Amphibians and Reptiles of the Rainforests of Southern El Peten, Guatemala.
by William E. Duellman.
INTRODUCTION
Early in 1960 an unusual opportunity arose to carry on biological field work in the midst of virgin rainforest in southern El Peten, Guatemala.
At that time the Ohio Oil Company of Guatemala had an air strip and camp at Chinaja, from which place the company was constructing a road northward through the forest. In mid-February, 1960, J. Knox Jones, Jr.
and I flew into El Peten to collect and study mammals, reptiles, and amphibians. While enjoying the comforts of the fine field camp at Chinaja, we worked in the surrounding forest and availed ourselves of the opportunity to be on hand when the road crews were cutting the tall trees in the forest, thereby bringing to the ground many interesting specimens of the arboreal fauna. We stayed at Chinaja until late March, with the exception of a week spent at Toocog, another camp of the Ohio Oil Company located 15 kilometers southeast of La Libertad and on the edge of the savanna. Thus, at Toocog we were able to work both in the forest and on the savanna. In the summer of 1960, John Wellman accompanied me to El Peten for two weeks in June and July. Most of our time was spent at Chinaja, but a few days were spent at Toocog and other localities in south-central El Peten.
Many areas in Guatemala have been studied intensively by L. C. Stuart, who has published on the herpetofauna of the forested area of northeastern El Peten (1958), the savannas of central El Peten (1935), and the humid mountainous region to the south of El Peten in Alta Verapaz (1948 and 1950). The area studied by me and my companions is covered with rainforest and lies to the north of the highlands of Alta Verapaz and to the south of the savannas of central El Peten. A few specimens of amphibians and reptiles were obtained in this area in 1935 by C. L. Hubbs and Henry van der Schalie; this collection, reported on by Stuart (1937), contained only one species, _Cochranella fleischmanni_, not present in our collection of 77 species and 617 specimens.
Acknowledgments
I am grateful to L. C. Stuart of the University of Michigan, who made the initial arrangements for our work in El Peten, aided me in the identification of certain specimens, and helped in the preparation of this report. J. Knox Jones, Jr. and John Wellman were able field companions, who added greatly to the number of specimens in the collection. In Guatemala, Clark M. Shimeall and Harold Hoopman of the Ohio Oil Company of Guatemala made available to us the facilities of the company"s camps at Chinaja and Toocog. Alberto Alcain and Luis Escaler welcomed us at Chinaja and gave us every possible a.s.sistance. Juan Monteras and Antonio Aldana made our stay at Toocog enjoyable and profitable. During our visits to southern El Peten, Julio Bolon C.
worked for us as a collector, and between March and June he collected and saved many valuable specimens; his knowledge of the forest and its inhabitants was a great a.s.set to our work. Jorge A. Ibarra, Director of the Museo Nacional de Historia Natural in Guatemala a.s.sisted us in obtaining necessary permits and extended other kindnesses. To all of these people I am indebted for the essential parts that they played in the completion of this study.
Field work in the winter of 1960 was made possible by funds from the American Heart a.s.sociation for the purposes of collecting mammalian hearts. My field work in the summer of 1960 was supported by a grant from the Graduate Research Fund of the University of Kansas.
DESCRIPTION OF THE AREA
A vast lowland region stretches northward for approximately 700 kilometers from the highlands of Guatemala to the Gulf of Mexico. The northern two-thirds of this low plain is bordered on three sides by seas and forms the Yucatan Peninsula. The lowlands at the base of the Yucatan Peninsula make up the Departamento El Peten of Guatemala. The area with which this report is concerned consists of the south-central part of El Peten.
