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, México

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, México

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, México

BY
WILLIAM E. DUELLMAN

CONTENTS

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 México 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, César Fárjas of Donají, and Juan Mayol of San Andrés Tuxtla. Profesor Jordi Juliá Z. of the Laboratorio de Entomología, Comisión del Papaloapan, Ciudad Alemán, 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 Ganadería, 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 Río Jaltepec and Matías 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 Alemán. 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 México.

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 Jesús Carranza. For the past two decades Thomas MacDougall, a resident of New York City, has spent his winters collecting biological specimens in southern México. 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 México 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 Juárez, Sierra de los Míjes, 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 Río Coatzacoalcos, which flows in a northerly course to the Gulf of Mexico. The western part is drained by the Río San Juan, the principal tributary of the Río Papaloapan. Behind the coastal dunes are frequent, and sometimes large, lagoons. Immediately inland from Coatzacoalcos and along the lower stretches of the Río Papaloapan are extensive marshes. Essentially the entire coastal plain, with the exception of the coastal dunes, consists of rich alluvial deposits.

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 Río 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 Río Coatzacoalcos; the principal ones from north to south are—Río Jaltepec, Río Tortuguero, Río Sarabia, and Río 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 Río 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 (Minatitlán 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 Minatitlán it is September. There is little variation in temperature throughout the isthmus; the average annual temperature at Salina Cruz is 26.6° C.; that at Minatitlán is 26.2° 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.

Fig. 3. Climatographs for Minatitlán, 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 Río Coatzacoalcos and Río 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 Minatitlán 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 Mogoñe southward to about Matías Romero.

In the vicinity of Matías 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 Juchitán 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 Río Papaloapan and the Río Coatzacoalcos. Volcán San Martín, 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° 38'; long. 94° 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° 18'; long. 95° 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° 38'; long. 95° 07'; elev. 314 m. A village about 10 kilometers southwest of Matías Romero; transition between scrub forest and broadleaf hardwood forest [38].

Bisilana.—Lat. 16° 20'; long. 95° 13'; elev. 35 m. A place name for a former ranch at the edge of Tehuantepec; open arid scrub forest [62].

Chivela.—Lat. 16° 20'; long. 95° 01'; elev. 195 m. A village on the Trans-isthmian Railroad, 26 kilometers by rail south of Matías Romero and on the western edge of the semi-arid Plains of Chivela [40].

Concepción.—Lat. 16° 17'; long. 95° 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."

Donají.—Lat. 17° 13'; long. 95° 02'; elev. 90 m. A village at Km. 155 on the Trans-isthmian Highway; rainforest [29].

Escurano.—Lat. 16° 25'; long. 95° 27'; elev. 500 m. A ranch about 25 kilometers west-northwest of Tehuantepec; arid scrub forest [51].

Guichicovi, San Juan.—Lat. 16° 58'; long. 95° 06'; elev. 250 m. A village on the north slopes of the isthmus, 12 kilometers north-northwest of Matías Romero; cleared hardwood forest and coffee plantations [33].

Huilotepec.—Lat. 16° 14'; long. 95° 09'; elev. 30 m. A small village on the Río Tehuantepec, 13 kilometers south-southeast of Tehuantepec; open arid scrub forest [69].

Ixtepec.—Lat. 16° 34'; long. 95° 06'; elev. 60 m. A town and railroad junction on the northwestern edge of the Plains of Tehuantepec; arid scrub forest [45].

Juchitán.—Lat. 16° 26'; long. 95° 02'; elev. 15 m. A town on the Plains of Tehuantepec, 22 kilometers by road east-northeast of Tehuantepec; arid scrub forest [50].

Limón.—Lat. 16° 20'; long. 95° 29'; elev. 600 m. A former agrarian colony and now a small ranch about 27 kilometers west of Tehuantepec; arid scrub forest [60].

Matías Romero.—Lat. 16° 53'; long. 95° 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° 24'; long. 95° 18'; elev. 40 m. A village on the Río Tehuantepec, northwest of Tehuantepec; dense scrub forest [57].

