Correspondence Anna M. Taylor, Mammal Vocal Communication and Cognition Research, School of Psychology, University of Sussex, Brighton BN1 9QH, UK. Email: firstname.lastname@example.org
The field of animal vocal communication has benefited greatly from improved understanding of vocal production mechanisms and specifically from the generalization of the source–filter theory of speech production to non-human mammals. The application of the source–filter theory has enabled researchers to decompose the acoustic structure of vocal signals according to their mode of production and thereby to predict the acoustic variation that is caused by anatomical or physiological attributes of the caller. The source–filter theory states that vocal signals result from a two-stage production, with the glottal wave generated in the larynx (the source), being subsequently filtered in the supralaryngeal vocal tract (the filter). This theory predicts that independent indexical information such as body size, weight, age and sex can be contained in both the glottal wave (mostly characterized by its fundamental frequency), and the spectral envelope of the radiated vocalization (mostly characterized by the vocal tract resonances or formant frequencies). Additionally, physiological fluctuations in emotional or motivational state have been found to influence the acoustic characteristics of signals in a reliable and predictable manner that is perceptually available to receivers. While animal vocalizations contain some dynamic attributes, their static attributes are sufficient to provide an effective means of acoustic individual discrimination both within and across call types. In this paper, we draw together a wealth of experimental work conducted within the source–filter framework over the last decade and we review how such experiments have elucidated the communicative value of animal vocalizations.
Understanding communication systems is essential to the study of animal behaviour and ecology, as the progression of interactions between individuals is mediated by visual, olfactory and vocal signals (Bradbury & Vehrencamp, 1998). In particular vocal signals have been found to play a crucial role in determining the outcome of intra- and inter-sexual competition and to mediate agonistic or affiliative interactions between individuals (Owings & Morton, 1998; Fitch & Hauser, 2002). In mammals, early research on communication focused primarily on the more conspicuous features of acoustic signalling such as call occurrence, calling rate and loudness and signaller/receiver interactions (Clutton-Brock & Albon, 1979; McComb, 1991; Owings & Morton, 1998; McElligott & Hayden, 1999), providing valuable insights into our understanding of the function and evolution of sound signals. In recent years, vocal communication research has benefited from the application of the ‘source–filter theory’ (Fant, 1960; Titze, 1994), a framework initially developed for the study of human speech, which fits the requirements of a model linking vocal production, acoustic structure and functional discrimination/perception.
The aim of the present paper is thus to highlight how the source–filter theory has contributed to the current state of knowledge on vocal production mechanisms and its impact on animal vocal communication. Coupled with the development of modern digital techniques of signal analysis, the source–filter theory has enabled researchers to develop specific hypotheses within a testable framework. It is possible to investigate the detailed structure and variation of the acoustic parameters that compose vocalizations in relation to their origin, to describe and quantify their variation using clearly defined terminology, and finally to test their function using playback experiments (Fitch, 1997; Reby & McComb, 2003a,b; Charlton, Reby & McComb, 2007b; Charlton, McComb & Reby, 2008a; Koren & Geffen, 2009). We here present the key advances in the application of this framework. In the first section, we introduce the source–filter framework itself, define the acoustic parameters according to their mode of production and make broad predictions about the information they are likely to encode in mammal signals. In subsequent sections, we review the impact of this approach on different aspects of mammal vocal signals categorized according to the nature and function of the information encoded in signals: static cues to fitness (second section), motivational and referential cues (third section) and cues to individual identity (fourth section). In each section, we present two types of studies: correlational approaches that examine the covariation of acoustic parameters with traits or events, and experimental approaches, where playbacks of acoustic stimuli are used to examine the perceptual and/or functional relevance of these parameters.
