Emergence of variation between groups through time in fish shoal collective motion

Despite extensive interest in the dynamic interactions between individuals that drive collective motion in animal groups, the dynamics of collective motion over longer time frames are understudied. Using three-spined sticklebacks, Gasterosteus aculateus, randomly assigned to twelve shoals of eight fish, we tested how six key traits of collective motion changed over shorter (within trials) and longer (between days) timescales under controlled laboratory conditions. Over both timescales, groups became less social with reduced cohesion, polarisation, group speed and information transfer. There was consistent inter-group variation (i.e. collective personality variation) for all collective motion parameters, but groups also differed in how their collective motion changed over days in their cohesion, polarisation, group speed and information transfer. This magnified differences between groups, suggesting that over time the typical collective motion cannot be easily characterised. The minimal sample size of independent groups and their divergence over time need to be considered in future studies.