Social bond dynamics and the evolution of helping

Empiricists often struggle to apply game theory models to real-life cases of animal cooperation. One reason is that many examples of cooperation occur in stable groups, where costs and benefits of helping depend on several factors. Among these are variable investments, fitness interdependencies, learning, memory, reciprocity, and partner choice, including the formation of social bonds with specific group members. Here, we present a game theory model exploring the conditions under which social bonds between group members can promote cooperation, with reciprocal acts of helping. In the model, bonds build up from exchanges of help in a similar way as the strength of association increases in learning, as in the Rescorla-Wagner rule. The bonds in turn affect partner choice and influence helping amounts. The model has a mechanism of reciprocity for bonded pairs, which can evolve towards either loose or strict reciprocation. Several aspects of the model are inspired by observations of food sharing in vampire bats. We find that small social neighbourhoods are required for the evolutionary stability of helping, either as small group sizes, or if members of larger groups can form temporary (daily) smaller groupings. The evolutionary outcome is a fairly low cost helping, while the benefit of receiving help can be substantial. Individuals in need request help based on bond strength, but there is also an evolved preference for initiating bonds with new group members. In contrast, if different groups come into temporary contact with each other, the evolved tendency is to avoid forming bonds between groups. Popular science summaryThe search for evolutionary explanations of cooperation between members of social groups has long been a high profile endeavour. A case of particular interest is when individuals develop a network of friends and exchange help through these social bonds. The question of helping between friends was given emphasis already from the start of the evolutionary study of cooperation, more than 50 years ago, but it has remained without any decisive progress since that time. Here we present a game-theory analysis of helping through the build-up of social bonds. We find that there is reciprocity in socially bonded pairs, which is neither immediate nor very strict, and that relatively small social neighbourhoods are required for the evolutionary stability of helping.