The scaling of social interactions across animal species

Abstract

Social animals self-organise to create groups to maximise protection against predators, productivity and fitness. One-toone interactions are the building blocks of these emergent social structures and may correspond to friendship, grooming, communication, among other social relations. These structures should be robust to failures and provide efficient communication to compensate the costs of forming and maintaining the social contacts but the specific purpose of each social interaction regulates the evolution of the respective social networks. We collate 637 animal social networks and show that the number of social interactions scales with group size as a super-linear power-law for various species of animals, including humans, other mammals and non-mammals. We identified that the super-linear exponents fall in 3 broad classes depending on the social function, i.e. cooperation, spatial proximity, and friendship. By fitting a hierarchical model to our data, we observed that social hierarchy must be absent to create efficient network structures for cooperation. This is not the case for friendship ties where network performance is not driven by group survival. Social interactions measured by spatial proximity fall in an intermediary category, not as efficient as cooperation, and is likely more relevant for the spread of infectious diseases than for social phenomena.