The Vocalizations of Some Hybrid Treefrogs: Acoustic and Behavioral Analyses

The vocalizations of natural hybrids between various species of treefrogs (Hylidae) are compared with the vocalizations of the parental species : 1. In most respects, the calls of two males of H. avivoca X H. chrysoscelis and of six males of H. cinerea X H. gratiosa are intermediate. 2. The vocalizations of three males of H. femoralis X H. clarysoscelis tend to be similar to those of males of H. chrysoscelis. 3. The calls of each of the hybrids are composed, however, of some features which are intermediate, some which are similar to one of the parental species, and others which are unique to hybrid calls. Differentiation of the calls of males of H. avivoca, H. chrysoscelis, H. femoralis, and hybrids in this group is mainly in the temporal domain. Females of H. chrysoscelis and H. femoralis responded only to the calls of males of their own species in discrimination experiments where hybrid calls were alternative stimuli. The calls of males of H. cinerea, H. gratiosa and hybrids between these two differ primarily in their spectral composition : the two spectral peaks have different locations in each kind of call. In discrimination experiments, females of H. gratiosa responded only to conspecific calls when the calls of hybrids were equally accessible. Females of H. cinerea showed partial discrimination against the calls of hybrids. Females of both kinds often responded to the calls of hybrids when conspecific signals were unavailable. In general, the closer the spectrum of a natural call resembled that of conspecific calls, the more attractive was the acoustic signal.