In humans, it has long been established that production of voiced sounds commences in the larynx, a cartilaginous structure that is situated low in the throat where the tract of the pharynx splits into the trachea and the oesophagus. The largest structures of the larynx are the thyroid cartilage (which is attached to the hyoid bone by the thyrohyoid membrane) and the cricoid cartilage (which forms the inferior wall of the larynx and attaches to the top of the trachea). The vocal folds are located at the superior border of this cricoid cartilage. They are attached at the back to the arytenoid cartilages and at the front to the thyroid cartilage. The vocal folds themselves consist of three layers: muscle, vocal ligament and the epithelium. They are sometimes referred to as ‘vocal cords’, however, the term ‘vocal folds’ is preferred when discussing mammals as is it more anatomically correct (Titze, 1994; Fitch, 2006). Together with the spacing between them, the vocal folds form the glottis, where voiced sounds are generated. As air from the lungs forces its way through the closed glottis, the vocal folds are pushed apart. Biomechanical forces cause the vocal folds to snap shut again, and this sequence of opening and closing of the glottis causes a cyclic variation in air pressure across the larynx. Earlier accounts of vocal production stated that vocal fold vibration was predominantly driven by Bernouilli forces building up from sub-glottal pressure (van den Berg, Zantema & Doornenbal, 1957; Fant, 1960; Lieberman, 1977); however, systems of mechanical vibration invoked by Bernouilli forces are subject to dampening out, resulting in a gradual decrease in mechanical activity (Fung, 1981; Chan & Titze, 2006). A better understanding of tissue biomechanics has enabled researchers to determine that the continuous energy provided by the airflow from the lungs as it passes through the vocal folds creates a self-sustaining system of ‘flow-induced oscillation’. In such a system no additional mechanical forces are necessary to maintain a continuous rate of vibration (see Chan & Titze, 2006 for a detailed account of flow induced oscillation). The resulting waveform constitutes the source signal or glottal wave. While the vocal anatomy of all non-human mammals is fundamentally the same, most non-human mammals have a more elevated laryngeal position than humans with the larynx attached to the skull in a static position at the back of the oral cavity (Fig. 1).
The rate of opening and closing of the glottis determines the fundamental frequency (henceforth ‘F0’) of the glottal wave, also sometimes referred to as the glottal pulse rate. In human speech, F0 is the main factor determining the perceived pitch of a voice (however, it should be noted that the term ‘pitch’ is essentially perceptual and is better avoided when describing acoustic variation in vocal signals). F0 is determined primarily by the length and mass of the vocal folds: longer and heavier vocal folds vibrate at a slower rate than smaller vocal folds (Titze, 1994; Fitch, 1997). In humans, these properties can to a certain extent be manipulated by the muscles that form the vocal folds: flexion or relaxation of the cricothyroid or thyroarytenoid muscles control the lengthening and shortening of the vocal folds, and flexion or relaxation of the cricoarytenoid muscles control adduction and abduction of the vocal folds (Fink, 1975; Hardcastle, 1976). 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 (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).
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. The vocal tract consists of all the air cavities between the larynx and the opening of the mouth and/or nostrils through which the source signal travels before it radiates into the environment (Fig. 1). The vocal tract acts as a bank of bandpass filters, selectively dampening and/or enhancing specific ranges of frequencies from the source signal, corresponding to the resonant properties of its physical structures. The resonant frequencies form spectral peaks called formants (from the Latin formare, to shape; Fant, 1960; Titze, 1994).
In humans, the two largest cavities of the vocal tract are the pharynx and the mouth (Titze, 1994). Sophisticated vertical and horizontal movements of the tongue and lower jaw in the pharynx and the mouth influence the resonant properties of the vocal tract, thereby affecting the relative frequency position of formants, and particularly that of the lower formants (Fant, 1960; Lieberman, Klatt & Wilson, 1969; Hauser, Evans & Marler, 1993; Titze, 1994). Modulation of the lower formants of the voice spectrum results in the production of the different phonemes we perceive as vowels (Fant, 1960; Titze, 1994). In non-human animals, the vocal tract is usually not as flexible and thus its resonant properties are often static and more predictable (Fitch, 1994; 2000a,b, 2002). In particular the length of the vocal tract is directly reflected in the formants of many animal vocal signals (Fitch, 1997).
We have so far stressed that an important assumption of source–filter theory lies in the independence of source and filter, enabling researchers to relate specific acoustic parameters to their mechanism of production. However, it should be noted that in some circumstances interactions between source and filter components have been observed when the source or the filter influences or interferes with the output of the other (Titze, 2008). The contribution of source–filter interactions to the diversity of mammal vocal signals remains to be fully investigated.
