Social Group Size Predicts the Evolution of Individuality

SummaryDiscriminating among individuals is a critical social behavior in humans and many other animals [1, 2 and 3] and is often required for offspring and mate recognition, territorial or coalitional behaviors, signaler reliability assessment, and social hierarchies [4, 5, 6, 7, 8 and 9]. Being individually discriminated is more difficult in larger groups, and large group size may select for increased individuality–signature information–in social signals, to facilitate discrimination [4, 10, 11, 12, 13 and 14]. Small-scale studies suggest that more social species have greater individuality in their social signals, such as contact calls [4, 12 and 13]. However, this relationship has not been evaluated in a broader-scale evolutionary context or in social signals other than contact calls. It is not yet known whether social group size may be viewed as a general evolutionary driver of individuality. Here we show a strong positive evolutionary link between social group size in sciurid rodents and individuality in their social alarm calls. Social group size explained over 88% of the variation in vocal individuality in phylogenetic independent contrasts. Species living in larger groups, but not in more complex groups, had more signature information in their calls. Our results suggest that social group size may promote the evolution of individual signatures and that the sociality-individuality relationship may be a general phenomenon in nature.

► Individual recognition is important for social behaviors in humans and other animals
► Individual recognition is harder in larger groups, but increased individuality helps
► In evolutionary analyses, social group size predicts individuality in social signals
► Social group size promotes the evolution of individualistic signatures