Experiment of the month

Natural History, Oct, 2002 by Stephan Reebs

When attacked by a predator, a group of animals often becomes more compact as each individual tries to reach the relative safety of the center. Well documented in vertebrates, this so-called selfish herd effect has now been reported for an invertebrate, the fiddler crab (Uca pugilator). Equipped with a video camera, Steven V. Viscido and David S. Wethey, of the University of South Carolina, ventured onto the mudflats of a small tidal, creek to measure the cohesion of crab flocks before and after an attack. They mapped the position of individual crabs relative to a reference grid system made of white golf balls placed strategically in the dark mud. When a natural predator (a clapper rail) or a simulated one (a person) ran toward a flock, the crabs scurried about but always ended up forming a tighter aggregation. So strong was the pull of the center, with its "promise" of greater safety, that the crabs farthest away from the predator also rushed to join the group, even though this meant moving toward the source of danger. Obviously, for wary crabs, living on the edge is not considered a viable option. ("Quantitative analysis of fiddler crab flock movement: evidence for `selfish herd' behaviour," Animal Behaviour 63, 2002)

Stephan Reebs is a professor of biology at the Universite de Moncton in New Brunswick, Canada, and the author of Fish Behavior in the Aquarium and in the Wild (Cornell University Press).