The scaling of social interactions across animal species

TL;DR

This study analyzes 611 animal social networks, revealing that social contact numbers scale super-linearly with group size, with the scaling exponent varying by social function, impacting social structure and disease spread.

Contribution

It provides a comprehensive empirical analysis of social network scaling laws across multiple species and links social function to network structure and costs.

Findings

Social contacts scale as a super-linear power-law with group size.

The scaling exponent varies with social interaction type, from 1 to 2.

Proximity interactions may facilitate disease spread similarly to friendships.

Abstract

Social animals self-organise to create groups to increase protection against predators and productivity. One-to-one 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 611 animal social networks and show that the number of social contacts $E$ scales with group size $N$ as a super-linear power-law $E=CN^{\beta}$ for various species of animals, including humans, other mammals and non-mammals. We identify that the power-law exponent $\beta$ varies according to the social function of the interactions as $\beta = 1+a/4$, with $a \approx {1,2,3,4}$. By fitting a multi-layer model to our data, we observe that the cost to cross social groups also varies according to social function. Relatively low costs are observed for physical contact, grooming and group membership which lead to small groups with high and constant social clustering. Offline friendship has similar patterns while online friendship shows weak social structures. The intermediate case of spatial proximity ($\beta=1.5$ and clustering dependency on network size quantitatively similar to friendship) suggests that proximity interactions may be as relevant for the spread of infectious diseases as for social processes like friendship.