Modelo, El.—Lat. 17° 07'; long. 94° 43'; elev. 200 m. An old rubber plantation on the Río Chalchijapa, a tributary to the Río Coatzacoalcos; rainforest [31].

Nanches, Portillo Los.—Lat. 16° 35'; long. 95° 37'; elev. 500 m. A place name, about 4 kilometers southeast of Totolapilla; scrub forest [44].

Nizanda.—Lat. 16° 42'; long. 95° 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° 43'; long. 94° 40'; elev. 300 m. A small ranch on the west base of Cerro Atravesado; scrub forest [39].

Papaloapan.—Lat. 18° 11'; long. 96° 06'; elev. 25 m. A small village on the Río Papaloapan in northern Oaxaca; low evergreen forest and savanna [11].

Princesa, La.—Lat. 16° 56'; long. 95° 02'; elev. 150 m. A ranch on the northern slopes of the isthmus, 6 kilometers by road north of Matías Romero; poorly developed rainforest [34].

Quiengola, Cerro de.—Lat. 16° 24'; long. 95° 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° 25'; long. 95° 20'; elev. 45 m. A ranch on the Río Tehuantepec, northwest of Tehuantepec; dense scrub forest [52].

Salina Cruz.—Lat. 16° 10'; long. 95° 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° 15'; long. 95° 22'; elev. 40 m. A ranch about 25 kilometers west-southwest of Tehuantepec; arid scrub forest [68].

San Pablo.—Lat. 16° 24'; long. 95° 18'; elev. 40 m. A ranch on the Río 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° 18'; long. 95° 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° 25'; long. 94° 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 Lucía.—Lat. 16° 18'; long. 95° 28'; elev. 800 m. A place name for a former ranch on the east slopes of Cerro Arenal; scrub forest [63].

Santa María Chimalapa.—Lat. 16° 55'; long. 94° 42'; elev. 296 m. A village on the Río de los Milagros, a tributary to the Río Coatzacoalcos; rainforest [35].

Santiago Chivela.—Lat. 16° 42'; long. 94° 53'; elev. 200 m. A village on the Trans-isthmian Highway, 13.4 kilometers by road south of Matías 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° 50'; long. 95° 08'; elev. 225 m. A village about 13 kilometers west-southwest of Matías Romero; semi-arid scrub forest [37].

Sarabia.—Lat. 17° 04'; long. 95° 02'; elev. 100 m. A village 25 kilometers north of Matías Romero on the Trans-isthmian Highway; rainforest [32]. Collections were made in the vicinity of the village and in the rainforest along the Río Sarabia, 5 kilometers north of the village at an elevation of 80 meters.

Tapanatepec.—Lat. 16° 32'; long. 94° 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° 20'; long. 95° 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° 16'; long. 95° 30'; elev. 1100 m. A town in the mountains about 40 kilometers west-southwest of Tehuantepec; scrub forest [67].

Tequisistlán.—Lat. 16° 24'; long. 95° 37'; elev. 190 m. A village in the valley of the Río Tequisistlán, a tributary to the Río Tehuantepec; dense scrub forest [54]. Most collections were made about one kilometer north of the village where the Pan-American Highway crosses the Río Tequisistlán.

Tolosita.—Lat. 17° 12'; long. 95° 03'; elev. 80 m. A village on the Río Tortuguero near the Trans-isthmian Highway; rainforest [30].

Tres Cruces.—Lat. 16° 26'; long. 95° 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° 06'; long. 96° 05'; elev. 80 m. A town on the Río Papaloapan in northern Oaxaca; low evergreen forest [12].

Ubero.—Lat. 17° 18'; long. 95° 00'; elev. 80 m. A lumber camp and railroad station, 8.5 kilometers south of the Río Jaltepec on the Trans-isthmian Highway; rainforest [28].

Unión Hidalgo.—Lat. 16° 27'; long. 94° 48'; elev. 7 m. A village on the railroad, 20 kilometers east-northeast of Juchitán; open scrub forest [48].

