Indicators of age, body size and sex in goat kid calls revealed using the source–filter theory

The source–filter theory is an important framework recently applied to the study of animal vocalisations, which links the mode of vocal production to call parameters. Vocalisations can be good indicators of a sender's characteristics, such as identity, body size, age, and even hormonal status and affective states. For these reasons, applied vocal communication research would greatly benefit from adopting the source–filter theory approach to identify key call parameters linked to physical and physiological characteristics of domestic animals. Here, we introduce the source–filter theory through a detailed analysis and interpretation of goat contact calls during development. In mammals, vocal development is mostly influenced by maturation. Maturational processes of vocalisations are linked to growth or sex hormone effects on the larynx and vocal tract. We investigated changes to the parameters of goat contact calls during ontogeny, according to age, body size and sex. We recorded goat kids from birth to four months old and analysed their calls by considering the shape and functioning of the vocal apparatus. We found age and sex-related changes to most of the measured vocal parameters, suggesting a direct or indirect effect of sex hormones on vocal ontogeny. Furthermore, body size growth was negatively correlated with most frequency parameters, indicating that vocal production is constrained by body size throughout development. Therefore kid vocalisations provide information about age, sex and body size. We suggest that similar analyses applied to the study of vocal correlates of affective states, could greatly help the discovery of convenient and non-invasive indicators of animal welfare.