A behavioral and neurobiological study of the responses of gray treefrogs, Hyla versicolor, to the calls of a predator, Rana catesbeiana

The bullfrog, Rana catesbeiana, is a predator of the gray treefrog, Hyla versicolor. Recent research with a variety of taxa has demonstrated that females may be sensitive to changes in their risk of predation and may respond behaviorally during mate choice in ways that reduce this risk. In a three-speaker female choice experiment, we tested whether gravid female treefrogs would avoid approaching a source of conspecific advertisement calls when it was adjacent to a speaker broadcasting bullfrog calls. We also assessed the hearing sensitivity of female gray treefrogs to bullfrog calls with neurophysiology and tested whether calling male bullfrogs would attack a simulated treefrog. A final playback experiment tested whether males would reduce their calling in response to bullfrog calls. Because calling male bullfrogs often attacked our model frog, the calls of this ranid might be a useful indicator of predation risk to H. versicolor. Nevertheless, we found that neither female phonotaxis nor male calling was influenced by broadcasts of bullfrog vocalizations. While auditory thresholds of H. versicolor were about 17 dB SPL higher to the heterospecific than to the conspecific call, thresholds to the bullfrog call were well below sound pressure levels used in the tests of phonotaxis. Possible explanations for our behavioral findings are discussed.