A Distributional Study of the Amphibians of the Isthmus of Tehuantepec, Mexico.

by William E. Duellman.

INTRODUCTION

Few regions in Middle America are so important zoogeographically as is the Isthmus of Tehuantepec, that neck of land connecting North America with Central America, separating the Pacific Ocean from the Gulf of Mexico by a distance of only about 220 kilometers (airline), and forming a low break between the highlands of Mexico and those of Central America. Before World War II the isthmus could be reached readily only by railroad or by ocean vessel to Salina Cruz or Coatzacoalcos. With the advent of roads, princ.i.p.ally 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 ill.u.s.trated 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 compet.i.tion exists among several species of tree frogs, all of which breed in the same ponds? What factors in the environment permit certain amphibians, but not others, to live in the humid rainforests, as well as in the arid tropical scrub forest? The answers to these questions and many others must await additional field studies.

The purpose of this paper is to make known the species of amphibians living in the Isthmus of Tehuantepec, to describe the environments in which they live, and to discuss their distribution in the isthmus. With respect to the distribution of animals in the Isthmus of Tehuantepec I will attempt to explain the present patterns of distribution with special reference to climatic fluctuation in the Pleistocene.

_Acknowledgments_

My extensive field work in the Isthmus of Tehuantepec was made possible by grants from the Penrose Fund of the American Philosophical Society (1956) and the Bache Fund of the National Academy of Sciences (1958).

Furthermore, my field work received the hearty support of the Museum of Zoology at the University of Michigan; for their cooperation I am indebted to Norman Hartweg, T. H. Hubbell, and Henry van der Schalie.

In the course of my studies I received helpful suggestions from Norman Hartweg, L. C. Stuart, and Charles F. Walker, to whom I am grateful.

For permission to examine specimens in their care I thank Doris M.

Cochran, Hobart M. Smith, and Richard G. Zweifel. I am deeply indebted to Thomas MacDougall for many suggestions and for aid in preparing the gazetteer. I am most grateful for the efforts of my field companions, Richard E. Etheridge, Jerome B. Tulecke, John Wellman, and especially my wife, Ann S. Duellman, who spent many long days and nights gathering much of the data on which this report is based. Our work in the isthmus was furthered by the generous help and hospitality of many residents, especially the late Wilbur Barker of Tehuantepec, Fortunado Delgado of Rancho Las Hojitas near Acayucan, Cesar Farjas of Donaji, and Juan Mayol of San Andres Tuxtla. Profesor Jordi Julia Z. of the Laboratorio de Entomologia, Comision del Papaloapan, Ciudad Aleman, Veracruz, helped make possible my field work in 1959; for this he has my sincere thanks. In conclusion I express my grat.i.tude to Ing. Juan Lozano Franco, Secretaria de Agricultura y Ganaderia, for providing me with the necessary permits.

_Field Studies in the Isthmus of Tehuantepec_

I first visited the Isthmus of Tehuantepec and collected on the Pacific lowlands of the isthmus in July, 1955. At that time heavy rains and impa.s.sable roads restricted travelling. In February and March of 1956 my wife and I concentrated our efforts in the central region between the Rio Jaltepec and Matias Romero, but also made several trips across the isthmus to gather ecological data in the dry season. In July of the same year, accompanied by Richard E. Etheridge, we again crossed the isthmus several times in order to gather ecological data in the wet season, and studied especially hylid frogs, most of which had not been seen in the dry season. Accompanied by Jerome B. Tulecke and John Wellman, I collected again in the isthmus in July, 1958, between Salina Cruz and Tehuantepec, and between Coatzacoalcos and Cosoleacaque. In March and April, 1959, I collected at Ciudad Aleman. Nearly 1200 specimens of 30 species of amphibians were thus collected in the Isthmus of Tehuantepec; all specimens are now in the Museum of Zoology at the University of Michigan. Of other species known from the isthmus, I have had field experience with all but one (_Bolitoglossa veracrucis_) in other parts of Mexico.

_Sources of Material_

There are in museum collections nearly 3000 specimens of amphibians with reliable data from the Isthmus of Tehuantepec. Among the first herpetological specimens collected in the isthmian region are those a.s.sembled 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 ca.n.a.liferus_ 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, princ.i.p.ally in the vicinity of Jesus Carranza. For the past two decades Thomas MacDougall, a resident of New York City, has spent his winters collecting biological specimens in southern Mexico. He makes his headquarters at Tehuantepec, but his compulsion to see the "back country" has taken him to many remote parts of southern Oaxaca. His earlier collections are in the American Museum of Natural History; the later ones are in the University of Illinois Museum of Natural History.

Minor collections include those made by Matthew W. Stirling at San Lorenzo, Veracruz, February-April, 1946 (United States National Museum), by Fred G. Thompson on a trip across the isthmus in December, 1955 (University of Michigan Museum of Zoology), by the University of Kansas Museum of Natural History field party under the direction of Rollin H. Baker at Tolosita, Oaxaca, and by David A. Langebartel and a.s.sociates 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 pa.s.s, which separates the mountain ma.s.ses of Mexico proper from those of Central America, and at the same time provides a continuum of lowlands from the Gulf of Mexico to the Pacific Ocean. This topography combines with the climatic conditions to create extremely diverse environments, the distribution of which can be adequately understood only after an acquaintance with the topography and climate of the region.

_Physiography_

In east-central Oaxaca the mountain ma.s.ses comprising the Sierra Madre Oriental and the Sierra del Sur terminate in a series of ranges--Sierra de Juarez, Sierra de los Mijes, and Sierra de Choapam. From lofty peaks, such as Cerro de Zempoaltepetl (3400 meters), the highlands diminish eastward to succeedingly lower ridges, until in the middle of the Isthmus of Tehuantepec the continental divide is about 250 meters above sea level. Eastward from this low divide the land rises to form the Sierra Madre de Chiapas, which is continuous with the highland ma.s.ses of Guatemala.

