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.

We investigated whether the growls of domestic dogs, Canis familiaris, exhibit predictable variations in a range of key acoustic parameters when they are given in two contrasting experimental contexts, and whether humans are sensitive to such variation. In a standardized experimental paradigm, an experimenter visited 32 domestic dogs and generated an aggression context and a play context. In these contexts, 204 isolated growls were recorded and subsequently acoustically analysed. Contrary to previous findings on barks, fundamental and formant frequencies of growls did not vary between the two contexts. However, growls from the aggression context were significantly longer than growls from the play context. Additionally, the temporal structure of the vocal sequences containing growls differed significantly between the contexts. In a series of psychoacoustic experiments, human listeners were not able to discriminate between growls recorded in the two contexts, however both aggressiveness and playfulness ratings were strongly influenced by dog weight, with larger dogs being perceived as more aggressive than smaller dogs. However, participants were able to attribute aggressiveness and playfulness to synthesized sequences in which growls occurred at a two rates, typical of either the aggression or the play context. We conclude that context‐related variation in growling behaviour resides in the temporal structure, rather than in the acoustic composition of isolated growls, and furthermore that human listeners are able to attribute context on the basis of growling rate. In addition, human appear to have an intuitive acoustic bias toward perceiving larger dogs as more aggressive.

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Vocal expressions of emotions follow simple rules to encode the inner state of the caller into acoustic parameters, not just within species, but also in cross-species communication. Humans use these structural rules to attribute emotions to dog vocalizations, especially to barks, which match with their contexts. In contrast, humans were found to be unable to differentiate between playful and threatening growls, probably because single growls' aggression level was assessed based on acoustic size cues. To resolve this contradiction, we played back natural growl bouts from three social contexts (food guarding, threatening and playing) to humans, who had to rate the emotional load and guess the context of the playbacks. Listeners attributed emotions to growls according to their social contexts. Within threatening and playful contexts, bouts with shorter, slower pulsing growls and showing smaller apparent body size were rated to be less aggressive and fearful, but more playful and happy. Participants associated the correct contexts with the growls above chance. Moreover, women and participants experienced with dogs scored higher in this task. Our results indicate that dogs may communicate honestly their size and inner state in a serious contest situation, while manipulatively in more uncertain defensive and playful contexts.

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