Interference risk and the function of dynamic shifts in calling in the gray treefrog (Hyla versicolor).

Male gray treefrogs call to attract females under challenging acoustic conditions. At higher chorus densities, there is considerable background noise and a good chance that a male's calls will often be overlapped by calls of other individuals. Call overlap may reduce the probability of mating because females prefer calls with internal pulse structure that is not obscured. With increases in acoustic stimulation, males lengthen call duration while simultaneously reducing call rate such that "pulse effort" changes little. In our study, we tested the "interference risk hypothesis." This proposes that males change call duration so that, on average, there will be a sufficient number of pulses and interpulse intervals clear of acoustic interference per call to attract a female. However, female choice experiments employing call alternatives of different durations, rates, intensity levels, and degrees of call overlap refuted this hypothesis. Our results leave open the possibilities that the dynamic shift in the two components of pulse effort are related to the problem of call detection in a noisy environment or are responses by males to the perceived threat of competition for females.