Physiography
Immediately south of Chinaja is a range of hills, the Serrania de Chinaja, having an almost due east-west axis and a crest of about 600 meters above sea level. South of the Serrania de Chinaja are succeedingly higher ridges building up to the Meseta de Coban and Sierra de Pocolha and eventually to the main Guatemalan highlands. The northern face of the Serrania de Chinaja is a fault scarp dropping abruptly from about 650 meters at the crest to about 140 meters at the base. From the base of the Serrania de Chinaja northward to the Rio de la Pasion at Sayaxche the terrain is gently rolling and has a total relief of about 50 meters. North of the Rio de la Pasion is a low dome reaching an elevation of 170 meters at La Libertad; see Stuart (1935:12) for further discussion of the physiography of central El Peten. The rocks in southern El Peten are predominately Miocene marine limestones; there are occasional pockets of Pliocene deposits. There is little evidence of subterranean solution at Chinaja, but northward in central El Peten karsting is common. The upper few inches of soil is humus rich in organic matter; below this is clay.
Climate
The climate of El Peten is tropical with equable temperatures throughout the year. Temperatures at Chinaja varied between a night-time low of 65 F. and a daytime high of 91 F. during the time of our visits. In the Koppen system of cla.s.sification the climate at Chinaja and Toocog is Af.
Rain falls throughout the year, but there is a noticeable dry season. To anyone who has traveled from south to north in El Peten and the Yucatan Peninsula, it is obvious from the changes in vegetation that there is a decrease in rainfall from south to north. There is a noticeable difference between Chinaja and Toocog. Although rainfall data are not available for Chinaja and Toocog, there are records for nearby stations (Sapper, 1932). At Paso Caballos on the Rio San Pedro about 40 kilometers northwest of Toocog the average annual rainfall amounts to 1620 mm.; the driest month is March (21 mm.), and the wettest months are June (269 mm.) and September (265 mm.). At Cubilquitz, Alta Verapaz, about 35 kilometers south-southwest of Chinaja and at an elevation of 300 meters, the average annual rainfall is 4006 mm.; the driest month is March (128 mm.), and the wettest months are July (488 mm.) and October (634 mm.).
During the 18 days in February and March, 1960, that we kept records on the weather at Chinaja moderate to heavy showers occurred on seven days.
During our stay there in June and July rain fell every day, as it did in Toocog. However, during the week spent at Toocog in March no rain fell.
Vegetation
The vegetation of northern and central El Peten has been studied by Lundell (1937), who made only pa.s.sing remarks concerning the plants of the southern part of El Peten. No floristic studies have been made there. The following remarks are necessarily brief and are intended only to give the reader a general picture of the forest. I have included names of a few of the commoner trees that I recognized.
Chinaja is located in a vast expanse of unbroken rainforest. In this forest there is a noticeable stratification of the vegetation. Three strata are apparent; in the uppermost layer the tops of the trees are from 40 to 50 meters above the ground. The spreading crowns of the trees and the interlacing vines form a nearly continuous canopy over the lower layers. Among the common trees in the upper stratum are _Calophyllum brasiliense_, _Castilla elastica_, _Cedrela mexicana_, _Ceiba pentandra_, _Didalium guianense_, _Ficus_ sp., _Sideroxylon lundelli_, _Swietenia macrophylla_, and _Vitex_ sp. (Pl. 1, fig. 1). The middle layer of trees have crowns about 25 meters above the ground; these trees in some places where the upper canopy is missing form the tallest trees in the forest. This is especially true on steep hillsides. Common trees in the middle layer include _Achras zapote_, _Bombax elliptic.u.m_, _Cecropia mexicana_, _Orbignya cohune_, and _Sabal_ sp. The lowermost layer reaches a height of about 10 meters; in many places in the forest this layer is absent. Common trees in the lower stratum include _Crysophila argentea_, _Cymbopetalum penduliflorum_, _Casearia_ sp., and _Ha.s.seltia dioica_.
The ground cover is sparce; apparently only a few small herbs and ferns live on the heavily shaded forest floor. Important herpetological habitats include the leaf litter, rotting stumps, and rotting tree trunks on the forest floor and the b.u.t.tresses of many of the gigantic trees, especially _Ceiba pentandra_ (Pl. 2). Epiphytes, especially various kinds of bromeliads, are common. Most frequently these are in the trees in the upper and middle strata.
