The role of social attraction and its link with boldness in the collective movements of three-spined sticklebacks

Abstract

Social animals must time and coordinate their behaviour to ensure the benefits of grouping, resulting in collective movements and the potential emergence of leaders and followers. However, individuals often differ consistently from one another in how they cope with their environment, a phenomenon known as animal personality, which may affect how individuals use coordination rules and requiring them to compromise. Here we tracked the movements of pairs of three-spined sticklebacks, Gasterosteus aculeatus, separated by a transparent partition that allowed them to observe and interact with one another in a context containing cover. Individuals differed consistently in their tendency to approach their partner's compartment during collective movements. The strength of this social attraction was positively correlated with the behavioural coordination between members of a pair but was negatively correlated with an individual's tendency to lead. Social attraction may form part of a broader behavioural syndrome as it was predicted by the boldness of an individual, measured in isolation prior to the observation of pairs, and by the boldness of the partner. We found that bolder fish, and those paired with bolder partners, tended to approach their partner's compartment less closely. These findings provide important insights into the mechanisms that govern the dynamics and functioning of social groups and the emergence and maintenance of consistent behavioural differences.

Keywords: animal personality; behavioural syndrome; boldness; collective behaviour; coordination; leadership; sociability; social attraction; three-spined stickleback.

Figure 1

(a) Schematic overview of the experimental tank with a transparent partition, with all movement trajectories (black lines) of a representative pair of fish when not in the cover area (15 cm length) during the first day of the pairing stage. (b) Distance to the partner's lane (mm) for focal fish bolder and shyer than their partner, during the isolation (N = 19; N = 19) and pairing stages (N = 14; N = 15). ** P < 0.01; *** P < 0.005.

Figure 2

Distance of focal fish to their partner's lane (mm) in relation to (a) their own boldness and (b) the boldness of their partner during collective movements during the pairing stage. Boldness scores are square-root transformed.

Figure 3

Plot showing the distance of focal fish to the lane of their partner (mm) during the isolation stage when out of cover, and during the pairing stage, separate for each of four possible states in which focal fish could be out of cover: when focal fish initiated trips later joined by their partner (initiations), when focal fish and their partner were out together (collective movements), when focal fish were still out of cover after a collective trip but their partner was already back under cover (returns) and focal fish going out and back under cover during solo trips. Average scores are shown with SEs. The distance of focal fish to their partner's lane during collective movements is depicted in white.