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In the evolution of species recognition mechanisms, the differential reactions of receivers towards a signal may be as important as variation in the signal features. In many animals, the auditory system's most sensitive range often matches the dominant frequency range of the signals. The degree of spectral matching, however, can be modified by stimulus intensity and, therefore, patterns detected in the laboratory should ideally be confirmed by field experiments. Territorial males of the dart-poison frog,
Epipedobates femoralis, approach vocally active conspecific intruders. Since phonotaxis (followed by fighting) affects male fitness, the auditory system is predicted to discriminate against heterospecific calls. We tested the rôle of spectral and temporal call features in call discrimination by conducting playback experiments in the field with synthetic calls. To study the effect of heterospecific interference on the male's phonotactic reaction, we measured signal parameters and signalling activity of four sympatric species. The highest probability of response coincided with the spectral and temporal features of the population's advertisement call and the probability of approach was matched to the average frequency of this call. In addition, the degree of matching was sensitive to the number of notes per call. The decrease in the probability of response, however, was steeper at lower than average frequencies, where the probability of heterospecific interference was highest. Our results suggest that under field conditions frequency tuning interacts with temporal call features in maximizing the detection of conspecific signals and reducing the probability of heterospecific interference.