At Toocog there is sharp break between savanna and forest (Pl. 7, fig.
2). The forest is noticeably drier and more open than at Chinaja (Pl.
9). The crowns of the trees are lower, and there is no nearly continuous canopy between 40 and 50 meters above the ground. Although _Swietenia macrophylla_ and other large trees occur, they are less common than at Chinaja. Especially common at Toocog are _Achras zapote_, _Brosimum alicastrum_, and various species of _Ficus_.
GAZETTEER
The localities from which specimens were obtained are cited below and shown on the accompanying map (Fig. 1).
[Ill.u.s.tration: FIG. 1. Map of El Peten, Guatemala, showing localities mentioned in text.]
Chinaja.--Lat. 16 02", long. 90 13", elev. 140 m. Camp of the Ohio Oil Company of Guatemala and formerly a small settlement. On some maps Chinaja is located just to the north of the Alta Verapaz--El Peten boundary; recent surveys place the location just to the south of the imaginary line through the rainforest. Field work was conducted in the immediate vicinity of the camp, on the lower slopes of the Serrania de Chinaja, and at several sites to the northwest and north-northwest of Chinaja, where the forest was being cleared. The entire area supports rainforest.
La Libertad.--Lat. 16 47", long. 90 07", elev., 170 m. A town on the savannas in central El Peten; although we collected there in the rainy season, the specimens obtained on the savannas are not included in this report.
Paso Subin.--Lat. 16 38", long. 90 12", elev. 90 m. A small settlement on the Rio Subin, a tributary of the Rio de la Pasion. Specimens were obtained in rainforest in the immediate vicinity of the settlement.
Rio de la Pasion.--A large river flowing northward through southern El Peten and thence westward into the Rio Usumacinta.
Specimens were obtained along the river between the Rio Subin and Sayaxche.
Rio San Roman.--A river flowing northward in south-central El Peten to the Rio Salinas (Usumacinta). We collected along the river at a place about 16 kilometers north-northwest of Chinaja, approximately at Lat. 16 10", long. 90 17", elev.
110 m. In the dry season the river was clear; it is surrounded by rainforest.
Sayaxche.--Lat. 16 31", long. 90 09", elev. 80 m. A town on the southern bank of the Rio de la Pasion. Specimens were obtained in the rainforest and in cleared areas in the immediate vicinity of the town.
Toocog (formerly Sojio).--Lat. 16 41", long. 90 02", elev.
140 m. A camp of the Ohio Oil Company of Guatemala located at the rainforest-savanna edge, 15 kilometers southeast of La Libertad. Although we collected on the savannas as well as in the forest, especially to the east of the camp, only species obtained in the forest are considered in this report.
THE HERPETOFAUNA OF THE RAINFOREST
In presenting an account of the herpetofauna of southern El Peten three items need to be considered: (1) The composition of the fauna; (2) the ecology of the fauna; (3) the relationships of the fauna. Each of these topics is discussed briefly below. Logically a discussion of the origin of the fauna should follow, but this is being withheld for inclusion in a report on the herpetofauna of the entire El Peten by L. C. Stuart and the author; at that time the above topics will be expanded to cover the herpetofauna of the whole region.
Composition of the Fauna
TABLE 1.--COMPOSITION OF THE HERPETOFAUNA IN SOUTHERN EL PETeN, GUATEMALA.
=============+============+============+============ Group | Families | Genera | Species -------------+------------+------------+------------ Gymnophiona | (1)[A] | (1) | (1) Caudata | 1 | 1 | 2 Salientia | 6 | 10 (1) | 19 (1) Crocodilia | 1 | 1 | 1 Testudines | 4 | 7 | 8 Sauria | 6 | 13 (1) | 19 (1) Serpentes | 4 | 21 (7) | 29 (10) +------------+------------+------------ Total | 22 (1) | 53 (10) | 78 (13) -------------+------------+------------+------------
[Footnote A: Numbers in parenthesis indicate the number of additional taxa that probably occur.]