Familiarity dampens the effect of boldness on coordination in three-spined sticklebacks

Group-living animals must coordinate with other group members to reap the benefits and avoid the costs of social living, but precisely how individuals should respond to one another is mediated by interacting social and environmental factors. In stable groups, familiarity (i.e. prior experience with particular conspecifics) often improves group coordination, with familiar groups outperforming unfamiliar groups in foraging efficiency and predator evasion. Group coordination is also affected by individual behavioural tendencies, and consistent individual differences such as boldness drive specific coordination behaviours including leadership. Furthermore, in stable groups these consistent differences in behaviour (or ‘personalities’) facilitate individuals predicting each other’s behaviour as they can learn their groupmates’ behavioural tendencies over time. Few studies have investigated how familiarity and personality interact to influence behaviour in social groups. Here, we assess how familiarity differentially affects ‘bold’ and ‘shy’ individuals in pairs of three-spined sticklebacks ( Gasterosteus aculeatus ). Our results show that familiarity modifies how individuals coordinate with their partners, and that leadership dynamics are determined by boldness levels in unfamiliar pairs, but not in familiar pairs. This suggests that familiarity with others in social groups alters the leadership tendencies of individuals, with implications for how personality interacts with the social environment to determine individual coordination behaviour.