Matching and symmetry in the frequency recognition curve of the poison frog Epipedobates trivittatus.

@enIn territorial species, males use signals to advertise territory ownership to other males. In species with acoustic communication, masking interference by heterospecific signals may impede male-male communication and affect the reproductive success of males. Frogs are thought to minimize masking interference by using species-specific frequency channels for communication. For this strategy to work, a frequency match is expected between the advertisement call and the auditory sensitivity. A previous field study on the Amazonian frog Epipedobates femoralis supported this prediction, but also revealed an asymmetric decrease in the probability of male reaction towards synthetic calls. That males of E. femoralis reacted less towards low-frequency (compared with high-frequency) calls was interpreted as a mechanism that reduces masking interference by E. trivittatus, a species calling within a lower, partially overlapping, frequency range. If this hypothesis holds, then males of E. trivittatus should exhibit the opposite asymmetry pattern, i.e. react less towards high-frequency (compared with low-frequency) calls. We tested this prediction by conducting 25 playback experiments on 22 males of E. trivittatus. Male phonotactic reaction towards synthetic calls of various frequency values was evaluated as a binary variable (the male either approached or not the loudspeaker), by measuring the latency until first jump, and by calculating the linear approaching speed. As in E. femoralis, the maximum probability of positive reaction was matched to the call frequency. Against our expectations, the response curve was symmetric. We discuss whether these results reflect a lack of selective pressures, or a compromise between natural selection and physiological constraints on the shape of the frequency recognition curves.