Speed-mediated properties of schooling

Collectively moving animals often display a high degree of synchronisation and cohesive group-level formations, such as elongated schools of fish. These global patterns emerge as the result of localised rules of interactions. However, the exact relationship between speed, polarisation, neighbour positioning and group structure has produced conflicting results and is largely limited to modelling approaches. This hinders our ability to understand how information spreads between individuals, which may determine the collective functioning of groups. We tested how speed interacts with polarisation and positional composition to produce the elongation observed in moving groups of fish as well as how this impacts information flow between individuals. At the local level, we found that increases in speed led to increases in alignment and shifts from lateral to linear neighbour positioning. At the global level, these increases in linear neighbour positioning resulted in elongation of the group. Furthermore, mean pairwise transfer entropy increased with speed and alignment, implying an adaptive value to forming faster, more polarised and linear groups. Ultimately, this research provides vital insight into the mechanisms underlying elongation of moving animal groups and highlights the functional significance of cohesive and coordinated movement.