Assortative interactions and the evolution of cooperation during predator inspection in guppies (Poecilia reticulata)

One possible mechanism for the evolution of cooperation/altruism is assorta tive interactions, in which cooperators interact with each other by choice and cheaters are forced to interact with each other by default. This mechan ism has been regarded as implausible in the past, but more recent models ha ve revealed its likelihood when cooperative behaviour is a quantitative tra it that can be observed directly by members of the population. Assortative interactions are likely to occur in guppies (Poecilia reticulata) because t hey cooperate in the context of predator inspection, their inspection behav iour is a quantitative trait, and they are known to monitor each other's be haviour and choose to associate with fish that inspect. Despite these preco nditions, our experiments failed to demonstrate assortative interactions in free-swimming guppies. We offer our negative evidence as an incentive for others to look for what remains a theoretically plausible mechanism for the evolution of cooperation/altruism.