, 1980; Seyfarth & Cheney, 1990). Functionally referential calls, at least in some primate Rapamycin species, appear to evolve along a continuum whereby purely reflexive/affective calls come under more volitional control (Macedonia & Evans, 1993; Evans, 1997). Thus

innate distress calls may have become more and more specific throughout evolution, driven by audience effects and the receiver comprehension, and culminating in voluntarily alarm calling (Sherman, 1977; Cheney & Seyfarth, 1985; Seyfarth & Cheney, 2003a,b). In acoustic terms, using both natural and resynthesized stimuli, it has been found that the discrimination between ‘snake’ and ‘eagle’ alarm calls by conspecifics in vervet monkeys is most reliable when made using spectral cues, even though temporal and fundamental frequency cues also vary between the two calls (Owren, 1990a,b; Owren & Bernacki, 1998; Seyfarth & Cheney, 2003a,b). In fact, the active modulation Caspase inhibitor of the first

two formants during vocalizations appears to play the greatest role in referential communication, with deviations from what would be expected of a uniform vocal tract ranging from 23% for F1 to 60% for F2 (Riede & Zuberbühler, 2003; Riede et al., 2005, 2008). This is perhaps not surprising as F1 and F2 are dependent on those parts of the vocal tract that have the most potential for volitional manipulation. Rudimentary modulation of the first two formants is reminiscent of the process seen in the acoustic differentiation of vowel sounds in human speech, as the vocal tract is manipulated in order to filter the source signal specifically to encode external events (Fant, 1960; Lieberman & Blumstein, 1988). These results support the hypothesis that the shaping of spectral patterns in alarm calls is likely to have evolved specifically for communicative reasons, and may be paramount in the transition from purely affective calls (all mammals) to functionally referential calls (some non-human primates),

and ultimately to intentionally referential calls (humans) (see Evans, 1997). We have seen that source and filter components can provide varying levels of affective and functionally referential information for in many mammalian species. In human speech, the combination of source and filter characteristics is vital for language as both intonation and semantic content are necessary for successful communication (Lieberman & Blumstein, 1988). In non-human mammals, the potential inter-play and communicative effects of interactions between source and filter is less well understood (but see Charlton et al., 2008b), although recent research has shown that hyrax songs simultaneously encode body weight, size, current condition, hierarchical status and current hormonal state of the singer (Koren & Geffen, 2009). It is likely that several levels of information may be similarly present within the signals of other mammals, and this largely unexplored branch of animal vocal communication merits further investigation.