Ecology of the Herpetofauna

Our two visits to ChinajÁ and Toocog afforded the opportunity to gather data on the ecology of the rainforests of southern El PetÉn and to study the relationships between the environment and members of the herpetofauna. Tropical rainforests present the optimum conditions for life, and it is in this environment that life reaches its greatest diversity. Here, too, biological inter-relationships are most complex. This complexity is illustrated by the presence of many species of some genera, all of which are found together in the same geographic region. In the rainforests of southern El PetÉn there are six species of Anolis, five of Hyla, four of Bothrops, and three of Coniophanes. Obviously, the diversity of ecological niches in the rainforest is sufficient to support a variety of related species. Of the examples mentioned above, fairly adequate ecological data were obtained for most of the species of Anolis, which will be used to show the ecological diversity and vertical stratification of sympatric species in the rainforests.

Of the six species of Anolis, all except A. sericeus are typically found in humid forests. Anolis sericeus sericeus is poorly represented in the collections from southern El PetÉn, where it may be in competition with Anolis limifrons rodriguezi that resembles Anolis s. sericeus in size, coloration, and habits. Therefore, Anolis sericeus sericeus is excluded from the following discussion. The common terrestrial species is Anolis humilis uniformis; sometimes this small species perches or suns on the bases of small trees or buttresses of some large trees. When disturbed it takes to the ground and seeks cover in the leaf litter or beneath logs or palm fronds. Anolis lemurinus bourgeaei is about twice the size of Anolis humilis uniformis and is usually observed on buttresses of large trees or on the lower two meters of tree trunks. Individuals were seen foraging on the ground along with Anolis humilis uniformis. At no time were Anolis lemurinus bourgeaei observed to ascend the trunks of large trees; they always took refuge near the bases of trees. Anolis limifrons rodriguezi is found on the stems and branches of bushes. It is a small species that sometimes is observed on the ground but was never seen ascending large trees. Anolis capito is about the same size as Anolis lemurinus bourgeaei and lives on the trunks of large trees. In the tops of the trees lives a large green species, Anolis biporcatus.

Similar segregation habitatwise can be demonstrated for other members of the herpetofauna. The avoidance of interspecific competition in feeding is well illustrated by three species of snakes that probably are the primary ophidian predators on frogs. Drymobius margaritiferus margaritiferus is diurnal and terrestrial; it feeds on frogs at the edges of breeding ponds by day. Also during the day Leptophis mexicanus mexicanus feeds on frogs in bushes and trees. At night the activities of both of these species is replaced by those of Leptodeira septentrionalis polysticta, which not only feeds on the frogs in the trees and bushes, but descends to the ground and even enters the water in search of food.

From the examples discussed above, the importance of the three dimensional aspect of the rainforest is apparent. The presence of a large and diverse habitat above the ground is of great significance in the rainforest, for of the non-aquatic components of the herpetofauna in the rainforests of southern El PetÉn, 42 per cent of the species spend at least part of their lives in the bushes and trees. Another important part of the forest is the subterranean level—the rich mulch, underground tunnels, and rotting subterranean vegetation. Of the 78 species of amphibians and reptiles in southern El PetÉn, seven are primarily fossorial, and half-a-dozen others are secondarily fossorial. Probably the fossorial members of the fauna are the least well represented in the collection, for such widespread species as Dermophis mexicanus mexicanus, Rhadinaea decorata decorata and Tantilla schistosa schistosa were expected, but not found.

In the following discussion of the ecological distribution of amphibians and reptiles in the rainforest I have depended chiefly on my observations made in southern El PetÉn, but have taken into consideration observations made on the same species in other regions, together with reports from other workers. The reader should keep in mind that the evidence varies from species to species. Of some species I have observed only one animal in the field; of others, I have seen scores and sometimes hundreds of individuals. For species on which I have few observations or rather inconclusive evidence, the circumstance of inadequate data is mentioned.

In analyzing the ecological distribution within the forest, it is convenient to recognize five subdivisions (habitats); each is treated below as a unit.

