Introduction

How and why species emerge, persist, and become extinct is a central and controversially debated topic in evolutionary biology (Martin, 1990; Meyer, 1993; Masters, 1999; Mayr, 1999; see also Chapter 1). Acoustic signaling systems are known to mediate discrimination within and between species and play a major role in speciation and evolution (Wilkins et al., 2013). However, empirical research on this topic for primates is currently in its infancy.

Most research related to this topic comes from various non-mammalian animal groups, such as birds (e.g. Irwin et al., 2008; Podos, 2010), frogs (e.g. Amézquita et al., 2011; Willink et al., 2014), and insects (e.g. McNett and Cocroft, 2008), although singing mice present a rare exception for mammals (Campbell et al., 2010). Current comparative bioacoustic research suggests that three major selective forces drive acoustic divergence within and among species, favoring speciation and evolution (see Figure 21.1): adaptive mechanisms such as ecological and sexual selection as well as neutral mechanisms such as drift. For instance, in Darwin finches (Geospiza ssp.), climate constraints shape feeding ecology and thereby bill shape and indirectly the structure of trill calls in their song (Podos, 2010). Ecological selection for beak size results in acoustic and population divergence through assortative mating. Sexual selection following host shifts is strongly supported for treehoppers (McNett and Cocroft, 2008), in which the frequency of a pure tone mating signal favors species divergence. Sexual selection for song complexity along a gradient and drift are proposed to explain song divergence in greenish warblers (Irwin et al., 2008), whereas only drift is suggested to be the driving force for divergence in singing mice (Campbell et al., 2010).

Disentangling the relative contributions of these selective forces for the evolution of acoustic divergence and species diversity is urgently needed (Wilkins et al., 2013). For primates, a first and important step in this direction is to study the extent that acoustic divergence in communication is present and biologically relevant. It is also important to examine these aspects in genetically characterized species with known sympatric and allopatric distribution and varying in ranges, habitat, and body mass.