Frogs of the genus Ptychohyla are ecologically associated with mountain streams at elevations between 650 and 2200 meters; in the geographic region where these frogs occur the vegetation between those elevations consists of cloud forest or pine-oak forest. In some places the frogs have been found in a mixture of oak and semi-deciduous scrub forest. At Vista Hermosa, Oaxaca, P. leonhardschultzei and P. ignicolor were found in cloud forest, whereas at Agua del Obispo, Guerrero, the former species was found in pine-oak forest. Ptychohyla schmidtorum is known only from cloud forest; P. euthysanota euthysanota and P. spinipollex generally are found in cloud forest, but in some places they live in pine-oak forest. Ptychohyla euthysanota macrotympanum has been found in pine-oak forest and in a mixture of oak and semi-deciduous scrub forest. With the possible exception of the members of the Ptychohyla schmidtorum group, which has been found only in cloud forest, it seems as though the type of vegetation is not the controlling factor in the ecological distribution of these frogs.
Ptychohyla has been found only where there are clear, cascading streams overhung by vegetation, on which adults and young perch at night, or even by day. The presence of these streams, in which the tadpoles live, seems to be an important factor in the distribution of Ptychohyla. As has been shown previously, the tadpoles of Ptychohyla are adapted for existence in torrential streams, where the water is cool, and the amount of oxygen is high. Clearly these tadpoles are unsuited for life in ponds or sluggish streams in the lowlands, where the temperature of the water is high, a layer of silt on the bottom is deep, and the amount of oxygen is low. The tadpoles cling to rocks on the bottom of the streams; there they move slowly across the rocks, apparently feeding on the thin covering of algae. Tadpoles were not observed on rocks having a thick covering of algae or moss. The tadpoles were observed to swim against the current in torrential streams, in which no fishes were found. Therefore, it seems as though the presence of the stream-habitat for the tadpoles is a significant factor in the ecological distribution of the species of Ptychohyla.
Interspecific Competition
At localities where two species of Ptychohyla occur sympatrically (P. ignicolor and P. leonhardschultzei at Vista Hermosa, Oaxaca, and P. euthysanota euthysanota and P. schmidtorum schmidtorum at Finca La Paz, Depto. San Marcos, Guatemala) effort was made to determine what, if any, ecological interspecific relationships existed. Although adults of the sympatric species were found on adjacent leaves or branches of bushes overhanging the streams at both localities, segregation at the time of breeding seems to be maintained by the notably different breeding calls in sympatric species (see discussion of breeding calls). Thus, as has been shown by Blair (1956), Fouquette (1960), and others working on a variety of pond-breeding frogs and toads, the breeding call in Ptychohyla acts as an important reproductive isolating mechanism.
At no locality were Ptychohyla and associated species of hylids found so abundantly as were species of pond-breeding hylids in the lowlands. Apparently reproductive activity is not concentrated in a short breeding season, and it is highly doubtful if the populations of these frogs are as large as those of the lowland pond-breeders. The continual humid conditions and abundance of insect food throughout the year in the cloud forest are perhaps indicative of little interspecific competition among adults of Ptychohyla and other sympatric hylids.
At Finca La Paz, Guatemala, tadpoles of two species of Ptychohyla were ecologically segregated. The tadpoles of P. euthysanota euthysanota were found in riffles in the streams, whereas those of P. schmidtorum schmidtorum were found in slower water, chiefly in small pools in the streams. At Vista Hermosa, Oaxaca, tadpoles of P. leonhardschultzei were found in riffles, and tadpoles of the sympatric P. ignicolor were found in a small pool in a stream. Similar ecological relationships were observed for several species of Costa Rican hylids. Throughout the range of Ptychohyla east of the Isthmus of Tehuantepec, members of the genus occur with species of Plectrohyla, all of which are larger than Ptychohyla, and all of which have tadpoles that live in torrential streams. Tadpoles of Ptychohyla spinipollex have been found in streams inhabited by the tadpoles of Plectrohyla guatemalensis and P. quecchi; tadpoles of Ptychohyla euthysanota euthysanota and P. schmidtorum schmidtorum were found in streams inhabited by tadpoles of Plectrohyla guatemalensis, P. matudai, and P. sagorum. In some streams great numbers of tadpoles occur. The habitat is rather restricted, and the food supply is limited. Consequently, interspecific competition among the various species of hylids whose tadpoles live in the torrential streams probably is highest during the larval stage. Unfortunately, this aspect of salientian population ecology has received no intensive study.
Reproduction and Development
Since the cloud forests inhabited by Ptychohyla are daily bathed in clouds and have a fairly evenly distributed rainfall throughout the year, the frogs living in these forests are active throughout the year. At least some of the species evidently have a long breeding season, for I found calling males of P. leonhardschultzei in February, March, and August, and found tadpoles in February, March, June, and August. Tadpoles of the various species have been obtained throughout much of the year, as follows: P. euthysanota euthysanota, February, March, May, and July; P. euthysanota macrotympanum, March, June, and August; P. spinipollex, February, March, April, June, July, and August; P. schmidtorum schmidtorum, March, May, June, July, and August; P. schmidtorum chamulae, June and August; P. ignicolor, June. I suspect that this temporal distribution more accurately reflects the seasonal activities of collectors than of the frogs.