Effects of personality and social context on movement tendencies in three-spined sticklebacks

Assessing social effects on individual behaviour is challenging because knowledge of an individual's social environment is often difficult to gain, especially for animals that form dynamic social groups. We here report on an experiment where the social environment could be manipulated. To this end, we capitalized on the availability of populations of migrant and resident sticklebacks, Gasterosteus aculeatus, that exhibit strong differences in movement tendencies. This allowed us to create mixed shoals with different proportions of migrants and residents, to investigate the occurrence of social effects between migrant and resident fish. In a mesocosm consisting of linearly connected ponds, we found that the overall movement patterns, as quantified by the number of pond transitions, remained consistently higher in migrants than in residents, regardless of the social group composition. The fish's intrinsic movement tendency was the main predictor of the observed variation between populations. However, at the start of the experiment, when social effects were expected to be strongest, residents were more likely to leave the starting pond in the presence of migrants than in their absence. As this increase in movement tendency was not related to the frequency of migrants in the mixed groups it cannot solely be ascribed to social conformity. Additionally, we found an effect of being part of the majority on movement: the first fish to leave the starting pond was almost always a member of the majority group (be it migrants or residents). In conclusion, we found little evidence for social modulation of movement based on differences in the populations' movement tendencies, but rather an effect of being in a majority group.