Strong personalities, not social niches, drive individual differences in social behaviours in sticklebacks.

Understanding the mechanisms responsible for consistent individual differences in behaviour is a recent challenge for behavioural ecology. Although theory is rapidly developing in this area, there are few empirical tests. There are at least two hypotheses to explain why individuals behave differently from one another in a dynamic social environment. The social niche specialization hypothesis proposes that repeated social interactions generate consistent individual differences in social behaviour. The behavioural type hypothesis proposes that an individual's social behaviour reflects its behavioural type. We tested these two hypotheses by manipulating the opportunity for repeated social interactions in groups of three spine stickleback, Gasterosteus aculeatus, and by measuring the behavioural types of the same individuals in three contexts: when in a novel environment, when presented with an opportunity to associate with conspecifics and when confronted by an intruder. We found no evidence that repeated social interactions increased between-individual variation in social foraging behaviour. Instead, individuals' social foraging behaviour was related to their behavioural type, specifically their shoaling behaviour. In addition, the behavioural types of the members of a group strongly influenced a group's average foraging behaviour. Together, these results do not support the hypothesis that social dynamics within groups generates individual differences in behaviour. Instead, they suggest the reverse: individual differences in behaviour drive group-level dynamics.