Production constraints and cues to physical attributes
Animals use vocal communication to mediate crucial interactions such as sexual competition, territorial maintenance, partner or parent/young recognition and coordination of defence against predators (Owings & Morton, 1998). The outcome of many of these interactions depends on the physical attributes of individuals, such as their body size, physical condition, age or sex (Schmidt-Nielsen, 1975; Peters, 1986; Andersson, 1994). A comprehensive discussion of how acoustic signals may have the potential to provide accurate and reliable information about the physical attributes of individuals is given in a seminal paper by Fitch & Hauser (2002). Here we update the notion of ‘honest signalling’ (Fitch & Hauser, 2002) with a range of empirical tests conducted within the source–filter framework. Acoustic cues to physical attributes are often referred to as ‘indexical’ (Ghazanfar et al., 2007), for they provide receivers with reliable information on specific attributes (e.g. size) of callers (Fitch, 1997; Fitch & Reby, 2001; Maynard Smith & Harper, 2003). 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.
Indices generated at the source
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 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 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 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. 400 kg) produces calls with a F0 up to 2080 Hz (Feighny, Williamson & Clarke, 2006; Riede & Titze, 2008), despite having a vocal fold length of about 3 cm, which would normally be expected to produce a F0 of c. 50 Hz (Riede & Titze, 2008). While to date it is unclear how the wapiti is able to produce such a high F0 (vocal fold elasticity alone cannot explain this extreme divergence from biomechanical predictions: Riede & Titze, 2008), this example provides a clear illustration of the independence of F0 from body size and even in this case from vocal fold length.
Realizing the importance of filter-induced variation in animal vocalizations has been one of the most exciting recent developments in bioacoustics. Unlike the vocal folds, the vocal tract cannot grow independently of the rest of the body for its development is anatomically constrained by skeletal structures (Fitch, 2000b,c). The vocal tract length is thus directly dependent on body size. Investigations have confirmed a strong negative correlation between vocal tract length and body size (domestic dogs X-rays: Riede & Fitch, 1999; red deer dissections: Fitch & Reby, 2001; rhesus macaque radiographs: Fitch, 1997). This means that, unlike F0, formant frequencies have the potential to provide accurate or ‘honest’ information about the caller (Fitch, 1997, 2000c; Fitch & Reby, 2001; Fitch & Hauser, 2002; Reby & McComb, 2003b).
The overall spacing between formants appears to play the greatest role in providing an acoustic correlate of caller size. This relationship is quantified under the term ‘formant dispersion’ (Titze, 1994; Fitch, 1997; Reby & McComb, 2003a), literally referring to the pattern of dispersion of formants in the spectrum of the call. A direct negative correlation between formant dispersion and body size (Japanese macaques: Fitch, 1997; red deer: Reby & McComb, 2003a; domestic dogs: Riede & Fitch, 1999; Taylor et al., 2008; pandas: Charlton, Zhang & Snyder, 2009) has been confirmed in many species. Figure 2 illustrates the relationship between the formant dispersion calculated from growl vocalizations in 30 domestic dogs of different breeds and their respective body weight.
When the importance of formant dispersion as a size code was first identified, it was calculated as the ‘average distance between each adjacent pair of formants’ (Fitch, 1997, p. 1216). An alternative method of calculation is to run a linear regression plotting observed formant locations against predicted formant spacing. When the vocal tract is modelled as a straight uniform tube that is closed at one end and open at the other, the spacing between any two successive formants (Δf ) can be approximated as a constant, and formant frequencies can be plotted as , as illustrated in Figure 3 (Reby & McComb, 2003a). Regardless of which method of calculation is used, formant dispersion can be used to estimate vocal tract length by the equation , where c is the speed of sound in air approximated as 350 m s−1 and Δf is the formant dispersion (Titze, 1994; Fitch, 1997).
In some species, callers have evolved anatomical adaptations that enable them to alter the relationship between body size and formant frequency dispersion in their vocal signals. Both red and fallow deer show an anatomical peculiarity that was previously believed to be unique to humans: instead of the larynx resting in an elevated position at the back of the oral cavity as seen in most non-human mammals, the larynges of male red and fallow deer rest in an unusually low position in the neck (Fig. 1; red deer: Fitch & Reby, 2001; fallow deer: McElligott, Birrer & Vannoni, 2006). This causes the vocal tracts of these animals to be longer than would normally be expected for their size. Consequently, their vocalizations contain lower formant dispersions relatively to other species lacking this anatomical innovation, in effect resulting in the projection of a relatively exaggerated impression of their body size. As illustrated in Fig. 4, the larynges of male red and fallow deer can be retracted even further into the throat during the production of mating calls, allowing further vocal tract elongation (Fitch & Reby, 2001; McElligott et al., 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 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 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). A recent mate choice experiment demonstrated that red deer hinds preferentially approached a loudspeaker emitting stag roars resynthesized to have more closely spaced formants mimicking a larger male, over a loudspeaker emitting roars resynthesized with more widely spaced formants mimicking a smaller male (Charlton et al., 2007b). This suggests that sexual selection for anatomical adaptations mediating acoustic size exaggeration may be a driving factor in the evolution of these production mechanisms (Fitch & Reby, 2001; Charlton, 2008). It has been hypothesized that the lowered resting position of the larynx in humans may have evolved through similar selection pressures, predating the development of speech (Ohala, 2000; Fitch & Reby, 2001; Fitch, 2002).