For the purposes of this description, the lowlands of the isthmus may be divided into three parts--the Gulf Coastal Plain, the central ridges, and the Pacific Coastal Plain, which in the isthmus is called the Plains of Tehuantepec (Figs. 1 and 2).

The Gulf Coastal Plain is broad and fairly level near the coast, but rolling in the interior. The plain, throughout most of its length in the isthmus, is at least 75 kilometers wide. The majority of the region in the isthmus is drained by the Rio Coatzacoalcos, which flows in a northerly course to the Gulf of Mexico. The western part is drained by the Rio San Juan, the princ.i.p.al tributary of the Rio Papaloapan. Behind the coastal dunes are frequent, and sometimes large, lagoons.

Immediately inland from Coatzacoalcos and along the lower stretches of the Rio Papaloapan are extensive marshes. Essentially the entire coastal plain, with the exception of the coastal dunes, consists of rich alluvial deposits.

[Ill.u.s.tration: FIG. 1. Map of the Isthmus of Tehuantepec based on the American Geographical Society's "Map of Hispanic America on the Scale of 1:1,000,000."

The localities shown are numbered in the gazetteer; the numerical sequence of localities is an arrangement whereby north takes precedence over south and west over east. 1. Alvarado. 2. Lerdo de Tejada. 3.

Tlacotalpan. 4. Tula. 5. Tecolapan. 6. Amat.i.tlan. 7. Cosamaloapan. 8.

Chacaltianguis. 9. Novillero. 10. Ciudad Aleman. 11. Papaloapan. 12.

Tuxtepec. 13. Cuatotolapam. 14. Hueyapan. 15. Berta. 16. Coatzacoalcos.

17. Ayentes. 18. Rio de las Playas. 19. Cosoleacaque. 20. Minat.i.tlan.

21. Acayucan. 22. Aquilera. 23. San Lorenzo. 24. Naranja. 25. Suchil.

26. Jesus Carranza. 27. La Oaxaquena. 28. Ubero. 29. Donaji. 30.

Tolosita. 31. El Modelo. 32. Sarabia, 33. Guichicovi. 34. La Princesa.

35. Santa Maria Chimalapa. 36. Matias Romero. 37. Santo Domingo Petapa.

38. El Barrio. 39. Palmar. 40. Chivela. 41. Santiago Chivela. 42.

Nizanda. 43. Agua Caliente. 44. Portillo Los Nanches. 45. Ixtepec. 46.

La Ventosa. 47. Zanatepec. 48. Union Hidalgo. 49. Tres Cruces. 50.

Juchitan. 51. Escurano. 52. Salazar. 53. Santa Efigenia. 54.

Tequisistlan. 55. Cerro de Quiengola. 56. San Pablo. 57. Mixtequilla.

58. Tapanatepec. 59. Zarzamora. 60. Limon. 61. Tehuantepec. 62.

Bisilana. 63. Santa Lucia. 64. Cerro de Arenal. 65. Cerro de San Pedro.

66. La Concepcion. 67. Tenango. 68. San Antonio. 69. Huilotepec. 70.

Salina Cruz.]

[Ill.u.s.tration: FIG. 2. Topographic profile of the Isthmus of Tehuantepec showing major localities along the Trans-isthmian Highway and major types of vegetation. Vertical exaggeration approximately 165 times.]

The central ridges extend from the Rio Jaltepec southward to within 40 kilometers of the Pacific coast. It is in this area that the continuity of the high ridges and volcanic peaks, which extend nearly the entire length of the Americas, is interrupted at a point almost directly in line with the shortest distance between the two oceans. The northern part of this central region consists of hills dissected by tributaries of the Rio Coatzacoalcos; the princ.i.p.al ones from north to south are--Rio Jaltepec, Rio Tortuguero, Rio Sarabia, and Rio Malatengo. The plains of Chivela are south of these rivers and lie at an elevation of about 200 meters; at the southern edge of these plains a range of hills rises to 250 to 400 meters above sea level. These hills drop abruptly to the Plains of Tehuantepec. In the northern and central parts of this central region the rocks are granitic; the hills to the south of the Plains of Chivela are limestone.

The Pacific Coastal Plain or Plains of Tehuantepec have a maximum width of about 30 kilometers. From the base of the hills at an elevation of about 75 meters the plains slope gradually to the sea. To the west of the Rio Tehuantepec and to the east of the Plains of Tehuantepec at the base of the Sierra Madre de Chiapas, the coastal plain becomes much narrower; in these places the continuity of the plain is frequently interrupted by low north-south ridges extending outward from the mountains or by isolated hills. The soil is poor, varying from volcanic rock to gravel and sand.

_Climate_

The prevailing winds are from the north across the Gulf of Mexico.

These moisture-laden winds precipitate most of their moisture north of the central ridges. This results in high rainfall on the northern slopes and Gulf Coastal Plain and relatively little rainfall on the southern slopes and the Pacific Coastal Plain. Precipitation is cyclic; there is a marked wet and a dry season throughout the region, but this is most noticeable on the Pacific lowlands (Fig. 3). At Salina Cruz on the Pacific Ocean the average annual rainfall is 1040 mm. (Contreras, 1942); of this amount, only 15 mm. falls from November through April.

On the Gulf Coastal Plain (Minat.i.tlan 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 Minat.i.tlan 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 Minat.i.tlan is 26.2 C. During the winter when ma.s.ses 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.

[Ill.u.s.tration: FIG. 3. Climatographs for Minat.i.tlan, 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 a.n.a.lyze 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.