While the role of such cues often originates in the physical relationship that ties them to the dimension they express, selection pressures have often led to senders being able to alter their values to some extent (Ohala, 1984; Maynard Smith & Harper, 2003), an aspect that is discussed in more detail below. Understanding the origin, nature and function of such acoustic indexical cues is therefore one of the most active areas of current vocal communication research. Initially, studies of animal vocal signals tended to focus on understanding the control and variability of F0 (Cohen & Fox,

1976; Tembrock, 1976; Morton, 1977; August & Anderson, 1987; Masataka, 1994). This focus is likely to have been a reflection of the salience of ‘pitch’ to human speakers and listeners (Ohala, 1984). F0 is perceptually identifiable by non-specialists and it is easy to measure on spectrograms APO866 supplier or oscillograms (see Boersma, 1993). Moreover, F0 is highly variable within and between calls both across individuals and across species

(Morton, 1977; August & Anderson, 1987; Hauser, 1993; Yin, 2002; Reby & McComb, 2003a,b; Rendall et al., 2005). In an influential paper based on a comparative study of vocalizations used in agonistic displays in a range of mammalian and avian species, Morton (1977) suggested that audible frequency differences in vocalizations reflect ritualized signalling: animals with aggressive motivation produce low-pitched, broadband vocalizations (such as growls and hisses), while animals with a friendly or submissive motivation produce high-pitched vocalizations (such as whimpers and whines). This theory, known Selleck INK128 as Morton’s motivation-structural code, is based on the observation across several species that aggressive and dominant animals seek to project (both visually and acoustically) a larger impression of body size whereas friendly or submissive animals seek to project a smaller impression of body size (Morton, 1977; Ohala, 1984; Owings & Morton, 1998). It is well documented that larger sized individuals are at an advantage over smaller individuals during agonistic encounters, and individuals benefit from avoiding

escalation of unmatched encounters, due to the great risk of injury or even death MCE caused by fighting (Schmidt-Nielsen, 1975; Clutton-Brock & Albon, 1979; Peters, 1986). We have seen that the F0 of vocal signals is determined by the physical properties and adjustments of the vocal folds. However, due to their soft tissue anatomy, the growth of the vocal folds is not stringently constrained by the body size of an individual (Fitch, 1997, 2000c). A good illustration of this can be seen in the comparison of subspecies of cervidae. While adult Scottish red deer stags (weighing 160–250 kg) produce calls with a mean F0 of 112 Hz (Reby & McComb, 2003a), the smallest red deer subspecies (the Corsican deer, weighing c. 80 kg) produces calls with a mean F0 of 34 Hz and the largest red deer subspecies (the wapiti, weighing c.