Distinct decision-making properties underlying the species specificity of group formation of flies.

Many animal species form groups. Group characteristics differ between species, suggesting that the decision-making of individuals for grouping varies across species. However, the actual decision-making properties that lead to interspecific differences in group characteristics remain unclear. Here, we compared the group formation processes of two Drosophilinae fly species, and , which form dense and sparse groups, respectively. A high-throughput tracking system revealed that flies formed groups faster than flies, and the probability of remaining in groups was far higher than that of . flies joined groups even when the group size was small, whereas flies joined groups only when the group size was sufficiently large. flies attenuated their walking speed when the inter-individual distance between flies became small, whereas such behavioural properties were not clearly observed in . Furthermore, depriving flies of visual input affected grouping behaviours, resulting in a severe reduction in group formation. These findings show that decision-making regarding grouping, which greatly depends on vision, is significantly different from , leading to species-specific group formation properties.