William Edward Duellman

“ But it soon became evident that in order to understand the relationships of the herpetofauna of the Sierra de Coalcomán, the species inhabiting the Tepalcatepec Valley and adjacent mountain ranges would have to be studied. In the course of making that study I examined all specimens from Michoacán already in museums.
There have been few detailed herpetofaunal studies in México. The first such study of any consequence was that by Bogert and Oliver (1945) on the herpetofauna of Sonora. ”

“ The small yellow tree frogs, Hyla microcephala and its relatives, are among the most frequently heard and commonly collected frogs in the lowlands of southern México and Central America. The similarities in size, proportions, and coloration of the different species have resulted not so much in a multiplicity of specific names, but in differences of opinion on the application of existing names to the various taxa. For example, the populations on the Atlantic lowlands have been known by three names, two of which have been applied to other taxa. ”

“ 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. ”

“ The difference in the color of the throat fan, which is apparent only in living individuals, is more striking. Obviously more than one species is represented, as is borne out by the differences in the color of the throat fan and in the osteology, but there is uncertainty about the correct name for each species. On the strength of Bocourt's diagnosis of keeled snout scales in A. nebuloides, I am applying that name to the population in Oaxaca and A. nebulosus to the specimens from Michoacán. ”

“ According to Savage's (1966) interpretation of the origins and history of the American herpetofauna, Agalychnis and Pachymedusa are members of the Mesoamerican fauna, and Phyllomedusa is part of the Neotropical fauna. Perhaps the phyllomedusines arose in South America; from there a primitive stock spread northward and survived as Pachymedusa in México, whereas the stock in Central America and South America evolved into Agalychnis and Phyllomedusa, respectively. ”

“ Justification of Synonymy.—The holotype of Osteocephalus taurinus is a female having a snout-vent length of 103.9 mm. The diameter of the tympanum is 6.8 mm, 77.3 percent of the diameter of the eye. The skull is strongly exostosed, and the lateral edges of the frontoparietals are elevated so as to form distinct ridges. The skin on the dorsum is smooth. When we examined the type on 5 August 1969, the specimen was soft and badly faded to a pale creamy tan with pale brown transverse bars on the hind limbs and spots on the flanks. ”

“ Probably no ecological group of hylid frogs (some Hyla plus Plectrohyla and Ptychohyla) in Middle America is so poorly known as those species that live in the cloud forests on steep mountain slopes and breed in cascading mountain streams. During the last half of the nineteenth century most of the species of hylids living in the lowlands of southern México and northern Central America were named and described. ”

“ 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 most respects Hyla microcephala is distinct from the other species, and with the exception of the amount of ossification of the frontoparietals, the other species can be easily derived from a microcephala-like ancestor. Possibly the slightly increased ossification of the frontoparietals in robertmertensi, phlebodes, and sartori is secondary, or possibly after differentiation of the species the amount of ossification was further reduced in microcephala. If so, the species fall into a reasonable phylogenetic scheme that has microcephala as the extant species most like the ancestral stock. ”

“ The affinities of Ptychohyla apparently are not with any of the other groups that have been generically separated from Hyla. Of the daughter genera in Middle America only Plectrohyla has stream-adapted tadpoles, but these large frogs are not closely related to Ptychohyla. Stuart (1954:169) suggested that certain montane species of Hyla in lower Central America and Hyla salvadorensis in El Salvador may be related to Ptychohyla or even congeneric. I have had experience with most of these species in the field and believe that Stuart was correct in his suggestion of relationships. ”

“ Webbing between fingers one and two is basal in all species. Webbing between fingers two, three, and four is most extensive in taurinus, in which the three fingers are about one-half webbed (Fig. 5) . Osteocephalus buckleyi, pearsoni, and verrucigerus have reduced webbing between fingers two and three, and leprieurii has reduced webbing between fingers two, three, and four. All members of the genus have well-developed subconical subarticular tubercles on the fingers and toes; there is a tendency for the distal tubercle on the fourth finger to be weakly bifid. ”

“ The acquisition and study of additional material from throughout the range of the species is necessary to clarify the picture of geographic variation. Until then, I prefer to consider all of the specimens from Michoacán as Gerrhonotus imbricatus imbricatus.
The largest specimen is a male having a snout-vent length of 136 mm. Two juveniles collected in July 24, 1960, have snout-vent lengths of 36 and 42 mm. A specimen having a snout-vent length of 127 mm. and a tail length of 145 mm. was regurgitated by a Crotalus pusillus, which had a body length of 550 mm. ”

“ 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. ”

“ We visualize the evolutionary history of the group to have followed a course that began with the invasion of Central America by a microcephala ancestral stock that differentiated into two populations in lower Central America—a microcephala-like frog on the Pacific lowlands and a phlebodes-like frog on the Caribbean lowlands. ”

“ If the presence of ventrolateral glands in breeding males is ignored, a student of salientian systematics might derive the Ptychohyla euthysanota group from a hylid stock containing Hyla miotympanum and Hyla mixomaculata. Likewise, Ptychohyla schmidtorum could be placed with Hyla uranochroa and related species in Costa Rica. Nonetheless, the fact remains that all of the species assigned to the genus Ptychohyla have ventrolateral glands in the breeding males; furthermore, ventrolateral glands are unknown in other hylids. ”

“ Ontogenetic change in coloration is slight or non-existent in buckleyi, pearsoni, and taurinus, except that juveniles lack ventral markings. A dark blotch on the back and distinct transverse bars on the limbs are evident in juveniles of verrucigerus; these markings are obscured in the adults. Juveniles of leprieurii are olive-brown with yellow dorsolateral stripes; the transverse dark marks, characteristic of the adults, appear before the stripes are lost. ”

“ In Michoacán the rainfall is heaviest on the southern slopes of the Sierra de Coalcomán and somewhat less so on the southwestern slopes of the Cordillera Volcánica. At these relatively low elevations (150 to 600 meters) there is tropical semi-deciduous forest, characterized by relatively dense shade throughout the year and by a leaf mulch on the ground. This type of forest forms the gallery forest along the larger streams in the Balsas-Tepalcatepec Basin and on the Pacific Coastal Plain. ”

“ 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 [Pg 41] species are those that are found near water. A possible exception is Syrrhophus pipilans. ”

“ Despite the similarity in color pattern, size, and structure, we are reluctant to place the two taxa in the same species until data on coloration in life, mating calls, and life history are available for Hyla misera and compared with those of Hyla microcephala. ”

“ Some species have a complete quadratojugal-maxillary arch; others do not. Assuming that the parental stock that gave rise both to the Hyla uranochroa group and to Ptychohyla was widespread in Central America at a time of cooler, more humid conditions, it is possible that with subsequent warming temperatures and seasonal rainfall in the lowlands the parental stock was restricted to the Costa Rican highlands, where the Hyla uranochroa group developed, and to the Chiapas-Guatemala highlands, where Ptychohyla evolved. ”

“ 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. ”