There is compelling evidence that formant information is also perceived across species, presumably because the fundamental similarities across mammal vocal production systems have led to comparable similarities in the perception of acoustic signals. Several animals have been trained to discriminate vowel-like sounds using operant conditioning techniques (Chacma baboons: Hienz & Brady, 1988; Chinchilla: Burdick & Miller 1975; domestic dogs: Baru, 1975; Japanese macaques: Sinnott, 1989; Sinnott & Kreiter, 1991; Sommers et al., 1992). Using resynthesized formants, researchers furthermore demonstrated that human listeners were able to reliably rate the size of domestic dogs based on an acoustic signal alone (Taylor et al., 2008), providing direct evidence for interspecific perception and assessment of size-related variation in formant frequencies. The use of formants as indices of body size may be widespread in mammals with potential implications for interspecific interactions such as eavesdropping predator/prey contexts.
Finally, Fitch (1997) notes that reliability of size information in formants is dependent on the quality of the source signal. Formants are perceptually easier to discriminate in harsh, broadband calls (such as grunts, groans or growls) than in high F0, tonal calls with wide inter-harmonic intervals and little inter-harmonic energy. The impact of some source characteristics on formant perceptibility is little investigated and remains an area of interest for future empirical work.
Affective and referential information
In the previous section, we have shown how acoustic signals are frequently dependent on static physical attributes, and also how anatomical or behavioural adaptations may effectively provide a means of vocal control. In the context of social interactions, the significance of vocal signals may go beyond the encoding of caller attributes and may provide a secondary level of information relating to the current motivational or emotional state of individuals (Ohala, 1984). Additionally, in a limited number of mammals, the acoustic structure of signals can be arranged in a predictable manner, so as to provide functionally referential information (Hauser, 1993, 1998), defined here as signals providing reliable stimulus-specific information about an external object or event (Macedonia 1993) that are produced only in an appropriate context (Macedonia 1993; Hauser, 1993, 1998; Evans, 1997). The investigation of affective basis and referential content in animal vocalizations is highly relevant in the light of understanding the evolution of human speech and how meaning has become encoded in phonetic variability, bringing the source–filter theory to the centre of this topic (Fitch, 2000a, 2002; Ohala, 2000; Slocombe & Zuberbühler, 2005).
Dynamic encoding at the source
In many species, there are significant differences between calls recorded in different social situations (baboons: Owren et al., 1997; Rendall et al., 1999; Seyfarth & Cheney, 2003a,b). This is true both between call types (i.e. specific types of vocalizations occur consistently in specific contexts; Morton, 1977) and within call types, where the acoustic structure of call varies according to context (domestic dogs barks: Yin, 2002; Yin & McCowan, 2004). Indeed, several characteristics of F0 (such as mean F0, peak F0 and F0 modulation) have been linked to the context in which calls are emitted (baboons: Fischer et al., 2002; domestic dogs: Yin, 2002; Taylor et al., 2009a; pandas: Charlton et al., submitted; wapiti: Feighny et al., 2006; also see Ohala, 1984). Classification methods such as discriminant function analysis are useful in confirming the acoustic categorization of vocalizations emitted in different contexts. For example, Yin (2002) found that domestic dogs barks occurred on a graded scale, showing a continuum of acoustic gradations on several frequency parameters depending on the situation in which they were emitted. It was confirmed that barks could be statistically divided into different context-specific subsets on the basis of the co-variation of their peak, mean fundamental frequency, duration and inter-bark interval (Yin & McCowan, 2004). These parameters furthermore enabled human listeners to reliably categorize barks in function of their recording context (Pongrácz et al., 2005).
