The contribution of source–filter theory to mammal vocal communication research

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.