, 2006). Laryngeal retraction is

made possible by the evolution of a highly elastic thyrohyoid membrane linking the larynx to the hyoid apparatus and strong sternothyroid and sternohyoid muscles that pull the larynx down the throat toward the sternum (Fitch & Reby, 2001). As the sternothyroid and sternohyoid muscles are attached to the sternum, the larynx cannot be pulled lower than the sternum, putting an anatomical limitation on laryngeal retraction and thereby maintaining the proximate honesty of this signal (Fitch & Reby, 2001; Fitch & Hauser, 2002). A similar anatomical adaptation enabling laryngeal retraction during mating calls has also been observed in Mongolian gazelles (Frey et al., 2008). Moreover, as already noted by Fitch (2000b, 2006), several other behavioural and anatomical adaptations may be involved in acoustic size exaggeration. For example, male saiga antelopes are able to increase the length of their vocal tract while producing mating calls by means of a specific vocal posture involving a strongly tensed and extended trunk (Volodin, Volodina & Efremova, 2009).

Furthermore some species possess a pronounced proboscis, elongating the nasal region of the vocal tract and potentially influencing the spacing of formant frequencies (elephant seals: Sanvito, Galimberti & Miller, 2007). Similarly, black and white colobus monkeys have evolved a subhyoid airsac that is inflated to act as an additional resonator Thiamine-diphosphate kinase during roars, thereby lowering their formants in comparison to what would normally be observed for animals of the same body size (Harris et al., 2006; also see Riede et al., 2008 for an experimental test of the effect of laryngeal airsacs on formant frequencies). On a comparative note, at least 60 species of birds possess elongated tracheas,

and the evolution of this has been discussed in the context of the size exaggeration hypothesis (see detailed review by Fitch, 1999). Vocalizations are an integral part of male competitive signalling (Bradbury & Vehrencamp, 1998; Owings & Morton, 1998). The size-related variation in formants can thus provide receivers with valuable information about potential competitors, and enable functional decisions about PD0332991 concentration whether or not to escalate an agonistic interaction with another individual, based on the assessment of the caller’s body size relative to that of the receiver (red deer: Fitch & Reby, 2001; Reby et al., 2005; fallow deer: McElligott et al., 2006; domestic dogs: A. M. Taylor, D. Reby & K. McComb, 2009b). As well as affecting interactions linked to male–male competition, acoustic size exaggeration (or maximization) also appears to play an important role in mate choice (Charlton, 2008).