Balancing quality and quantity of social relationships in volatile social environments

Cooperative interactions do not occur in a vacuum, but develop over time in social groups that undergo demographic changes. Intuition suggests that stable social environments might favour individuals that develop few but strong reciprocal relationships (a ‘focused’ strategy), while volatile social environments do the opposite and favour individuals with more but weaker social relationships (a ‘diversifying’ strategy). We model reciprocal investments under a tradeoff between quantity and quality of social relationships, and reveal that this intuition is fallible. We find that volatile social environments particularly favour a focused cooperative strategy. This result can be explained by applying the theory of antagonistic pleiotropy, originally developed for senescence, to cooperative strategies. Diversifying social investments benefits late-life individuals that already have their social network built up, but is detrimental early in life when networks must be built from scratch. Under volatile social environments, the age structure of a population remains generally young, and this emphasizes strategies that do well early in life: a focused strategy which makes the individual form its first few social bonds happen quickly. Overall, our model highlights the importance of pleiotropy and population age structure for the evolution of cooperative strategies and other social traits. ### Competing Interest Statement The authors have declared no competing interest.