Dynamic changes in F0 providing cues to affective state are most likely mediated by changes in physiological arousal such as rate of respiration or muscular (cricoarytenoid) tension in the vocal folds (Scherer, 1986; Titze, 1994; Hauser, 2000; Bachorowski & Owren, 2008). Generally speaking, the motivational information provided by F0 fits the framework of the motivation-structural rules and frequency code theory: thus, the barks of domestic dogs recorded in an aggressive context have been found to have a significantly lower F0 than barks recorded in a playful setting (Yin, 2002; Yin & McCowan, 2004; Pongrácz et al., 2005; Taylor et al., 2009). Similarly, wapiti bugle calls emitted in aggressive contexts are lower in frequency (both F0 and formants) than bugle calls emitted during non-aggressive interactions (Feighny et al., 2006). In fallow deer, males with a low F0 have a higher reproductive success, and likewise human men with a low F0 report more sexual partners, and are more attractive to women, than males with higher F0 (fallow deer: Vannoni & McElligott, 2008; humans: Puts 2005; Apicella, Feinberg & Marlowe, 2007; Apicella & Feinberg, 2009). However, the effect of motivational state on F0 does not always follow a predictable direction. For example, male baboons with a high dominance status produce calls with a higher F0 than lower ranked males (Fischer et al., 2004), presumably because they have a high reproductive and territorial motivation and are in a higher state of physiological arousal. Similarly, red deer stags with a higher F0 are known to have a greater reproductive success than stags with lower F0 (Reby & McComb, 2003b). It thus seems prudent to propose that variations in F0 are species-specific and should be documented across several species before generalized assumptions across species can be made.
Another dimension of the source implicated in the communication of motivational state is calling rate. Calling rate can be linked to rate of respiration, and typically provides immediate information about the current condition or motivation of an individual (red deer: Clutton-Brock & Albon, 1979; McComb, 1991). During the rutting season, fallow deer bucks call at a rate of 3000 groans h−1– groaning in this species appears to be aimed at other males by advertising a measure of fighting motivation rather than at attracting females (McElligott & Hayden, 1999, 2001). Interestingly, fallow deer bucks may perform less laryngeal retraction in favour of maintaining a high groaning rate, as the latter plays a more important role in this species (Vannoni, Torriani & McElligott, 2005). In contrast, red deer stags, who retract the larynx to some degree for virtually all roars, are able to sustain a roaring rate of ‘only’ around 400–500 roars h−1 (Clutton-Brock & Albon, 1979; McComb, 1991). This trade-off between calling rate (indicating physical condition and fitness) and laryngeal retraction (indicating body size, as well as fitness) may be due to the dual role of roaring in intra-sexual competition and mate attraction which may differ between these two species.
Because of the overall reliability of formant frequencies as an acoustic correlate of body size, small variations in formant dispersion may have a secondary function of signalling motivational state. Effectively, the signalling of body size can become a ritualized advertisement of emotional or motivational state (Ohala, 1984; also see Morton's, 1977 motivation-structural rules). In several species, callers have been observed to retract the lips in positive situations or in encounters where it is beneficial for them to appease another individual (such as a smile or fear grin; canines: Fox, 1970; humans: Drahota, Costall & Reddy, 2008) and to protrude the lips during socially stressful or agonistic encounters where it is beneficial to appear larger or more dominant (baboons: Harris et al., 2006; wolves: Fox, 1970). Moreover, Ohala (1984) has proposed that the vocal gestures associated with different emotional/motivational states may have driven the evolution of facial expressions, a theoretical hypothesis that has received support from both observational and empirical studies (Harris et al., 2006; Chuenwattanapranithi et al., 2008; Drahota et al., 2008). The interaction between emotional/motivational state, acoustic output and facial expression is a largely unexplored branch of vocal communication (also see Ohala, 1984; Aubergé & Cathiard, 2003; Chuenwattanapranithi et al., 2008; Drahota et al., 2008) and further research in non-human mammals is required to determine the full extent and limitations of this phenomenon.
Complex control of filter components has also been observed for the encoding of context-specific information. Indeed, some non-human primate species produce acoustically distinct alarm calls for different classes of predators (Barbary macaques: Diana monkeys: Zuberbühler, Cheney & Seyfarth, 1999; Zuberbühler, 2002; vervet monkeys: Seyfarth, Cheney & Marler, 1980; Owren, 1990a; Owren & Bernacki, 1998; rhesus monkeys: Hauser, 1998; also see meerkats for a non-primate example: Manser, 2001; Manser et al., 2002), and other group members are able to respond in functionally appropriate ways to these calls (vervet monkeys look up or hide under bushes in response to an ‘eagle’ alarm call, but scan their surroundings and head for the trees in response to a ‘leopard’ alarm call: Seyfarth et al., 1980; Seyfarth & Cheney, 1990). Functionally referential calls, at least in some primate 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 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).