Ventosa, La.—Lat. 16° 30'; long. 94° 51'; elev. 25 m. A village at the junction of the Pan-American and Trans-isthmian highways; open scrub forest [46].

Zanatepec.—Lat. 16° 28'; long. 94° 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° 21'; long. 95° 48'; elev. 800 m. A ranch between La Reforma (16 kilometers west of Tequisistlán) and Santa María Ecatepec; scrub forest with oaks on higher ridges [59].

Veracruz

Acayucan.—Lat. 17° 57'; long. 94° 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° 47'; long. 95° 47'; sea level. A fishing village at the mouth of the Río 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.

Amatitlán.—Lat. 18° 26'; long. 95° 45'; elev. 4 m. A village on the bank of the Río Papaloapan; savanna and sugar plantations [6].

Aquilera.—Lat. 17° 48'; long. 95° 01'; elev. 150 m. A village 21 kilometers southwest of Acayucan on the Trans-isthmian Highway; rainforest [22].

Ayentes.—Lat. 18° 10'; long. 94° 26'; elev. 2 m. A railroad station on the east bank of the Río Coatzacoalcos, across the river from the city of Coatzacoalcos; scrub forest and marshes [17].

Berta.—Lat. 18° 07'; long. 94° 27'; elev. 5 m. A ranch just south of Coatzacoalcos; scrub and low evergreen forest [15].

Chacaltianguis.—Lat. 18° 18'; long. 95° 52'; elev. 5 m. A village on the Río Papaloapan; savanna [8].

Ciudad Alemán.—Lat. 18° 13'; long. 96° 07'; elev. 30 m. A new government town, headquarters of the Comisión del Papaloapan; scrub and low evergreen forest [10].

Coatzacoalcos (formerly Puerto México).—Lat. 18° 10'; long. 94° 27'; elev. 2 m. A seaport at the mouth of the Río 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° 22'; long. 95° 50'; elev. 4 m. An agricultural town on the Río Papaloapan; savanna and sugar plantations [7].

Cosoleacaque.—Lat. 17° 59'; long. 94° 38'; elev. 55 m. A village 8 kilometers by road west of Minatitlán; savanna [19].

Cuatotolapam.—Lat. 18° 08'; long. 95° 16'; elev. 13 m. A village on the Trans-isthmian Railroad; savanna and low evergreen forest along streams [13].

Hueyapan.—Lat. 18° 08'; long. 19° 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.

Jesús Carranza (formerly Santa Lucrecia).—Lat. 17° 27'; long. 95° 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 Jesús Carranza along the Río Coatzacoalcos and its tributaries.

Minatitlán.—Lat. 17° 58'; long. 94° 32'; elev. 15 m. An oil refinery center on the Río Coatzacoalcos; savanna [20].

Naranjo.—Lat. 17° 35'; long. 95° 07'; elev. 100 m. A village on the Trans-isthmian Highway, 45 kilometers south of Acayucan; rainforest and palm forest [24].

Novillero.—Lat. 18° 16'; long. 95° 59'; elev. 10 m. A village on the Río Papaloapan; scrub forest and grassland [9].

Oaxaqueña, La.—Lat. 17° 26'; long. 94° 53'; elev. 80 m. A hacienda on the Río Coatzacoalcos about 12 kilometers east of Jesús Carranza; rainforest [27].

Playas, Río de las.—Lat. 18° 08'; long. 94° 07'; elev. 3 m. The river (sometimes known as the Río Tonolá) forming the boundary between the states of Veracruz and Tabasco; rainforest [18].

San Lorenzo.—Lat. 17° 44'; long. 94° 42'; elev. 25 m. A village on the Río Chiquito, about 30 kilometers southeast of Acayucan; rainforest [23].

Suchil.—Lat. 17° 31'; long. 95° 03'; elev. 40 m. A village on the Trans-isthmian Railroad, about 10 kilometers north of Jesús Carranza; rainforest [25].

Tecolapan.—Lat. 18° 24'; long. 95° 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° 37'; long. 95° 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° 37'; long. 95° 42'; elev. 3 m. A town at the confluence of the Río San Juan and Río Papaloapan; marshes and sugar plantations [3].

