Whirligig beetle aggregations: what are the costs and the benefits?

Abstract

Laboratory experiments investigated the benefits and costs of aggregation formation in the whirligig beetles, Gyrinus marinus and G. substriatus (Coleoptera: Gyrinidae). Different sized groups of whirligigs were exposed to fish predators, and capture rate per group and per individual were estimated. Attack rate per group increased with group size, suggesting that these aggregations behave as selfish herds. In another series of experiments in which whirligigs were exposed to top- and side-attacking predators, large groups were found to detect predators when they were further away than small groups. Video analysis of these groups showed that beetles tended to increase their swimming speed after exposure to a predator but did not show an increased tendency to circle. In natural aggregations, filmed in the field, swimming speed was found to be related to group size. Body condition of field collected beetles, as estimated from the regression of body mass on body length, was not found to be related to group size, implying that large groups do not suffer reduced condition.