Social environment affects inhibitory control via developmental plasticity in a fish

Living in a social group may impose cognitive demands, for example individual recognition, social memory and the inhibition of behaviour when it is not adaptive. As the neural substrates for these cognitive skills are metabolically expensive, cognitive abilities may be positively related to the complexity of the social system. Where there is large spatiotemporal variation in the ecological conditions experienced and hence in the social system exhibited by species, selection may favour adaptive phenotypic plasticity of cognitive abilities involved in social tasks rather than evolved differences across populations. Here, we tested this hypothesis in a social-living teleost fish, the guppy, Poecilia reticulata. We exposed new-born guppies to treatments that altered two parameters of social environmental complexity: group size (experiment 1) and group stability (experiment 2). Then, we assessed guppies' inhibitory control, the ability to withhold responding to a stimulus, a cognitive function that is critically involved in social interactions. In experiment 1, guppies reared alone showed higher levels of behavioural inhibition in a foraging task compared to guppies reared in pairs or in groups of six. In addition, we found that individuals' variance in performance was smaller for fish raised alone. In experiment 2, guppies reared in a stable social group showed greater inhibition than those from groups subjected to frequent fission–fusion events. These results reveal phenotypic plasticity of inhibitory control in guppies; however, contrary to prediction, they indicate greater inhibitory abilities developing in individuals exposed to ‘simpler’ social environments.