Emotional contexts influence vocal individuality in ungulates

Abstract

To group-living animals, such as most ungulates, being able to recognise the members of one's social groups is crucial. While vocalisations often carry cues to identity, they are also impacted by the affective state of the caller, with signals often becoming more chaotic in contexts of negative valence or high arousal. How might this influence vocal individuality - and is there a pattern across taxa? To understand how the individual information content is maintained over emotionally charged contexts, we studied putatively negative and positive contact calls of seven ungulate species: cattle, goats, sheep, pigs, wild boars, horses, and Przewalski's horses. Calls were classified to individuals using linear and machine learning methods, and their information content assessed using Beecher's statistic and the Potential of Individuality Coding. In most species, calls could be reliably classified to the caller within and across affective states. While there was no uniform pattern in information content change between valences across species, some species showed a pronounced increase in individuality in either positive or negative situations. In each of the species, at least one acoustic parameter was a reliable indicator of individuality across contexts. Our results suggest that different coding strategies may be present across taxa, and imply that individual vocal recognition requires acoustic stability of certain important parameters. These findings reveal a nuanced role of affective communication in maintaining social bonds among socially complex animals.