Behaviour 2019
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A Preference for Aggregating with Familiar Individuals Stabilizes Cooperation among Non-relatives
Aviva F. Blonder, Jeffrey C. Schank. University of California, Davis, Davis, CA, United States

Cooperation is essential to life as we know it, but paying a cost to provide a benefit to a potential competitor should be selected against. It is known that cooperation can evolve if cooperators are able to assort, whether by aggregating with kin, by finding other cooperators based on a tag or “green beard” trait, or through reciprocity. However, animals of many species cooperate in groups of unrelated familiar individuals, without the need for complex individual recognition. We employ an agent-based model where individuals disperse widely in space, resulting in at best random aggregation among relatives. In addition, all cooperation is indiscriminate, eliminating reciprocity and tag-based cooperation. We found that even under these conditions, which preclude known mechanisms for assortment among cooperators, a preference for aggregating with unrelated familiar individuals can make naive cooperation an evolutionarily stable strategy. This highlights the importance of interdependence and social structure without kinship in the evolution of cooperation and provides a promising explanation for why many species cooperate in groups of familiar individuals.