THE "ACOUSTIC ADAPTATION HYPOTHESIS" - A REVIEW OF THE EVIDENCE FROM BIRDS, ANURANS AND MAMMALS

The acoustic properties of the environment influence sound propagation. Many previous studies examined whether various species of anurans, birds and mammals adjust usage and / or structure of their vocal signals to limit degradation during propagation in this environment ("acoustic adaptation hypothesis"). The present review examines how widespread such adaptations actually are across taxa. First, evidence for environment-related adjustments in usage of vocal signals is collected from studies in birds and other vertebrates (i.e., anurans and mammals). Second, a meta-analysis conducted by Boncoraglio & Saino (2007) on the influences of the environment on the acoustic structure of avian vocalisations is taken as a reference, and results from additional studies in birds are reviewed and compared to its conclusions. Finally, evidence from similar studies conducted in anurans and mammals is collected and discussed. Concerning the usage of vocal signals, evidence of environment-related adaptations in the few studies found was more widespread in anurans and mammals than in birds. Regarding structure of vocal signals, evidence from additional studies in birds did not completely confirm results of the meta-analysis of Boncoraglio & Saino (2007). Pooling all bird studies together presented minimum frequency, frequency modulations and frequency range as acoustic variables most often adjusted to the environment, in contrast to temporal features, repetition rate and maximum frequency. The few studies conducted in anurans and mammals did not allow the identification of specific acoustic variables that typically show environment-related variations. Overall, evidence for the acoustic adaptation hypothesis was not as widespread as expected across taxa. The different aspects of vocal behaviour adapted to environmental conditions varied according to the species and local habitats. Environment-related adjustments in structure of vocal signals seem to be constrained by call function in anurans and mammals. This effect was not examined in birds, but vocal learning does not appear to be a pre-requisite to environment-related adjustment in this group.