Interspecific Interactions between Two Japanese Pond Frogs, Rana porosa brevipoda and Rana nigromaculata

Abstract

Male social interactions between Rana porosa brevipoda and Rana nigromaculata were investigated. Males of R. nigromaculata were significantly larger and heavier than those of R. porosa brevipoda. Mixed-species choruses were formed by male R. nigromaculata joining the choruses of R. porosa brevipoda. In the mixed-species choruses, males of both species defended their own calling spaces against both conspecifics and heterospecifics. Most of the interspecific agonistic interactions were initiated by male R. nigromaculata, and most of the interactions were terminated by the victory of R. nigromaculata, probably due to its larger body size. Males of the two species emitted similar calls: advertisement calls with multiple notes and encounter calls with a single note. Advertisement calls of R. nigromaculata consisted of fewer notes and were lower in frequency than those of R. porosa brevipoda, although there was a considerable spectral overlap between the two species. Males of both species with SVL of approximately 65mm emitted calls of similar dominant frequency. Playback experiments were performed to examine the response of the males to conspecific and heterospecific advertisement calls. Male R. porosa brevipoda tended to respond to only conspecific calls, whereas male R. nigromaculata tended to respond to both conspecific and heterospecific calls. This indicates that male R. nigromaculata do not distinguish heterospecific calls from conspecific ones. Heterospecific pairings and hybridization were observed within the mixed-species choruses. Two types of possible costs, energetic costs and hybridization, caused by the asymmetric interspecific interactions are discussed.