Vocal encoding of heritable sociability prevails over emotions in sheep bleats

Vocalizations of animals are good indicators of their emotions. Temperament is known to influence the regulation and expression of emotions. However, how animal temperament affects their vocalizations and particularly their vocal expressions of emotions remains largely unexplored. Sociability is often measured as the behavioural reactivity to social separation and is a temperament trait intrinsically linked to emotional reactivity. Most social species respond to such challenging situations using contact calls. Here, we investigated whether the acoustic structure of these calls reflects sociability, emotions or both. Among 152 recorded female lambs, 42 belonged to two diverging sheep lines selected for high or low sociability. High bleats were recorded both in isolation (social challenge, all lambs) and before receiving a food treat (non-social challenge, selected lambs) to investigate the link between vocalizations, emotions and heritable sociability. The acoustic features of isolation bleats differed between lines, which was not the case for pre-food treat bleats. However, surprisingly, the genetic selection index and social behaviour explained the structure of isolation bleats better than emotional arousal per se. Last, encoding of individuality in isolation bleats was impaired by the genetic selection. Our findings show that selecting for sociable animals affects vocal signatures in calls produced during a social challenge, leading us to hypothesize a socio-acoustic co-selection. This article is part of the theme issue 'Mechanisms, development, phylogeny and functions of emotional expressions'.