Environment-dependent benefits of sociality in Soay sheep.

Although strong evidence links social connectedness to improved fitness, there is pronounced among-individual variation in social behaviour. This variation may be maintained if different types of social relationships (i.e. having many versus strongly connected partners) have environment-dependent fitness benefits, but this has rarely been tested. We applied network analysis to 37 years of social association data in female Soay sheep to test the relative importance of relationship quantity (mean group size, network degree and participation coefficient) and quality (mean strength and social differentiation) for overwinter survival. We find that both relationship quantity and quality predict survival the following winter. Crucially, the relative importance of relationship quantity and quality depends on weather conditions: harsh conditions favour quantity (specifically large mean group sizes), likely owing to thermoregulatory benefits, while benign conditions favour quality (specifically mean association strength), possibly because conspecific familiarity reduces conflict. Our second measure of relationship quality-social differentiation-had a positive association with survival that was not environment-dependent, illustrating the value of strong, stable relationships in fluid societies. These findings advance our understanding of social benefits in fission-fusion systems and show that changing environmental conditions may be an important mechanism maintaining variation in social behaviour in natural populations.