Tula.—Lat. 18° 36'; long. 95° 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 Petén, Guatemala (Stuart, 1935 and 1958), we find that there are more species recorded from the isthmus than from El Petén. Stuart found only 20 species of amphibians in both forest and savanna habitats in El Petén. 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 México across the Isthmus of Tehuantepec into Central America, those that reach the isthmus from Central America but do not extend into México proper, and those that reach the isthmus from México 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 México 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 México and Central America; these are:

Gymnopis mexicanus
Rhinophrynus dorsalis
Bufo marinus
Engystomops pustulosus
Leptodactylus labialis
Leptodactylus melanonotus
Hyla baudini
Hyla staufferi
Rana pipiens

Four species occur on the Gulf lowlands to the east and to the west of the isthmus, but on the Pacific lowlands they occur only to the east; this group includes Bufo valliceps, Eleutherodactylus rhodopis, Phrynohyas modesta, and Phrynohyas spilomma. Three species live to the east and to the west of the isthmus on the Pacific lowlands, but only to the west on the Gulf lowlands; these include Eleutherodactylus rugulosus, Microbatrachylus pygmaeus, and Gastrophryne usta.

Six species that cross the isthmus live on the humid Gulf lowlands and on the humid lowlands of Chiapas and Guatemala, but not on the semi-arid Plains of Tehuantepec; these include Bolitoglossa occidentalis, Eleutherodactylus rhodopis, Microbatrachylus pygmaeus, Phrynohyas modesta, Phrynohyas spilomma, and Rana palmipes. Of these, Microbatrachylus pygmaeus also occurs in scattered humid environments to the west of the isthmus on the Pacific lowlands.

Two species are endemic to the isthmian region. Bolitoglossa veracrucis is known only from the humid northern slopes of the isthmus. Hylella sumichrasti occurs on the Pacific slopes of the isthmus and extends to the east into western Chiapas.

In analyzing the distribution of the amphibians with respect to those that are restricted to either the Pacific or Gulf lowlands or those that cross the continental divide in the isthmus, we find that 25 per cent of the species are restricted to the Gulf lowlands, 17 per cent are restricted to the Pacific lowlands, and 53 per cent cross the isthmus. In analyzing the distribution patterns with respect to those that extend across the isthmus of Tehuantepec from east to west, we find that 14 per cent of the species do not extend east of the isthmus into Central America and that 19 per cent do not range west of the isthmus into México proper; 61 per cent of the species range to the east and to the west of the isthmus. Of the 36 species of amphibians inhabiting the isthmus only nine species (25 per cent) range across the isthmus, that is, occur on the Gulf and Pacific lowlands, and also range to the east and to the west of the isthmus. To these wide-ranging species the diversified environments of the isthmus do not present a barrier to distribution. The other 27 species (75 per cent) either do not cross the isthmus from east to west or from north to south; thus, probably in one way or another the isthmus presents a barrier to their distribution.

THE AMPHIBIAN FAUNA OF THE FOOTHILLS AND ADJACENT HIGHLANDS

To amphibians inhabiting the foothills and mountains of southern México and northern Central America, the isthmus presents a great barrier to dispersal. For example, salamanders of the genus Thorius, the mexicanus and augusti groups of the genus Eleutherodactylus, the bistincta group of the genus Hyla, and the genus Tomodactylus occur on the Mexican Plateau and southward into the mountains of Oaxaca. Nevertheless, no members of these groups are present in the Guatemalan-Chiapan Highlands. The genera Chiropterotriton, Magnadigita, Pseudoeurycea, and Ptychohyla, as well as the eximia group of Hyla are represented by different species in the Guatemalan-Chiapan Highlands than in the mountains of México on the other side of the isthmus. Several species of Plectrohyla occur in the Guatemalan-Chiapan Highlands, but none is known from the Mexican Highlands, although one species occurs in the Tuxtlas.

