Repetition rate.—The repetition rate of the secondary notes, in calls consisting of more than one secondary, was measured for each form. A considerable amount of variation in this parameter was found in all of the taxa ([Table 5]). This variation probably is due in part to the effect of temperature differences. Repetition rate is the only parameter analyzed for which there is a correlation with the air-temperature, but even here the correlation is weak, probably due to the microenvironmental effects of humidity, air-movement, and other factors in addition to the ambient air temperature that influences the body temperature of the frogs. These rates are nearly alike in both subspecies of H. microcephala and in phlebodes. The repetition rates in H. robertmertensi and H. sartori are considerably faster than in the other three taxa. Hyla sartori has the fastest repetition rate of the group.
In all characteristics of the mating calls the two subspecies of H. microcephala agree closely, as might be expected, although the differences are statistically significant. Hyla robertmertensi is distinctive in call pattern and seems to be closer to microcephala in dominant frequency but closer to H. phlebodes in fundamental frequency. Thus, it is somewhat intermediate between microcephala and phlebodes. The identical pattern and similarity in fundamental and dominant frequencies of the calls of H. phlebodes and H. sartori possibly indicate close relationship.
Geographic variation in call.—Hyla m. microcephala has higher fundamental and dominant frequencies in Costa Rica than in Panamá. In Costa Rican H. m. underwoodi the fundamental and dominant frequencies are lower than in other parts of the range. Frogs of this subspecies recorded in Nicaragua and Honduras have slightly lower dominant frequencies and higher fundamental frequencies than those recorded in Guatemala or Oaxaca. The duration of both primary and secondary notes decreases to the south; samples from Nicaragua and Costa Rica have the shortest notes. Comparison of duration of notes in the two subspecies shows that the Panamanian H. m. microcephala have slightly longer notes than do any H. m. underwoodi; the more northern populations of H. m. underwoodi from México most closely approach H. m. microcephala in this characteristic.
The calls of H. robertmertensi in Oaxaca have higher dominant and fundamental frequencies and longer secondary notes than do those in Chiapas.
The calls of H. phlebodes recorded at Puerto Viejo, Costa Rica, have slightly lower dominant frequencies than do those recorded at Turrialba, Costa Rica, and in Panamá, whereas those recorded at Turrialba have lower fundamental frequencies than in other samples. The duration of notes is slightly shorter in both Costa Rican samples than in those recorded in Panamá.
LIFE HISTORY
The frogs of the Hyla microcephala group breed in shallow grassy ponds. In some places they breed in permanent ponds, but usually congregate around temporary pools, such as depressions in forests, flooded fields, and roadside ditches. At the height of their breeding season, usually in the early part of the rainy season, the congregations are made up of large numbers of individuals. In April, 1961, and in June, 1966, the senior author noted nearly continuous choruses of H. m. microcephala in roadside ditches along the 75 kilometers of road between Villa Neily and Palmar Sur, Puntarenas Province, Cost Rica; on June 20, 1966, at Puerto Viejo, Heredia Province, Costa Rica, he estimated approximately 900 Hyla phlebodes in one pond, and two nights later noticed that the number of individuals [had] increased substantially. Other observations by the first author on size of breeding congregations include nearly continuous choruses of H. m. underwoodi between Villahermosa and Teapa, Tabasco, in July of 1958, an estimated 400 Hyla robertmertensi in a road side ditch 7.2 kilometers west-northwest of Zanatepec, Oaxaca, on July 13, 1956, and approximately 150 Hyla sartori around a rocky pool in a riverbed, 11.8 kilometers west-northwest of Tierra Colorada, Guerrero, on June 28, 1958.
The length of the breeding season seemingly is more dependent on climatic conditions in various parts of Middle America than on behavioral differences in the various species. Thus, Fouquette (1960b) found in the Canal Zone that H. m. microcephala formed breeding choruses from May through January, the entire rainy season in that area. In the wetter coastal region of Puntarenas Province, Costa Rica, the species breeds as early as mid-March, whereas in the drier region encompassing Guanacaste Province, Costa Rica, and southwestern Nicaragua breeding activity is initiated by the first heavy rains of the season, usually in June.