of amphibians in each of these. Beginning with the rainforest, we find only one fossorial species, _Gymnopis mexicanus_. A large number of species are found on the forest floor; characteristic inhabitants of the leaf litter are: _Bufo valliceps_, _Eleutherodactylus rhodopis_, _Microbatrachylus pygmaeus_, and _Syrrhophus leprus_. Other terrestrial amphibians usually are not scattered throughout the rainforest, as are those named immediately above, but instead inhabit areas of forest adjacent to ponds or streams; these species include: _Bufo marinus_, _Eleutherodactylus natator_, _Eleutherodactylus rugulosus_, _Leptodactylus labialis_, _Leptodactylus melanonotus_, _Rana palmipes_ and _Rana pipiens_. The most striking ecological assemblage of amphibians in the rainforest is the arboreal group of species, including:

_Bolitoglossa occidentalis_
_Bolitoglossa platydactyla_
_Eleutherodactylus alfredi_
_Hyla baudini_
_Hyla ebraccata_
_Hyla loquax_
_Hyla microcephala martini_
_Hyla picta_
_Phrynohyas modesta_
_Phrynohyas spilomma_
_Phyllomedusa callidryas taylori_

In the savannas _Rhinophrynus dorsalis_, _Engystomops pustulosus_, and _Gastrophryne usta_ are fossorial species. _Bufo marinus_, _Leptodactylus melanonotus_, _Leptodactylus labialis_, _Rana palmipes_, and _Rana pipiens_ are found in the vicinity of permanent water in the savannas. Although the savanna habitat does not provide the ecological conditions for the existence of an arboreal fauna, many arboreal species from the surrounding rainforest utilize the extensive marshes and ponds in the savannas for breeding purposes. Thus, _Hyla baudini_, _Hyla microcephala martini_, _Hyla picta_, and _Phrynohyas spilomma_ have been found breeding in savannas. In parts of savannas where clumps of trees surround depressions containing water throughout the year, individuals of the species named above, together with _Hyla loquax_ and _Phyllomedusa callidryas taylori_, may not only breed, but remain throughout the year.

In the semi-arid scrub forest the same fossorial species as exist in the savannas are found. Likewise, _Bufo marinus_, _Leptodactylus labialis_, _Leptodactylus melanonotus_, and _Rana pipiens_ are found near permanent water. Terrestrial species in this semi-arid environment include _Bufo canaliferus_, _Bufo coccifer_, _Bufo marmoreus_, _Syrrhophus pipilans_, and _Diaglena reticulata_. Of these, _Syrrhophus pipilans_ sometimes inhabits low trees and bushes; the others may be fossorial. The arboreal species in the scrub forest include _Hyla baudini_, _Hyla robertmertensi_, _Hyla staufferi_, and _Phyllomedusa dacnicolor_.

_Eleutherodactylus rugulosus_ and _Hylella sumichrasti_ live along streams in the scrub forest. _Hylella sumichrasti_ lays its eggs in these streams.

In comparing the ecological differences in the amphibian assemblages in the three major habitats, the most obvious difference is the great percentage of arboreal species in the rainforest as compared with savanna and scrub forest. Only four arboreal species are found in the scrub forest, none in the savannas, but eleven in the rainforest. Likewise, there is an absence of ground-dwelling forms in the arid habitats; in the latter the only terrestrial species are those that are found near water. A possible exception is _Syrrhophus pipilans_.

From the above analysis of ecological distribution we may see that the rainforest provides a variety of habitats for amphibians and that these habitats are suitable for amphibian life throughout the year. On the other hand, the savannas and scrub forests are characterized by extreme conditions of desiccation, a factor of considerable importance in limiting the ecological distribution of amphibians. However, there still is a diversity of amphibians in these semi-arid environments. Obviously, these species are adapted in various ways for survival during the dry season, at which time environmental conditions are such that the animals cannot carry on their normal activities.