Living in the humid forests of the foothills are salamanders of the genus Lineatriton, frogs of the spatulatus group of Eleutherodactylus, Anotheca coronata, Hyla miotympanum, and Phyllomedusa moreleti. All of these occur in the foothills of the Sierra Madre Oriental in eastern México and in Los Tuxtlas. Lineatriton, Hyla miotympanum, and the spatulatus group of Eleutherodactylus do not occur in the foothills of the Guatemalan-Chiapan Highlands; those amphibians reach the end of their ranges at the isthmus. Phyllomedusa moreleti and Anotheca coronata are found in the northern foothills of the Guatemalan-Chiapan Highlands, and Phyllomedusa moreleti is found in the foothills on the Pacific slopes of the Chiapan Highlands.

Although the above analysis is not so detailed as that of the lowland inhabitants, it does show that all of the genera and species of amphibians known to inhabit the foothills and highlands adjacent to the isthmus, only two species of amphibians cross the isthmus from one highland mass to the other. Thus, it is evident that the Isthmus of Tehuantepec presents a great barrier to dispersal of these groups of amphibians.

ESTABLISHMENT OF PRESENT PATTERNS OF DISTRIBUTION

From the foregoing analysis of geographical and ecological distribution in the Isthmus of Tehuantepec we may strive for an interpretation of the events that led to the establishment of patterns of distribution displayed not only by the amphibians, but other terrestrial vertebrates as well. The thesis that I am proposing below is based on the premise that in southern México and northern Central America climatic fluctuation during the Pleistocene was of sufficient magnitude to cause vegetational shifts, both vertically and latitudinally, resulting in the establishment of alternating continuous and discontinuous lowland and highland environments, although this climatic fluctuation was not so great as to eliminate tropical lowland environments from the region. I feel that the present patterns of distribution of the amphibians in the Isthmus of Tehuantepec may be explained on this premise.

Many authors dealing with the herpetofauna of Middle America have followed Schuchert's (1935) suggestion of a seaway in the isthmus during the Cenozoic. Thus, Burt (1931), Duellman (1956, 1958a), Gloyd (1940), Oliver (1948), Smith and Laufe (1946), and Stuart (1941) employed the presence of a seaway to explain distribution and speciation in various genera. Durham, Arellano, and Peck (1952), Olson and McGrew (1941), and Stirton (1954) have provided geological evidence that there probably was no Cenozoic seaway in the Isthmus of Tehuantepec. Even if there were a seaway in the Pliocene or Miocene (the dating of this possible seaway is open to question), its presence is not necessary to explain the present patterns of distribution in the isthmus.

In recent years the study of natural biotic environments, palynology, and Pleistocene chronology in Middle America has produced a wealth of data, which although still fragmentary begins to form a picture of past climatic events in that part of the world. Sedimentary studies by Hutchinson, Patrick, and Deevey (1956) and Sears, Foreman, and Clisby (1955) have provided evidence of drastic climatic shifts in México during the Pleistocene. Further evidence of bioclimatic fluctuation is provided by Martin and Harrell (1957) and Martin (1958); the latter has suggested that there was a displacement of the tropical zones in southern México and northern Central America by as much as 3000 feet during the glacial maximum. Much of the evidence of such drastic vertical shifts in environments is based on the presence of Pleistocene montane glaciers on Mexican volcanoes (White, 1956) and Chirripo in Costa Rica (Weyl, 1955). Dorf (1959) supports this idea of drastic climatic change.

In his studies of the avifauna of México and Guatemala Griscom (1932 and 1950) made an important issue of the continuity of the bird fauna in what he called the Subtropical Life-zone, which essentially consists of cloud forest, a widespread, but discontinuous, habitat on the Gulf (windward) slopes of the Mexican and Central American highlands at elevations between 1000 and 2000 meters. To account for this apparent uniformity in the avifauna Griscom hypothesized a continuity of cloud forest environment in the Pleistocene; this would result in the depression of cloud forests to the coastal lowlands and the displacement of tropical lowland environments far to the south in Central America. Stuart (1951) objected to this displacement of lowland tropical rainforest; he stated that a descent to sea level of a subtropical zone would have brought about either widespread extermination of the tropical fauna or acclimatization of that fauna to subtropical conditions.

