Varietas delectat: groups with diverse personalities mitigate physiological stress in a songbird

Social groups often consist of diverse phenotypes, including personality types. Social selection theory hypothesizes that group composition (i.e. social environment) influences the performance of group members. However, this hypothesis remains experimentally unexplored, and it is still contentious whether a homogeneous or a diversely composed group (i.e. social heterosis) is more beneficial to the individual. We experimentally formed groups of house sparrows with high and low diversity of personality, and found that their physiological state (body condition, physiological stress, and oxidative stress) improved with increasing group-level diversity of personality. These findings demonstrate that group composition affects the condition of group members and individuals benefit from social heterosis (i.e. associating with a diverse set of behavioural types). This aspect of the social life can play a key role in the evolutionary coexistence of different personalities in nature and has implications for human teams and animal welfare.