Allometric Convergence,Acoustic Character Displacement,and Species Recognition in the Syntopic Cricket Frogs Acris crepitans and A. gryllus

Evidence for reproductive character displacement (RCD) has accumulated more slowly than for ecological character displacement, perhaps because sampling scales and environmental covariates can obscure the role of RCD in speciation. We examined an early example of RCD in an anuran species group, the vocalizations of the sympatric cricket frogs Acris crepitans and A. gryllus. With a relatively fine spatial scale, we compared mixed-species choruses (syntopy), nearby locations where A. gryllus is recently extirpated (historic sympatry), and surrounding areas without secondary contact (allopatry). In each of these areas, we evaluated variation in dominant frequency, click rate, and mass of males. In addition, we determined the acoustic preferences of syntopic females. Temperature influenced dominant frequency of vocalizations in A. crepitans, but not in A. gryllus. Body size varied more and had a stronger influence on dominant frequency in A. crepitans than in A. gryllus. Consequently, the decrease in mass of A. crepitans in syntopy resulted in convergence of body size and divergence of dominant frequencies of the two species. In contrast, dominant frequency of A. crepitans did not differ between historic sympatry and allopatry. Females of both species used fine temporal structure to discriminate between conspecific and heterospecific vocalizations and showed no preferences for dominant frequency. Chorus noise limited the ability of A. gryllus females to detect and discriminate vocalizations, so convergence in mass might have resulted from RCD in dominant frequency to reduce heterospecific acoustic interference. However, influences other than RCD might have caused syntopic convergence in body size.