The role of appropriate social group size and composition on animal welfare.

Billions of animals worldwide live in captivity across diverse settings, with the vast majority being inherently social species whose welfare fundamentally depends on appropriate social interactions with conspecifics. This review examines the critical role of social environments in captive animal welfare across laboratory, zoo, farm, and companion animal contexts. We first demonstrate how inappropriate group sizes and compositions negatively impacts animal welfare through multiple pathways: impaired physical health and cognitive development, compromised emotional states and altered behavioural expression. Next, we analyse whether current legislative frameworks account sufficiently for social needs. Although regulations acknowledge the danger of overcrowding and the importance of a partner, they predominantly emphasise density requirements without adequately addressing species-specific social structures. Finally, to address these shortcomings, we propose that observations of natural groups are needed to inform a wide variety of sociality characteristics. This should be guided by an established sociality framework to provide a benchmark for social needs in captivity. These should then be refined by preference tests and integrated in flexible management practices. Much progress has been made to improve the physical environment of captive animals yet advances in addressing their social environment have not kept pace. Bridging this gap will be essential if we want to ensure animals in human care can experience lives that meet their needs.