1. Aquatic.—This habitat includes permanent streams and rivers (Pl. 10, fig. 1), some of which are clear and others muddy. In the rainy season temporary ponds form in depressions on the forest floor (Pl. 10, fig. 2); these are important as breeding sites for many species of amphibians. Aquatic members of the herpetofauna are here considered to be those species that either spend the greatest part of their lives in the water or usually retreat to water for shelter. Seven species of turtles and one crocodilian are aquatic. Of these, Dermatemys mawi, Staurotypus triporcatus, and Pseudemys scripta ornata inhabit clear water, whereas Chelydra rossignoni, Claudius angustatus, Kinosternon acutum, and K. leucostomum inhabit muddy water. Crocodylus moreleti apparently inhabits both clear and muddy water, for in the dry season it lives along the clear rivers, but in the rainy season inhabits flooded areas in the forest as well.

2. Aquatic Margin.—Extensive marshes were lacking in the part of southern El PetÉn that I visited; consequently, the aquatic margin habitat is there limited to the edges of rivers and borders of temporary ponds. Bufo marinus, Rana palmipes, and Rana pipiens are characteristic inhabitants of the aquatic margin, although in the rainy reason Bufo marinus often is found away from water. Observations indicate that Tretanorhinus nigroluteus lateralis inhabits the margins of ponds and streams and actually spends considerable time in the water. Although Iguana iguana rhinolopha is arboreal, it lives in trees along rivers, into which it plunges upon being disturbed. Species included in this category are those that customarily spend most of their lives at the edge of permanent water. Frogs and toads that migrate to the water for breeding and the snakes that prey on the frogs at that time are not assigned to the aquatic-margin habitat.

3. Fossorial.—Characteristic inhabitants of the mulch on the forest floor are Bolitoglossa moreleti mulleri, Lepidophyma flavimaculatum flavimaculatum, Scincella cherriei cherriei, Ninia sebae sebae, Pliocercus euryzonus aequalis, and Micrurus affinis apiatus. Other species of snakes that spend most of their lives above ground often forage in the mulch layer; among these are Coniophanes bipunctatus biserialis, Coniophanes fissidens fissidens, Coniophanes imperialis clavatus, Lampropeltis doliata polyzona, and Stenorrhina degenhardti. Among the amphibians, at least Hypopachus cuneus nigroreticulatus, Eleutherodactylus rostralis, and Syrrhophus leprus are known to seek shelter in the mulch.

4. Terrestrial.—One turtle, Geoemyda areolata, is primarily terrestrial. Among the lizards, conspicuous terrestrial species are Anolis humilis uniformis and Ameiva festiva edwardsi; Anolis lemurinus bourgeaei and Basiliscus vittatus spend part of their lives on the ground, but also live on trees and in bushes. Eumeces schwartzei and E. sumichrasti apparently are terrestrial. The only terrestrial lizard that is nocturnal is Coleonyx elegans elegans, which by day hides in the leaf litter or below ground. Nocturnal amphibians that are terrestrial include Bufo marinus, Bufo valliceps valliceps, Eleutherodactylus rugulosus rugulosus, Syrrhophus leprus, and Hypopachus cuneus nigroreticulatus. A large number of active diurnal snakes are terrestrial; these include Boa constrictor imperator, Clelia clelia clelia, Dryadophis melanolomus laevis, Drymarchon corais melanurus, Drymobius margaritiferus margaritiferus, Pseustes poecilonotus poecilonotus, and Spilotes pullatus mexicanus. Nocturnal terrestrial snakes include three kinds of Bothrops (B. atrox asper, B. nasutus, and B. nummifer nummifer), all of which seem to be equally active by day.

5. Arboreal.—In this habitat the third dimension (height) of the rainforest probably is the most complex insofar as the inter-relationships of species and ecological niches are concerned. I have attempted to categorize species as to microhabitats within the arboreal habitat; in so doing, I recognize four subdivisions—bushes, tree trunks, tree tops, and epiphytes.

Bush inhabitants include several species of lizards and snakes, all of which have rather elongate, slender bodies, and long tails. Common bush-inhabitants in southern El PetÉn are Anolis limifrons rodriguezi, Basiliscus vittatus, Laemanctus deborrei, Leptophis mexicanus mexicanus, and Oxybelis aeneus aeneus. All of these are diurnal, and all but Laemanctus have been observed sleeping on bushes at night.

