WHIRLIGIG BEETLE AGGREGATIONS - WHAT ARE THE COSTS AND THE BENEFITS

Laboratory experiments investigated the benefits and costs of aggregat ion formation in the whirligig beetles, Gyrinus marinus and G. substri atus (Coleoptera: Gyrinidae). Different sized groups of whirligigs wer e exposed to fish predators, and capture rate per group and per indivi dual 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 sid e-attacking predators, large groups were found to detect predators whe n they were further away than small groups. Video analysis of these gr oups showed that beetles tended to increase their swimming speed after exposure to a predator but did not show an increased tendency to circ le. In natural aggregations, filmed in the field, swimming speed was f ound to be related to group size. Body condition of field collected be etles, as estimated from the regression of body mass on body length, w as not found to be related to group size, implying that large groups d o not suffer reduced condition.