Although there is not an abundance of data concerning the seasonal activity of the fauna, what is available shows some interesting correlations with the environments. During the dry season in the scrub forest there is essentially no amphibian activity; an occasional _Rana pipiens_ may be seen along a river, or a _Bufo marinus_ may be seen at night. In the rainforest the terrestrial-breeding amphibians are active during the dry season. _Eleutherodactylus rugulosus_ is found at night or by day along streams. _Eleutherodactylus rhodopis_, _Microbatrachylus pygmaeus_, and _Bufo valliceps_ are active during the day; these plus _Bolitoglossa occidentalis_, _Bolitoglossa platydactyla_, _Eleutherodactylus alfredi_, _Eleutherodactylus natator_, and an occasional _Hyla_ are active at night.

With the onset of the heavy summer rains and the subsequent formation of breeding ponds, amphibian activity reaches a peak. This is especially noticeable in the semi-arid environments, where during the dry season there is little activity.

Among the anurans in the isthmus the four species of _Eleutherodactylus_, the two species of _Syrrhophus_, and the one species of _Microbatrachylus_ are either known, or presumed, to lay eggs on the ground; these develop directly into small frogs. All of the other anurans deposit their eggs in water or attach them to objects over water (_Phyllomedusa_); these hatch into tadpoles, which later metamorphose into frogs. _Hylella sumichrasti_ is known to breed only in streams. All of the other species breed in ponds, but at times some species deposit their eggs in streams; in this last group are _Bufo valliceps_, _Bufo marmoreus_, _Phyllomedusa callidryas taylori_, and _Rana pipiens_.

Although the ecological data are incomplete, they do show that ecological conditions differ greatly in the three major environments, different species of amphibians inhabit these environments, and that the fauna is ecologically diversified in each environment.

_Distribution of the Fauna_

Plotting the distributions of species of amphibians known to live in the lowlands of the Isthmus of Tehuantepec results in an array of geographic patterns. These may be analyzed with respect to those species that are restricted either to the Gulf lowlands or the Pacific lowlands, or those that occur on both the Gulf and Pacific lowlands. Furthermore, the distributions may be analyzed with respect to those species whose ranges extend from Mexico across the Isthmus of Tehuantepec into Central America, those that reach the isthmus from Central America but do not extend into Mexico proper, and those that reach the isthmus from Mexico but do not extend into Central America. It should be kept in mind that the following analysis is of the lowland inhabitants only. Species inhabiting the foothills and mountains will be discussed later.

1. SPECIES RESTRICTED TO THE GULF LOWLANDS. Of the 36 species of amphibians recorded from the Isthmus of Tehuantepec, nine (25 per cent) are in this group. Four of these (_Eleutherodactylus alfredi_, _Syrrhophus leprus_, _Hyla loquax_, and _Hyla picta_) live in the Gulf lowlands to the east and to the west of the isthmus. Three others (_Hyla ebraccata_, _Hyla microcephala martini_ and _Phyllomedusa callidryas taylori_) are primarily Central American in their distribution and reach the northwestern limits of their ranges in the Gulf lowlands of the isthmus, whereas _Bolitoglossa platydactyla_ and _Eleutherodactylus natator_ reach the southern limits of their distributions in the isthmus.

2. SPECIES RESTRICTED TO THE PACIFIC LOWLANDS. This group includes six species, or 17 per cent of the amphibian fauna of the isthmus. Two of these (_Bufo coccifer_ and _Syrrhophus pipilans_) range to the east and to the west of the isthmus on the Pacific lowlands. Two others (_Bufo canaliferus_ and _Hyla robertmertensi_) range from the isthmus into Central America, and _Diaglena reticulata_ and _Phyllomedusa dacnicolor_ range on the Pacific lowlands of Mexico southeastward to the isthmus.

3. SPECIES THAT OCCUR ON THE PACIFIC AND GULF LOWLANDS. This group includes 19 species, or 53 per cent of the total amphibian fauna. Of these, nine species (25 per cent of the entire amphibian fauna) are widespread throughout the lowlands of Mexico and Central America; these are:

_Gymnopis mexicanus_
_Rhinophrynus dorsalis_
_Bufo marinus_
_Engystomops pustulosus_
_Leptodactylus labialis_
_Leptodactylus melanonotus_
_Hyla baudini_
_Hyla staufferi_
_Rana pipiens_