Flash expansion and the repulsive herd

The selfish herd hypothesis, as proposed by Hamilton (1971, Journal of Theoretical Biology, 31, 295-311), is a powerful hypothesis to explain emergent grouping behaviour by individuals acting in their own self-interest. However, immediately after prey detect a predator, the prey group may undergo a rapid disassembly, called a flash expansion, which might be considered a 'repulsive herd'. Although flash expansion occurs in bird flocks, fish schools and insect swarms, few empirical or simulation studies have directly examined it or tested whether there are differences among its members. In addition, although flash expansion is typically thought of as a near-simultaneous movement of individuals away from the group centre, little data has been collected to verify this. We performed an empirical study to test whether the overall movement of individuals within a flash expansion is away from (1) the first individual to startle, (2) the geometric centre or (3) the point of highest density. We videotaped replicate swarms of marked whirligig beetles (Gyrinidae: Dineutes) during flash expansion and determined their trajectories. Overall, individuals moved away from the geometric centre more strongly than from the density maximum or the first to respond (starter). The geometric centre hypothesis was also supported by the lack of polarization of the group and that the bearing angle was away from the geometric centre. The starter was more likely to be a female at the edge of a group, and she moved more quickly than others and favoured the centre of the group. This is one of the first detailed examinations of flash expansion and the individual differences within it. Future empirical and simulation studies of the movement rules and emergent properties of flash expansion are needed to better understand the collective motion of other animals.