Although palynological studies and some faunal studies of subtropical and temperate animals suggest a drastic climatic fluctuation that might have eliminated tropical environments in southern México and northern Central America, there is much biological evidence indicating the existence of tropical environments in this region even during the glacial maximum. Especially significant is the diversity of species inhabiting the present tropical environments; many of these have differentiated from related taxa to the south.

In the Pleistocene, climate fluctuated and vegetation shifted correspondingly in southern México and northern Central America. Most of the palynological studies and many studies of Pleistocene chronology deal with montane regions, either the Mexican Plateau or the mountains rising from the plateau. No such studies have been made in lowland tropical environments. During glacial advances the tropical lowland environments in México probably were not eliminated, for the great diversity of animals in these environments supports the hypothesis that they have been in existence for some time, although periodically they may have been discontinuous.

In order to understand the nature of bioclimatological events in the Pleistocene in lowland tropical environments of southern México, certain factors that are of little importance in the interpretation of Pleistocene chronology in the highlands must be considered. These factors are: 1) climatic moderation by oceans, 2) fluctuation in sea level, and 3) fluctuation in level of the water table as affected by sea level.

It is well-known that large bodies of water moderate the temperature on adjacent land. Furthermore, it is known that faunas of marine invertebrates shifted latitudinally in the Pleistocene; Trask, Phleger, and Stetson (1947) recorded cold-water Foraminifera then as far south as the Sigsbee Deep in the middle of the Gulf of Mexico. Large bodies of warm water, such as the Gulf of Mexico, Caribbean Sea, and Pacific Ocean of today, probably were not sufficiently cooled at the time of glacial advance to affect greatly the temperature of the winds blowing across them. Even if these bodies of water were somewhat cooler than now, the prevailing winds blowing from them onto the lowlands of México and northern Central America would have aided in maintaining relatively high temperatures there. These warm winds probably counteracted the cooling effect of glaciation in the lowlands and thereby maintained tropical conditions near the seas.

Although no adequate studies of Pleistocene beach lines have been made in southern México, such information is available for peninsular Florida on the other side of the Gulf of Mexico (Cooke, 1945). Fluctuation in sea level in the Pleistocene has been used by Hubbell (1954), Goin (1958), and Duellman and Schwartz (1958) to explain present patterns of distribution of animals in Florida. If Cooke's interpretations can be applied to the western side of the Gulf of Mexico, even generally, it would be supposed that sea level varied from about 300 feet lower than at present during the Illinoian Glacial Period to about 275 feet higher than at present during the Aftonian Interglacial Period. Lowering of sea level would expand the lowlands in the isthmus; rising sea level would restrict them, leaving only the central ridges and many islands in the isthmus, but never forming a seaway between the Gulf of Mexico and the Pacific Ocean.

Probably the level of the water table in the coastal lowlands and the gradients of the streams in the lowlands and foothills was closely correlated with fluctuation in sea level. If sea level fluctuated as much as 575 feet in the Pleistocene, changes in the level of the water table must have been of considerable magnitude.

During times of glacial advances the lowlands of the isthmus probably were more extensive and had more semi-arid tropical environments than at present, with patches of rainforest existing in sheltered valleys along the major streams. In the course of bio-climatic fluctuation the semi-arid environments (scrub forest and/or savanna) were continuous at times from the Pacific lowlands across the isthmus to the Gulf lowlands. At those times such typical inhabitants of the semi-arid environments as Rhinophrynus dorsalis, Engystomops pustulosus, and Hyla staufferi could have made their way across the isthmus. At times of most extensive glaciation, such as the Illinoian, temperatures in the isthmus probably were low enough to permit the growth of pine-oak forest and cloud forest continuously across the central ridges from the Mexican to the Chiapan-Guatemalan highlands. At those times such highland members of the fauna as Chiropterotriton, Pseudoeurycea, Magnadigita, and the eximia group of Hyla could have crossed the isthmus. During Wisconsin time, climate probably fluctuated less than during previous glaciations; probably no montane environments, except cloud forest, were represented in the isthmus during the Wisconsin. Even at this relatively late date such animals as Lineatriton lineola, Anotheca coronata, and Phyllomedusa moreleti could have crossed the isthmus.

