Coevolution of social network structure and life history in toothed whales

Toothed whales offer a 34 My-long natural experiment for the evolution of complex mammalian societies. However, quantitative comparative analyses of social structure in these species are lacking. Here, we draw on existing social network analyses to compare social structure across toothed whales. We consider published measures of two social network traits across all toothed whales: modularity ( Q ), which captures divisions or “cliques” in a social community, and social differentiation ( S ), which estimates how much whales vary in their relationships—whether they associate equally with others or form special bonds with specific individuals. Combining these with a recently published database of life history traits, we applied phylogenetic multilevel models with the objective of exploring the origins of social network structure. We identified 98 measures of modularity and 89 measures of social differentiation from 23 toothed whale species. Social network structure was more similar among closely related species (showing strong phylogenetic signal), despite substantial intraspecific variability. Toothed whales with longer lifespans and larger bodies tended to form more modular social networks, as did those where males were proportionally larger than females. Similar but weaker patterns were found between life history traits and social differentiation. Results from causal coevolutionary models provide preliminary evidence that social structure has been both a leader and follower of changes in life history. Our findings reveal the correlated evolution of social network structure and life history traits in toothed whales, shedding new light on the origins of social relationships.