Cooperation beyond the dyad: the emergence and stability of animal societies.

Cooperation is a fundamental driver of social evolution, yet we still lack an integrated framework explaining how interactions among individuals scale up to the formation and long-term stability of animal societies. Although major advances have been made, research has often focused on a narrow set of model taxa and on individual mechanisms in isolation, hindering a comparative understanding across species and theoretical approaches. In this review, we synthesise theoretical and empirical work to link the processes that initiate group living with those that sustain cooperation once groups are established. We first examine how cooperation promotes the emergence of social groups through byproduct benefits, mutualistic interactions, and population viscosity, highlighting their differing dependence on relatedness and their potential to operate simultaneously in nature. We then review mechanisms that maintain cooperation within established groups- including reciprocity, assortment and population structuring, division of labour, policing and punishment- and consider how their importance varies across socioecological contexts and levels of relatedness. Finally, we outline how emerging tools, such as long-term behavioural datasets, social networks, comparative analysis, simulations and experimental manipulations, can expand research beyond a small set of well-studied systems and support comparative tests to improve our understanding of the processes shaping cooperation and social complexity across diverse taxa.