During the interglacial periods, which in the isthmian region were characterized by warmer temperatures, higher sea level and consequently more restricted areas of lowlands, and possibly more rainfall than in the glacial periods, the continuity of pine-oak forest and cloud forest from east to west across the isthmus was interrupted. Probably, too, the semi-arid environments were restricted, and the rainforests were more widespread. At those times animals now inhabiting the rainforests of the Gulf lowlands and those inhabiting the Pacific lowlands of Chiapas and Guatemala could have crossed the isthmus. In this group are species such as Bolitoglossa occidentalis, Eleutherodactylus rhodopis, Microbatrachylus pygmaeus, and Rana palmipes.

The amount of differentiation in isolated populations of amphibians in southern México and northern Central America gives some idea of relative lengths of time of isolation from related populations. Those populations inhabiting high mountain environments on either side of the isthmus are specifically distinct. Some populations inhabiting cloud forests lower on the mountains are specifically distinct from related populations on the other side of the isthmus; between others there is no recognizable differentiation. Even though many populations are isolated from other populations of the same species in the lowlands of the isthmus, there is no apparent speciation. This indicates that the lowland environments and their inhabitants have been isolated from one another for a shorter time than have the highland environments and their inhabitants.

ACCOUNTS OF SPECIES

For each species of amphibian known to occur in the lowlands of the Isthmus of Tehuantepec, localities where one or more specimens were collected are listed, and variation, ecology, and life histories are discussed. A total of 2833 specimens has been examined for the purposes of this study. Individual specimens cited in the text are listed with catalogue numbers and abbreviations of the name of the museum, as follows:

AMNH American Museum of Natural History
KU University of Kansas Museum of Natural History
MCZ Museum of Comparative Zoology, Harvard College
UIMNH University of Illinois Museum of Natural History
UMMZ University of Michigan Museum of Zoology
USNM United States National Museum

Gymnopis mexicanus mexicanus Duméril and Bibron

Oaxaca: El Barrio (3); Matías Romero; Tehuantepec (2). Veracruz: Cosamaloapan; Cuatotolapam (2).

The two specimens from Cuatotolapam were collected by Ruthven in an area of mixed savanna and forest. The three specimens (USNM 30535-7) listed above from El Barrio were collected by Sumichrast; possibly they came from another locality. The city of Tehuantepec is divided into seven districts called "barrios." The two specimens listed from Tehuantepec (MCZ 1604) merely bear the data "Tehuantepec, Mexico." They may have come from the town, the district, or from anywhere in the isthmus. The specimen from Matías Romero has 109 primary and 67 secondary annuli, a length of 400 mm., and a diameter of 19 mm.; the one from Cosamaloapan has 106 primary and 58 secondary annuli, a length of 397 mm., and a diameter of 19 mm. Data on the other specimens were recorded by Dunn (1942:475).

Bolitoglossa occidentalis Taylor

Oaxaca: Río Sarabia (2); Ubero. Veracruz: La Oaxaqueña; 14 km. E of Suchil.

The specimens from Oaxaca are only tentatively assigned to occidentalis. All are immature and lack maxillary teeth. Taylor (1941:147) stated that the maxillary teeth are absent in young occidentalis. One from Río Sarabia is a male with a body-length of 29 mm. and a tail-length of 22 mm. The dorsum is reddish brown streaked with dark gray; the venter is dark gray. Two small individuals (one from Sarabia and one from Ubero) have body-lengths of 19 and 21 mm. and tail-lengths of 10.5 and 11 mm. In life they were pale yellowish tan above with a brown triangular mark on the occiput, but with no middorsal stripe. Both were found in the axils of elephant ear plants (Xanthosoma).

