In life dorsal coloration varied from pale green to olive-green with darker green or black flecks or reticulations, or pinkish tan to brown with dark brown flecks or reticulations. Some preserved specimens have relatively few dark flecks, whereas in most specimens the dorsum is heavily marked. All specimens have some white markings above the anus and on the posterodorsal surfaces of the thighs, but in some individuals the white flecks are expanded and interconnected forming an irregular white line.
Juveniles have a notably different coloration in life. The dorsum is uniform pale green; the anterior and posterior surfaces of the thighs, fingers, first three toes, and webbing are deep yellow. The anal stripe is creamy white, and the flanks are pale gray with black flecks. The upper lip, supratympanic fold, and canthal stripe are a bronze color. The belly is pale yellow with a silvery cast on the throat. Juveniles having snout-vent lengths from 24.5 to 36.6 mm. are so colored in life, and uniform dark bluish gray dorsally in preservative.
Comparisons.—The absence of a quadratojugal and the presence of a greatly enlarged, non-projecting prepollex place Hyla siopela in the Hyla bistincta group (see Duellman, 1964, and Adler, 1965). The presence of a rostral keel separates Hyla siopela from other members of the Hyla bistincta group, which is composed of two species having long anal sheaths (bistincta and pentheter), two small species having axillary membranes and lacking nuptial excrescences in breeding males (charadricola and chryses), and three species (crassa, pachyderma, and robertsorum) having short heads, round snouts, short anal sheaths, and nuptial excrescences in breeding males. Hyla siopela differs from the last three species in the shape of the snout and from each in certain structural features; H. crassa has fully webbed feet; H. pachyderma has large nuptial spines, and H. robertsorum has more webbing and a shorter tarsal fold. Furthermore, the venter in H. robertsorum is brown with creamy white flecks.
In structure and coloration H. arborescandens resembles siopela, but the former is smaller, and males of arborescandens have vocal slits.
Remarks.—This description brings to eight the number of species now recognized in the Hyla bistincta group. Hyla siopela is most closely related to Hyla robertsorum from the high mountains of the Sierra Madre Oriental in northern Puebla and eastern Hidalgo. Possibly the four species now recognized in the crassa subgroup (crassa, pachyderma, robertsorum, and siopela) are only subspecies of a single species, but differences in the amount of webbing in crassa and the nature of the nuptial excrescenses in pachyderma indicate that they are distinct species.
The type locality of Hyla siopela is a small stream cascading down the western slope of Cofre de Perote; the lower reaches of the stream can be reached by a dirt road leading east from the village of Perote for about 2 kilometers to a small park. The frogs were found in the stream at elevations of 50 to 100 meters higher than the park. The stream flows through a ravine supporting open, dry pine forest. Although the stream was searched thoroughly in February, 1966, no frogs were found. In July, 1966, adults and juveniles were found in crevices and under overhanging rocks behind small cascades and waterfalls by day and sitting on rocks and branches in the spray of cascades at night.
The specific name siopela is derived from the Greek siopelos, meaning silent, and alludes to the absence of a voice in this species.
Hyla altipotens new species
Plate 19
Holotype.—Adult male, KU 101001, from 37 kilometers (by road) north of San Gabriel Mixtepec (kilometer post 183 on road from Oaxaca to Puerto Escondido), Oaxaca, México, elevation 1860 meters; obtained on February 19, 1966, by William E. Duellman.