Oxytocin homogenizes horse group organization.

The oxytocinergic system has been suggested to make up an important part of the endocrine basis of group cohesion. However, controlled studies in open-group settings have not been performed. We here investigated the impact of exogenous intranasal oxytocin on the group-level social organization of 5 groups of horses ( = 58; 12 mares and 46 geldings) through GPS tracking and social network analysis. We find oxytocin flattened social differentiation across levels. Most strikingly, oxytocin did not simply reinforce existing bonds but selectively shifted social preferences toward homogenization - individuals and pairs who otherwise rarely associated spent more time close together, while individuals and pairs with the highest baseline association instead spent more time further apart. This resulted in a more distributed structure and lower clustering coefficient at the network level. These effects reinforce and extend oxytocin's role in collective behavior, social organization, and the evolution of group-based sociality across taxa.