Sex and the selfish herd: sexual segregation within nonmating whirligig groups

The fitness costs and benefits at different positions in fish shoals, bird flocks, and insect swarms can be asymmetric; a group's edge may provide more feeding opportunities, but also greater predator risk. Animals make trade-offs between these selection pressures based on individual differences in traits including satiation level, ability to avoid predators, and sex. Previous studies did not evaluate the impact of sex on group positioning in these types of nonhierarchical, nonmating groups called congregations. A controlled laboratory experiment was conducted, using marked whirligig beetles (Coleoptera: Gyrinidae), to test for sexual segregation and why different sexes might choose different positions. Soon after a disturbance, males often were found at the periphery and females at the center of groups. There was also an overlying influence of feeding on position; satiated individuals moved toward the center and hungry individuals toward the periphery. Several minutes after a disturbance, sexual segregation disappeared, but segregation due to hunger persisted. Sexual segregation in this study was best explained by the predator avoidance hypothesis, not the energy needs hypothesis. Females weighed less than males; this may make them more at risk to predation because of reduced swimming speed or less mechanical protection from their exoskeleton. No difference between the sexes was found in the volume of their defensive chemicals. This is one of the first studies to show that sex influences position of individuals within simple nonmating groups (congregations) and suggests that more attention should be given to positional sex differences within shoals, flocks, herds, and swarms. Copyright 2007, Oxford University Press.