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Why join a neighbour: fitness consequences of colony fusions in termites

The evolution of life is characterized by major evolutionary transitions during which independent units cooperated and formed a new level of selection. Relatedness is a common mechanism that reduces conflict in such cooperative associations. One of the latest transitions is the evolution of social insect colonies. As expected, they are composed of kin and mechanisms have evolved that prevent the intrusion of nonrelatives. Yet, there are exceptions an extreme case is the fusion of unrelated colonies. What are the advantages of fusions that have colonies with a high potential for conflict as a consequence? Here, we investigated fitness costs and benefits of colony fusions in a lower termite species, Cryptotermes secundus, in which more than 25% of all colonies in the field are fused. We found two benefits of colony fusion depending on colony size: very small colonies had an increased probability of survival when they fused, yet for most colony sizes mainly a few workers profit from colony fusions as their chance to become reproductives increased. This individual benefit was often costly for other colony members: colony growth was reduced and the current reproductives had an increased chance of dying when fusions were aggressive. Our study suggests that fusion of colonies often is the result of ‘selfish’ worker interests to become reproductives, and this might have been important for the termites' social evolution. Our results uniquely shows that selfish interests among related colony members can lead to the formation of groups with increased potential for conflict among less related members.