Hyla phlebodes inhabits regions having rainfall throughout the year. Although large breeding congregations are most common in the early parts of the rainy season, males probably call throughout the year. At Puerto Viejo in Costa Rica the senior author has heard Hyla phlebodes in February, April, June, July, and August. Charles W. Myers noted calling males of this species in the area around Almirante, Bocas del Toro Province, Panamá, in September, October, and February. An exception to the correlation between rainfall and breeding activity was noted by the junior author in Hyla phlebodes in the Canal Zone, where he noticed a decrease in activity of that species in October and November, when the rains are heaviest and most frequent. Furthermore, independent observations made by both of us indicate that H. phlebodes does not reach peaks of activity during or immediately after heavy rains, but instead builds up to peaks of activity two or three days after a heavy rain. This is in contrast to the other species, all of which characteristically inhabit drier environments than does H. phlebodes. Peaks of breeding activity in the other species occur immediately after, or even during, heavy rains.
The calling location of the males generally is on vegetation above, or at the edge of, the water. Hyla microcephala and H. phlebodes call almost exclusively from grasses and sedges; phlebodes usually calls from taller and more dense grasses than does microcephala. Except for some minor differences in calling location observed by the junior author (Fouquette, 1960b) in the Canal Zone, the differences in density and height of grasses utilized for calling-locations probably is dependent primarily on the nature of the available vegetation. Although bushes and broad-leafed herbs are usually present at the breeding sites, males of these species seldom utilize them for calling locations. Both H. robertmertensi and H. sartori have been observed calling from grasses, herbs, bushes, and low trees. Calling males of robertmertensi have been found two meters above the ground in small trees.
Daytime retreats in the breeding season sometimes are no more than shaded [clumps] of vegetation adjacent to a pond or in clumps of grass in a pond. Individuals of H. m. underwoodi were found by day under the outer sheaths of banana plants next to a water-filled ditch. Dry season refuges are unknown.
Amplexus is axillary in all four species. Egg deposition has been observed in H. m. microcephala, m. underwoodi, and phlebodes. In all three the eggs are deposited in small masses that float near the surface of the water and usually are at least partly attached to emergent vegetation. Each clutch does not represent the entire egg complement of the female.
Tadpoles are definitely known of only H. m. microcephala and phlebodes; these have been described in the preceding accounts of the species. The tadpoles of these two species can be distinguished readily ([Pl. 15]). The tadpole of H. microcephala has a uniformly white venter and nearly transparent tail, whereas in H. phlebodes the venter is flecked anteriorly and the tail is mottled. In life, H. microcephala is easily recognized by the orange posterior half of the tail, whereas the tail in H. phlebodes is mottled tan and grayish brown.
PHYLOGENETIC RELATIONSHIPS
The evidence already presented on osteology, external structure, coloration, mating call, and life history emphatically show that the four species under consideration are a closely related assemblage. Now the question arises: To what other groups in the genus is the Hyla microcephala group related? Furthermore, it is pertinent to this discussion to attempt a reconstruction of the phylogeny of the group as a whole and of the individual species in the Hyla microcephala group. With regard to the relationships of the group we must take into account certain species in South America. Our endeavors there are hampered by the absence of data on the mating calls and life histories of most of the relevant species.
As mentioned in the [account] of Hyla m. microcephala, the species microcephala possibly is subspecifically related to Hyla misera, a frog widespread in the Amazon Basin. Hyla misera resembles microcephala in coloration, external structure, and cranial characters. The frontoparietals are equally poorly ossified, and the frontoparietal fontanelle is extensive. Our principal reason for not considering the two taxa conspecific at this time is our lack of knowledge concerning the color of living H. misera, the structure of the tadpoles, and the characteristics of the mating call. Even with the absence of such data that we think essential to establish the nomenclature status of the taxa, we are confident that the two are sufficiently closely related that any discussion of the phylogenetic relationships of one species certainly must involve consideration of the other.
Hyla misera possibly is allied to other small yellowish tan South American Hyla that lack dark pigmentation on the thighs. Probable relatives are Hyla elongata, minuta (with goughi, pallens, suturata, velata, and possibly others as synonyms), nana, and werneri. The consideration of the interspecific relationships of these taxa is beyond the scope of this paper, but we can say that each of these species has a pale yellowish tan dorsum, relatively broad dorsolateral brown stripe, and narrow longitudinal brown lines or irregular marks on the dorsum. Furthermore, examination of the skulls of elongata, nana, and werneri reveals that they are like misera and microcephala in the nature of the frontoparietal fontanelle and in having a greatly reduced quadratojugal. Thus, on the basis of cranial and external characters the Hyla microcephala group can be associated with Hyla misera and its apparent allies in South America. This association can be only tentative until the mating calls, tadpoles, and chromosome numbers of the South American species are known.