Vocal production by terrestrial mammals: source, filter and function

In little over two decades, researchers have moved from a situation in which most studies of terrestrial mammal vocal signals focused on conspicuous characteristics, such as their rate of occurrence, and where the spectral acoustic variation was largely ignored or poorly quantified, to a field of study in which there is a much better understanding of the nature and function of the acoustic parameters that compose vocalizations. The source-filter theory, originally developed for the analysis of speech signals, has played a large role in this progress. Understanding how the acoustic variability of vocalizations is grounded within their mechanism of production has enabled researchers to predict the type of information that vocal signals are likely to contain, and to predict their co-variation with morphological and/or physiological attributes of callers. Moreover, the powerful theoretical platform derived from the source-filter theory not just conceptually supports the formulation of multilevel hypotheses, but also paves the way to develop the corresponding methodologies needed to address them. Although the full range of acoustic diversity of terrestrial mammal signals has yet to be explored, this chapter draws together a wealth of research conducted over the last two decades, and describes how source- and filter-related acoustic components encode functionally relevant information in the vocal communication systems of terrestrial mammal and how selection pressures have led to the evolution of anatomical innovations that enable animals to produce exaggerated vocal traits