This species has been noted by Goodnight and Goodnight (1956:146) on the Atlantic lowlands at Palenque, Chiapas, and by Shannon and Werler (1955:362) at several localities in Los Tuxtlas, Veracruz. I have collected it at Vista Hermosa on the eastern slopes of the Sierra Madre Oriental above Tuxtepec in northern Oaxaca. Both B. occidentalis and B. rufescens have been reported from Palenque, Chiapas (Taylor and Smith, 1945:547). Reëxamination of specimens from northern Chiapas and Tabasco is needed to verify the sympatric occurrence of these two similar species.

Bolitoglossa platydactyla Tschudi

Oaxaca: La Oaxaqueña; Tolosita (2). Veracruz: Acayucan; Cuatotolapam; 25 km. ESE of Jesús Carranza; 14 km. E of Suchil; 2.7 km. N of Tula.

Known only from the Gulf lowlands in the isthmian region, this species has been taken in a variety of habitats within the humid forest area: under outer leaves of banana plants, under a rock along a stream, under a log in a plowed field, and on a reed in a pond at night. Three adult males have an average snout-vent length of 44 mm. and a tail-length of 41 mm. In life the color of the dorsum varied from orange-yellow to orange-tan, usually being more orange on the tail. The iris was a reddish orange.

Bolitoglossa veracrucis Taylor

Veracruz: 35 km. SE of Jesús Carranza (21).

This species is known only from the type series collected at night on a limestone cliff by Walter W. Dalquest. If this salamander is restricted to this type of habitat, it should be found in the region of extensive limestone outcroppings in northern Chiapas and southern Tabasco.

Rhinophrynus dorsalis Duméril and Bibron

Oaxaca: Ixtepec; Limón; Salina Cruz (18); Tehuantepec (57); Tuxtepec (3). Veracruz: Amatitlán (3); Cosamaloapan (5); Novillero (2); San Lorenzo.

This species inhabits the scrub forests of the Pacific coastal plain and the savannas in southern Veracruz; apparently it does not occur in rainforest. Consequently, its distribution in the isthmus is discontinuous.

PLATE 1

Fig. 1. Savanna about 75 kilometers east of Coatzacoalcos, Veracruz. Photograph by L. C. Stuart.

Fig. 2. Low scrub forest near Alvarado, Veracruz. Photograph by L. C. Stuart.

PLATE 2

Fig. 1. Rainforest near Tolosita, Oaxaca. March, 1956.

Fig. 2. Rainforest along the Río Sarabia, Oaxaca. March, 1956.

PLATE 3

Fig. 1. Transition forest near La Princesa, Oaxaca. March, 1956.

Fig. 2. Palm Savanna on the Plains of Chivela, Oaxaca. March, 1956.

PLATE 4

Fig. 1. Scrub forest on the Plains of Tehuantepec in dry season. March, 1956.

Fig. 2. Scrub forest on the Plains of Tehuantepec in rainy season. View toward the north. In the distance is the Continental Divide in the hills of the Isthmus. July, 1958.

PLATE 5

Fig. 1. Low, dense scrub forest near La Ventosa, Oaxaca. July, 1958.

Fig. 2. Temporary pond in scrub forest north of Salina Cruz, Oaxaca. July 7, 1958. Rhinophrynus dorsalis, Bufo marmoreus, and Diaglena reticulata were breeding here the previous night.

PLATE 6

Fig. 1. Calling male of Rhinophrynus dorsalis, photographed in a pond north of Santa Cruz, Oaxaca, on July 6, 1958. × 2/3.

Fig. 2. Color pattern variation in two adults of Bufo canaliferus from Juchitán, Oaxaca. × 2/3.

PLATE 7

Fig. 1. Calling male of Engystomops pustulosus, photographed in a pond west of Tehuantepec, Oaxaca, on July 5, 1956. × 2.

Fig. 2. Foamy egg mass of Engystomops pustulosus at the edge of a pond west of Tehuantepec, Oaxaca. July 5, 1956. × 3/8.

PLATE 8

Fig. 1. Calling male of Diaglena reticulata, photographed at a pond north of Salina Cruz, Oaxaca, on July 6, 1958. × 1/2.