Other characteristics of the source signal include tempo, duration and amplitude contour, all of which are controlled by sophisticated muscular interactions and changes in airflow or sub-glottal pressure (Titze,

1994). Generally speaking, in both humans and non-human animals, the acoustic characteristics of the glottal wave are not reliably related to body size, because the organs that produce them are soft and unconstrained by skeletal structures (Fitch, 1997, 2000b). Source characteristics can thus vary between and within vocalizations from the same caller either on a volitional Selleck AZD6244 (intonation in human speech: Ohala, 1984; Banse & Scherer, 1996; frequency modulation in bats: Bastian & Schmidt, 2008) or on an involuntarily basis (emotional expression in humans: Ohala, 1996; Aubergé & Cathiard, 2003; affective state in baboons: Rendall, 2003b; stress in pigs: Düpjan et al., 2008). While the source signal is generally periodic, many recent studies report the presence of non-periodic elements (or ‘non-linear phenomena’) in the source component of mammalian vocal signals. Although in humans, non-linear phenomena can be related AZD6738 price to speech pathologies (e.g. Hirano, 1981), in many non-human animals they form

part of the normal vocal communication system. Examples of non-linear phenomena include subharmonics (additional harmonics visible in the spectrum beneath F0; African wild dogs: Wilden et al., 1998; chimpanzees: Riede, Owren & Arcadi, 2004), biphonation (two independent F0; African wild dogs:

Wilden et al., 1998; H. S. Webster et al., unpubl. data; chimpanzees: Riede et al., 2004; dholes: Volodina ifoxetine et al., 2006) and deterministic chaos (broadband signals with no particular harmonics; African wild dogs: Wilden et al., 1998; chimpanzees: Riede et al., 2004, red deer: Reby & McComb, 2003a,b). Bifurcations between linear and non-linear events are also often observed in species presenting non-linear phenomena (Wilden et al., 1998; Fitch, Neubauer & Herzel, 2002; Tokuda et al., 2002; Riede et al., 2004). Despite improvements in our understanding of the production process and role of non-linear phenomena in human speech (Titze, 2008), their place in animal communication systems is not yet well defined, although several hypotheses are discussed in the literature (Wilden et al., 1998; Fitch et al., 2002; Tokuda et al., 2002; Riede et al., 2004, 2005; Riede, Arcadi & Owren, 2007). The second stage of the source–filter theory is the filtering process that takes place in the vocal tract between the production of the signal at the source and its external radiation.