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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 , and forming a low break between the highlands of M?xico 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 summarized his collections from the vicinity of Cuatotolapam, Veracruz, and Hartweg and Oliver 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 . The zoogeographic significance of the Isthmus of Tehuantepec is exemplified by the works of Burt , Duellman , Gloyd , Oliver , and Stuart , 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.

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.

My extensive field work in the Isthmus of Tehuantepec was made possible by grants from the Penrose Fund of the American Philosophical Society and the Bache Fund of the National Academy of Sciences . 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, C?sar F?rjas of Donaj?, and Juan Mayol of San Andr?s Tuxtla. Profesor Jordi Juli? Z. of the Laboratorio de Entomolog?a, Comisi?n del Papaloapan, Ciudad Alem?n, 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 Ganader?a, for providing me with the necessary permits.

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 R?o Jaltepec and Mat?as 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 Alem?n. 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 in other parts of M?xico.

Minor collections include those made by Matthew W. Stirling at San Lorenzo, Veracruz, February-April, 1946 , by Fred G. Thompson on a trip across the isthmus in December, 1955 , 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 .

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 M?xico 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.

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 Ju?rez, Sierra de los M?jes, and Sierra de Choapam. From lofty peaks, such as Cerro de Zempoaltepetl , 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 .

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 R?o Coatzacoalcos, which flows in a northerly course to the Gulf of Mexico. The western part is drained by the R?o San Juan, the principal tributary of the R?o Papaloapan. Behind the coastal dunes are frequent, and sometimes large, lagoons. Immediately inland from Coatzacoalcos and along the lower stretches of the R?o Papaloapan are extensive marshes. Essentially the entire coastal plain, with the exception of the coastal dunes, consists of rich alluvial deposits.

The central ridges extend from the R?o 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 R?o Coatzacoalcos; the principal ones from north to south are--R?o Jaltepec, R?o Tortuguero, R?o Sarabia, and R?o 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 R?o 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.

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 . At Salina Cruz on the Pacific Ocean the average annual rainfall is 1040 mm. ; of this amount, only 15 mm. falls from November through April. On the Gulf Coastal Plain 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 Minatitl?n 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 Minatitl?n 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.

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 and Goldman , 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.

According to Beard 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 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.

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 Minatitl?n the average rainfall for March and April 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 . 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.

Central Ridges

In the vicinity of Mat?as Romero open pine-oak forest is found on some ridges as low as 250 meters above sea level.

Pacific Coastal Plain

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 ; 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.

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 R?o Papaloapan and the R?o Coatzacoalcos. Volc?n San Mart?n, 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 is followed by latitude, longitude, elevation, general description , and general type of habitat. Unless otherwise noted, distances are straight-line 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" . Numbers in brackets identify the position of a locality on the accompanying map .

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 .

Arenal, Cerro de.--Lat. 16? 18'; long. 95? 32'; elev. 925 m. . A ridge northeast of Tenango; scrub forest on slopes and pine-oak forest on top .

Barrio, El.--Lat. 16? 38'; long. 95? 07'; elev. 314 m. A village about 10 kilometers southwest of Mat?as Romero; transition between scrub forest and broadleaf hardwood forest .

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 .

Chivela.--Lat. 16? 20'; long. 95? 01'; elev. 195 m. A village on the Trans-isthmian Railroad, 26 kilometers by rail south of Mat?as Romero and on the western edge of the semi-arid Plains of Chivela .

Concepci?n.--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 .

Coyol.--Exact position unknown; according to Smith and Taylor , Coyol is "between San Antonio and Las Cruces."

Donaj?.--Lat. 17? 13'; long. 95? 02'; elev. 90 m. A village at Km. 155 on the Trans-isthmian Highway; rainforest .

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

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 Mat?as Romero; cleared hardwood forest and coffee plantations .

Huilotepec.--Lat. 16? 14'; long. 95? 09'; elev. 30 m. A small village on the R?o Tehuantepec, 13 kilometers south-southeast of Tehuantepec; open arid scrub forest .

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 .

Juchit?n.--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 .

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