Position preferences within groups: do whirligigs select positions which balance feeding opportunities with predator avoidance?

Abstract

The benefits and costs of group living are likely to be asymmetric within a group. Animals at the edge of a group are more at risk from predators, according to the selfish herd hypothesis, but are also more likely to obtain scattered food resources. Does an animal's choice between these two conflicting positions depend on its body reserves? The hunger level of marked whirligig beetles (Coleoptera: Gyrinidae) was manipulated and the positions of individuals relative to the rest of the group on the surface of the water were determined with image analysis software. In 12 out of 13 groups, of approximately 18 beetles each, hungry beetles were closer to the edge of a group and had a higher distance to their nearest neighbor than well-fed beetles. Hungry beetles at the edge obtained nearly all of the food particles dropped onto the surface of the water. These results show that position preferences within groups may involve a dynamic feedback between foraging, predator avoidance, and shortterm hunger levels.