Generic Relationships
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. The species concerned are four red-eyed stream-breeding Hyla in Costa Rica—H. alleei, H. legleri, H. rufioculis, and H. uranochroa, plus Hyla salvadorensis in the mountains of El Salvador. Morphologically all of the species are similar; Hyla uranochroa, H. legleri, and H. rufioculis have a lateral white stripe that is expanded to form a spot beneath the eye, as in Ptychohyla schmidtorum. The tadpoles of Hyla rufioculis and H. uranochroa have large funnel-shaped mouths and long slender tails like those of Ptychohyla schmidtorum. Lips of the tadpoles of H. legleri and H. salvadorensis are folded laterally, in this respect resembling those of the Ptychohyla euthysanota group. I do not know the tadpoles and the breeding call of Hyla alleei. The breeding calls of Hyla rufioculis and H. uranochroa consist of high melodious notes; the calls of H. legleri and H. salvadorensis consist of series of short notes that have the general characteristics of the call of Ptychohyla schmidtorum. Affinities of the genus Ptychohyla seem to me to be with the red-eyed species forming the Hyla uranochroa group in Costa Rica. All of the species in the Hyla uranochroa group have large frontoparietal fontanelles, rather small ethmoids, and small nasals that are not in contact with one another or with the ethmoid. 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.
Interspecific Relationships
Ptychohyla schmidtorum is thought to resemble more closely the parental stock of the genus than does any other species of Ptychohyla now extant. This parental stock is discussed above in the account of the generic relationships. Ptychohyla schmidtorum has a red eye, white lateral stripe, frontoparietal fontanelle, funnel-shaped mouth in tadpoles, and lacks nuptial spines; in all of these characters it resembles members of the Hyla uranochroa group. Probably during times of glaciation during the Pleistocene, when climates in México and Central America were depressed, the Ptychohyla stock was more widespread than it is now. Subsequent elevation of climatic zones during interglacial periods would have isolated populations as they are today in regions of cloud forests. Thus, through geographic isolation populations could have differentiated and evolved into the present species. Climatic fluctuation in the Pleistocene must have been of sufficient magnitude to permit the spread of cool, moist forests containing Ptychohyla across the Isthmus of Tehuantepec into the mountains of Oaxaca.
Because of its small nuptial spines, small triangular vomers, coloration, and absence of a rostral keel, Ptychohyla euthysanota, more than any of the other species in the P. euthysanota group, resembles P. schmidtorum. At the present time P. euthysanota and P. schmidtorum are sympatric.
As I have mentioned previously, ecological segregation and interspecific competition probably is highly developed in the tadpoles of Ptychohyla. If this ecological segregation resulted from intraspecific competition in a stock of Ptychohyla, possibly P. euthysanota and P. schmidtorum differentiated sympatrically in this way. Specific identity is maintained, at least in part, by different breeding calls in males.
Ptychohyla spinipollex and P. leonhardschultzei seem to be more closely related to one another than either is to P. euthysanota. Probably a stock of P. euthysanota was isolated on the Atlantic slopes of northern Central America from P. euthysanota on the southern slopes. The frogs on the Atlantic slopes differentiated and spread into the mountains of Oaxaca, where through isolation by the barrier of the Isthmus of Tehuantepec they developed into P. leonhardschultzei, while the stock on the northern slopes of Central America evolved into P. spinipollex. Subsequent to the differentiation of P. leonhardschultzei and P. spinipollex from P. euthysanota and during a time of cooler more equable climate than exists now, P. euthysanota and P. schmidtorum invaded the Central Highlands of Chiapas. Subsequent climatic changes isolated populations of each in the Central Highlands, where P. euthysanota macrotympanum and P. schmidtorum chamulae evolved. Ptychohyla ignicolor probably represents stock of P. schmidtorum that crossed the Isthmus of Tehuantepec and became isolated in Oaxaca on the western side of the isthmus.