The Friendship Paradox across animal social systems is governed by network structure and biological features

A population's social structure, often represented as a social network, shapes fundamental biological processes including the spread of disease, information, and behaviour. The 'Friendship Paradox' is a network phenomenon whereby the average individual has fewer 'friends' than their 'friends' do. This effect can be quantified as relationship disparity (the difference between an individual's connectedness and those they are connected to) which captures the local social environment. Previous work has shown that such relationship disparity can be exploited in effective outbreak monitoring, targeted health interventions and optimized contact tracing. Yet, how its magnitude varies across real-world social networks remains poorly understood. Here, we analyse relationship disparity across 391 empirical animal social networks to test how intrinsic network properties and biological attributes predict its extent. We find that smaller and sparser networks exhibit stronger relationship disparity, and it is further shaped by both species-specific differences and shared phylogenetic history. After controlling for variation in individual sociability, relationship disparity was more strongly accentuated in mammalian direct physical contact networks than indirect social association networks, whilst the corresponding finding was not detected among Sauropsids. Phylogenetic and species identity largely explained variation in relationship disparity indicating biological drivers. Together, these findings demonstrate that both network structure and biological attributes shape relationship disparity in animal social systems, providing a foundation for predicting how higher-order network architecture influences social processes such as contagion.