Relative telencephalon size does not affect collective motion in the guppy (Poecilia reticulata)

Collective motion is common across all animal taxa, from swarming insects to schools of fish. Collective motion requires intricate behavioural integration among individuals, yet little is known about how evolutionary changes in brain morphology influence the ability for individuals to coordinate behaviour in groups. In this study, we utilized guppies that were selectively bred for relative telencephalon size, an aspect of brain morphology that is normally associated with advanced cognitive functions, to examine its role in collective motion using an open field assay. We analyzed high-resolution tracking data of same-sex shoals consisting of eight individuals to assess different aspects of collective motion such as alignment, attraction to nearby shoal members, and swimming speed. Our findings indicate that collective motion in guppy shoals is not influenced by relative telencephalon size. Our study suggests that group dynamics in collectively moving animals are likely not driven by advanced cognitive functions but rather by fundamental cognitive processes stemming from relatively simple rules among neighboring individuals.