Predator-driven fragmentation of fiddler crab droves into selfish miniherds of biased composition

An explanation for animal groups is the selfish herd, characterized by aggregation as each member tries to shield itself from a predator by moving into a tight gap between other members. We test the hypotheses that: (1) droves, the large feeding groups of fiddler crabs, are selfish herds; (2) the miniherds that form when droves fragment on approach of a large predator are selfish herds; (3) selfish herds form when refugia are unlikely to be reached before an approaching predator arrives; and (4) the composition of selfish miniherds is biased toward individuals most vulnerable to predation. The study was conducted in South Carolina (USA) by videotaping the movements of sand fiddler crabs Uca pugilator when approached by a human predator. In both droves and miniherds, interindividual distance decreases with predator approach, consistent with behavior in a selfish herd. However, two other expectations for selfish herds—herd cohesion and sacrificing distance from the predator in order to get closer to other herders—are only met in miniherds. Crabs farther from refugia are more likely to form and remain in miniherds, indicating that selfish herding is only favored when refugia cannot be quickly reached. The composition of the smallest miniherds, consisting of 2-18 crabs, is biased toward females and small males. These individuals may be more vulnerable to predation because they lack the enlarged claw of large males that deters some predators. The small miniherds are relatively homogeneous with respect to the size and sex of their members, which may enhance cohesion and effectiveness as selfish herds. Miniherds will be effective selfish groups when predator attack has a significant vertical component and when the strike distance is large relative to both the size of the prey and the distance between group members. Droves are not selfish herds but permit crabs to flee feeding grounds as members of selfish miniherds.