Nonadditive effects of group membership can lead to additive group phenotypes for anti-predator behaviour of guppies, Poecilia reticulata.

Nonadditive effects of group membership are generated when individuals respond differently to the same social environment and may alter predictions about how behavioural evolution will occur. Despite this importance, the relationship between an individual's behaviour in two different social contexts and how reciprocal interactions among individuals within groups influence group behaviour are poorly understood. Guppy anti-predator behaviour can be used to explore how individuals behaviourally respond to changes in social context. Individuals from two strains were tested for response to a model predator alone and in groups to evaluate how individuals alter their behaviour in response to social context and how group phenotype relates to individual behaviour. Nonadditive effects of group membership were detected for a number of behaviours, revealing that the effect of being in a group differed among individuals. These nonadditive effects, however, yielded an additive group phenotype. That is, the average behaviour of the group was equal to the average of its parts, for all behaviours in both strains. Such an additive group phenotype may have resulted because all individuals within a group respond to the specific social environment provided by the other members of their group.