Socio-acoustic co-selection? Vocal encoding of sociability prevails over emotions in sheep bleats

Vocalisations 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 vocalisations 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 this challenging situation using contact calls. Here, we investigated whether the acoustic structure of these calls reflect sociability, emotions or both. We used 42 female lambs from two diverging sheep lines selected for high or low sociability. High bleats were recorded both in isolation (social challenge) and before receiving a food treat (non social context) to investigate the link between vocalisations, emotions and heritable sociability. The acoustic features of isolation bleats differed between the lines, but it was not the case for pre-feeding bleats. Surprisingly, the genetic selection index and social behaviour were better predictors of the structure of isolation bleats than the arousal. Last, encoding of individuality in isolation bleats was impaired by the genetic selection. Our findings suggest a socio-acoustic co-selection: selecting for sociable animals affects vocal signatures in calls produced during a social separation.