Optimal group positioning after a predator attack: the influence of speed, sex, and satiation within mobile whirligig swarms

The position of animals within fish shoals, bird flocks, and insect swarms is related to individual differences in hunger, body size, and defenses. These differences relate to the way that animals balance multiple selection pressures including food-distribution and predator-attack patterns. However, the role of drafting/slipstreaming (taking advantage of the vortices of those in front of you) and sex on the position of individuals within a polarized group has not been well studied. For example, although hungry fish have been found to prefer positions toward the front of a shoal on average, the mitigating factors of sex, recent predator exposure, and drafting have not been factored into this response. We conducted a controlled laboratory experiment with individually marked whirligig beetles (Coleoptera: Gyrinidae) where sex and feeding level were controlled and the position of beetles in a polarized group (in a flow tank) was analyzed at 2 different water speeds after exposure to a simulated predator. It was predicted that males and females would balance foraging and predator avoidance needs differently, as suggested by sexual segregation theory and that males might be likely to occupy front positions because of greater energetic needs. We found that in slow water males were more likely to occupy front positions, whereas in fast water females did, suggesting a different trade-off between the sexes in the need to forage versus save energy (draft). Additionally, we found that in slow water it was the hungry males that came to the group's front, whereas hungry females were more likely to move back. These are some of the first observations of the positional complexity with which individuals in congregations display, and several adaptive and nonadaptive explanations for the observed patterns are suggested.