Inter-play between source and filter
We have seen that source and filter components can provide varying levels of affective and functionally referential information 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.
Cues to individual identity in acoustic signals
Acoustic distinctiveness both at an individual and at a group level (Rendall, Rodman & Emond, 1996) has been shown to be essential for many different species; for example to maintain group cohesiveness when foraging in dense undergrowth or in otherwise reduced visibility, for the recognition of familiar versus unfamiliar individuals in neighbouring or overlapping territories, or for the identification of kin in large groups or in species where mother and offspring are not constantly together (Trivers, 1971; Halliday, 1983). Moreover, acoustic familiarity may play a role in inter-sexual interactions where the familiarity of males may be an indication of their investment in sexual displays toward females (Zimmermann & Lerch, 1993; Reby et al., 2001). Acoustic variation may originate in the source or in the filter, and understanding their relative contributions to individuality, and how selection pressures have differentially affected these contributions, has greatly assisted our understanding of the form and function of different types of vocal identity cues.
For many mammals however, the greatest contribution of the source to individual variation appears to be based on the dynamic modulation of F0 (baboons: Owren et al., 1997; bottlenose dolphins: Janik 2000; fallow deer: Torriani, Vannoni & McElligott, 2006; wolves: Palacios, Font & Márquez, 2007), which is furthermore linked to the existence of ‘vocal signatures’ (fur seals: Charrier et al., 2003b; wolves: Palacios et al., 2007; bottlenose dolphins: Janik, Sayigh & Wells, 2006). Signature calls essentially appear to serve a similar identifying purpose to ‘names’ in human interactions, although it should be noted that in human speech, names are words composed of phonemes produced by the manipulation of formants (Lieberman & Blumstein, 1988). F0 mediated acoustic distinctiveness has been identified between group members but also across individuals within a same group, so that all members within the group produce a recognizable call that is distinct from the signature call of other groups (hyenas: Holekamp et al., 1999; pigtail macaques: Gouzoules, Gouzoules & Marler, 1995). Moreover, F0 appears to be especially important for kin recognition in many species, specifically for reuniting mother and offspring. Different selection pressures have led to variations in acoustic kin recognition strategies. In species using a hider strategy where the young remain hidden in the undergrowth after birth (see Langbein & Putman, 1992), there is a selection pressure for young to be silent to avoid detection by predators. Because the offspring are mobile and may change hiding places, acoustic recognition of the dam is essential to maintain maternal care in these species (Fisher, Blomberg & Owens, 2002). Fallow deer fawns thus only leave their hiding place in response to the recognition of the distinctive fundamental frequency of their dam's call, while the dam does not recognize the call of the fawn (Vannoni et al., 2005; Torriani et al., 2006). On the other hand, in species using a follower strategy and in species where offspring are mixed with other same-aged offspring, mutual recognition between mother and young is vital for the survival of offspring (banded mongoose: Muller & Manser, 2008; northern fur seals: Insley, 2001; sheep: Searby & Jouventin, 2003). Recognition is likely to have evolved via different selection pressures on mother and young: for young animals, non-recognition of their mother may lead to death, whereas for the mother, non-recognition of their young may lead to the loss of one breeding season (Trivers, 1971). These differential pressures mean that acoustic recognition between mother and offspring may be asymmetrical (Insley, 2001). Thus in fur seals, pups attend to the harmonic structure and tempo of female calls to identify their mother (Charrier et al., 2003b), while mothers appear to attend to the properties of the energy spectrum (frequency modulation and amplitude contour) to identify their pup (Charrier, Mathevon & Jouventin, 2002). Playback experiments in which the harmonic structure of maternal calls has been modified have unambiguously shown that this manipulation irrevocably impairs recognition in fur seal pups (Charrier et al., 2003b).
