Why signal softly? The structure, function and evolutionary significance of low-amplitude signals

Acoustic signalling is a taxonomically widespread form of animal communication consisting of longrange, high-amplitude signals and short-range, low-amplitude signals. Research on acoustic communication has emphasized high-amplitude signals and often overlooked low-amplitude signals, even though they are produced in behavioural contexts that directly influence fitness. Low-amplitude signals are referred to by a variety of names such as soft songs, courtship songs, whispers, close calls, contact calls, grunts and moans, but all of these signals share a reduced amplitude and an active space that is limited to close-proximity receivers. In this review, we establish a general definition for low-amplitude signals and investigate the similarities and differences between low-and high-amplitude signals with respect to their acoustic structure and function. Then, we critically evaluate some proximate and ultimate evolutionary mechanisms that may explain why these signals are produced at low amplitude using examples from a variety of taxa. We conclude by suggesting priorities for future research on low-amplitude signals and highlighting how studying these signals will lead to a more complete understanding of how and why animals communicate acoustically. (C) 2015 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.