Tree-trunk inhabitants include five species of lizards. Thecadactylus rapicaudus lives on the trunks of large trees; Sphaerodactylus lineolatus lives beneath the bark on dead trees and on corozo palms. Anolis lemurinus bourgeaei lives on the bases and buttresses of large trees, from which it often descends to the ground. Corythophanes cristatus and Anolis capito were found only on tree trunks and large vines.

The least information is available for the species living in the tree tops. The following species were obtained from tops of trees when they were felled, or have been observed living in the tree tops: Anolis biporcatus, Iguana iguana rhinolopha, Celestus rozellae, Leptodeira septentrionalis polysticta, Leptophis ahaetulla praestans, Sibon dimidiata dimidiata, and Sibon nebulata nebulata.

Epiphytes, especially the bromeliads, provide refuge for a variety of tree frogs and small snakes. Of the tree frogs, Hyla picta, Hyla staufferi, Phyllomedusa callidryas taylori, Similisca baudini, and Similisca phaeota cyanosticta have been found in bromeliads; other species probably occur there. Among the snakes, Imantodes cenchoa leucomelas, Leptodeira frenata malleisi, Leptodeira septentrionalis polysticta, Sibon dimidiata dimidiata, and Sibon nebulata nebulata are frequent inhabitants of bromeliads; all of these snakes are nocturnal.

Relationships of the Fauna

Most of the 78 species of amphibians and reptiles definitely known from the rainforest in southern El PetÉn have extensive ranges in the Atlantic lowlands of southern MÉxico and Central America; many extend into South America. Sixty-two (80%) of the species belong to this group having extensive ranges in Middle America. Three species (Syrrhophus leprus, Leptodeira frenata, and Kinosternon acutum) are at the southern limits of their distributions in southern El PetÉn and northern Alta Verapaz, whereas Eleutherodactylus rostralis and Thecadactylus rapicaudus are at the northern and western limits of their distributions in El PetÉn. Nine (11%) species have the center of their distributions in El PetÉn and the YucatÁn Peninsula; representatives of this group include Claudius angustatus, Dermatemys mawi, Laemanctus deborrei, and Eumeces schwartzei.

In determining a measure of faunal resemblance, I have departed from the formulae discussed by Simpson (1960) and have analyzed the degree of resemblance by the following formula used to calculate an index of faunal relationships:

The herpetofauna of southern El PetÉn has been compared with that in the Tikal-UaxactÚn area (Stuart, 1958), that in the humid lowlands of Alta Verapaz (Stuart, 1950, plus additional data), and that in the Mexican state of YucatÁn (Smith and Taylor, 1945, 1948, and 1950). The herpetofaunas of lowland Alta Verapaz and YucatÁn are the largest, having respectively 94 and 91 species, where as there are 78 species known from southern El PetÉn and 64 from the Tikal-UaxactÚn area. An analysis of faunal relationships (Table 2) shows that the faunas of the rainforests of southern El PetÉn and lowland Alta Verapaz are closely related. The relationships between these two areas and the Tikal-UaxactÚn area in northern El PetÉn is notably less. Apparently the biggest faunal changes take place between southern El PetÉn and the Tikal-UaxactÚn area, and between the latter and YucatÁn. As stated by Stuart (1958:7) the Tikal-UaxactÚn is transitional between the humid rainforests to the south and the dry outer end of the YucatÁn Peninsula. The transitional nature of the environment is exemplified by a rather depauperate herpetofauna consisting of some species of both dry and humid environments and lacking a large fauna typical of either. Contrariwise, the continuity of the environment from southern El PetÉn to the lowlands of Alta Verapaz is reflected in degree of resemblance of the herpetofaunas.

Table 2.—Index of Faunal Relationships Between Southern El PetÉn and Other Regions.

Most of the species of amphibians and reptiles found in southern El PetÉn are found in humid tropical forests from the Isthmus of Tehuantepec southeastward on the Atlantic lowlands well into Central America.