Filter-induced individual variability
For some mammals and specifically for several primate species including humans, filter components play a substantial role in the acoustic distinctiveness of individuals (baboons: Owren et al., 1997; red deer: Reby et al., 1998; rhesus monkeys: Rendall et al., 1998; also see Fischer et al., 2001). This appears to be primarily an acoustic consequence of morphological individuality. For example, Rendall (2003a,b) used statistical algorithms to show that baboon grunts were individually distinct across several acoustic parameters (notably tempo, F0 and formant structure), but that formants provided the highest degree of differentiation between individuals due to the less reliable, dynamic nature of tempo and F0.
While most studies have focused on individual differences occurring within the same call types, there is some evidence that in some non-human mammals, individuals have idiosyncratic voices, like human speakers. Indeed, just as our voice is recognizable independently of the phoneme pronounced, in mammal calls the individuality of voice also has the potential to remain recognizable across call types, due to the physiological basis of the filtering process. This has been demonstrated in several species, giving credence to the concept of an ‘individual voice’ (baboons: Rendall et al., 1998; red deer: Reby et al., 2006). Red deer can be accurately individually identified across several call types (harsh roars, roars, chase barks and barks) due to inter-individual acoustic variation that most likely reflects individual differences in the morphology of the vocal tract (Reby et al., 2006). Similarly, rhesus monkeys retain distinctiveness across coos, grunts and noisy screams (Rendall et al., 1998), although the authors also note that individual distinctiveness across call types can sometimes be hampered by the broad structural differences between the calls.
Co-variation of source and filter
Several studies have now highlighted the importance of the inter-play of source and filter components for reliable identification of a caller (fallow deer: Reby et al., 1998; Vannoni & McElligott, 2007; rhesus monkeys: Rendall et al., 1998). In the case of mother–young recognition there is an interesting asymmetry: while adults do not typically vary in size, their offspring have growing bodies. Given the direct dependence of filter-related components on skeletal size, these acoustic parameters are expected to change allometrically in line with the physical development and growth of the offspring. Conversely, the relative independence of the source-related components from physical attributes means that they are potentially less subject to the developmental changes of the caller. In several pinniped species, it has been shown that mothers have long-term recognition of both the immature and adult vocalizations of their offspring from previous years (Insley, 2000; Charrier, Mathevon & Jouventin, 2003a). It would thus be of interest for future research to investigate the differential variation in source and filter characteristics throughout the lifetime of individuals and how this co-variation might definitely affect individual distinctiveness in adults versus immature animals. A point of interest that emerges from the literature is the apparent evolutionary convergence of bleat vocalizations. Bleats are stereotypical plaintive vocalizations that occur across several unrelated species in the context of individual recognition (seal pups: Schustermann & van Parijs, 2003; sheep: Searby & Jouventin, 2003; Sèbe et al., 2008). This highlights a potentially promising area for future research, as it seems likely that their acoustic characteristics are particularly favourable to individual and specifically mother–young recognition.
In this review, we have shown that the source–filter theory goes a long way in predicting, identifying and explaining the functional content of mammal acoustic signals and their evolution. We have presented many examples of how source- and filter-related components in vocalizations play independent roles in the transmission of different types of information, and specifically how they can function as indices of attributes such as body size, weight, age or sex. Attending to such cues, and using them to assess the physical attributes and condition both of potential competitors and mates, can have important implications for the reproductive opportunities and survival of receivers. In addition to static attributes, cues to transient qualities such as emotional or motivation state and dynamic qualities such as reproductive status or dominance rank can also be advertised in the source and filter components of vocal signals. Moreover, there is growing evidence that in some primate species callers are able to produce vocalizations containing information about events or objects in the external world encoded in their source and filter-related component characteristics. Finally, we have discussed how the inter-individual variation in anatomy/physiology reflected in the acoustic structure of vocal signals can lead to voice differences between individuals, and more specifically, how identity information can be given by frequency or amplitude contours, as is observed in the identifying whistles, and more generally in the ‘vocal signatures’, of several species.
In conclusion, this review has highlighted the important contributions of the source–filter paradigm to understanding mammal vocal communication. Understanding call production mechanisms has enabled the development of a testable framework for the investigation of the origin and function of signals. This conclusion is illustrated in Fig. 5, which provides an overview of the evolutionary feedback loop linking production mechanisms to the acoustic structure of signals and the ultimate effect this has on the perception by, and behaviour of receivers.
Many thanks to Karen McComb and Ben Charlton for their helpful comments on earlier versions of the paper, and to Alan McElligott for his support throughout the writing process. Thanks also to the contributions of Tim Halliday and one anonymous referee. Funded by a BBSRC studentship to the first author.