Coding for ‘Dynamic’ Information: Vocal Expression of Emotional Arousal and Valence in Non-human Animals

Emotions guide behavioural decisions in response to events or stimuli of importance for the organism, and thus, are an important component of an animal’s life. Communicating emotions to conspecifics allows, in turn, the regulation of social interactions (e.g. approach and avoidance). The existence of common rules governing vocal expression of affective states across species has been proposed as a function of the motivational state (i.e. intention of behaviour) of the emitter (‘motivation-structural rules’) and as a function the two main dimensions of emotions, valence (positive versus negative) and arousal (bodily activation). In this chapter, I review the potential for vocalisations to serve as universal non-invasive indicators of animal emotions, by considering the latest evidence for common rules existing across species according to the two dimensions of emotions (‘emotional-dimension rules’). Vocal indicators of emotional arousal have been relatively well studied. Cross-species comparison shows that, when arousal increases, vocalisations tend to be louder and are produced at faster rates, with higher frequencies (both source- and filter-related) and a more variable fundamental frequency (F0). In contrast, indicators of valence have only been investigated in a few species. The evidence so far indicates that, compared with negative vocalisations, positive vocalisations tend to be shorter, with a lower and less variable F0. Yet, comparison of vocal indicators of valence between closely related species suggests that these indicators are more species specific than indicators of arousal, which have clearly been conserved throughout evolution. To conclude, I further suggest a new set of rules that could explain the acoustic structure of vocalisations across species, which combine features predicted by the motivation-structural rules, the emotional-dimension rules, and characteristics of the social relationship involving the emitter and receiver.