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, andinsect swarms is related to individual differences in hunger,body size, and defenses. These differences relate to the waythat animals balance multiple selection pressures includingfood-distribution and predator-attack patterns. However, therole of drafting/slipstreaming (taking advantage of the vorticesof those in front of you) and sex on the position of individualswithin a polarized group has not been well studied. For example,although hungry fish have been found to prefer positions towardthe front of a shoal on average, the mitigating factors of sex,recent predator exposure, and drafting have not been factoredinto this response. We conducted a controlled laboratory experimentwith individually marked whirligig beetles (Coleoptera: Gyrinidae)where sex and feeding level were controlled and the positionof beetles in a polarized group (in a flow tank) was analyzedat 2 different water speeds after exposure to a simulated predator.It was predicted that males and females would balance foragingand predator avoidance needs differently, as suggested by sexualsegregation theory and that males might be likely to occupyfront positions because of greater energetic needs. We foundthat in slow water males were more likely to occupy front positions,whereas in fast water females did, suggesting a different trade-offbetween the sexes in the need to forage versus save energy (draft).Additionally, we found that in slow water it was the hungrymales that came to the group's front, whereas hungry femaleswere more likely to move back. These are some of the first observationsof the positional complexity with which individuals in congregationsdisplay, and several adaptive and nonadaptive explanations forthe observed patterns are suggested.