Estimating the distance to a source of sound: mechanisms and adaptations for long-range communication

Acoustic signals are used for long-range communication in many animals from insects to birds and mammals. Long-range signals are used primarily by males to advertise and defend their home ranges or territories or to attract mates. During transmission over long distances, the structure of signals becomes progressively degraded. At the position at which a receiver makes a decision to respond, the characteristics of signals can differ markedly from those at the source. This degradation impairs extraction of information coded in the signal but also allows receivers to assess the distance (range) of the signaller. Auditory distance assessment (called ‘ranging’) is particularly important in territorial species as the optimal initial response often depends on the distance of the signaller. Perception of distance is comparatively well studied in animals but this work is not well integrated with information from related fields such as sound perception in humans or other animals. Here we review recent advances in studies of distance estimation and relate these to fundamental issues in sound transmission and sound perception. We consider the different components of signal degradation and how they can be measured in a perceptually meaningful way. We discuss limitations on the perception of degradation and the possibility that signallers might send deceptive information about their actual distance from a receiver. By integrating studies of auditory distance perception and studies of sound perception in animals and humans, we provide a framework for understanding the evolutionary implications of sound degradation in communication.manufacture.