geometry for the selfish herd

sciencemag | To escape a hungry wolf, a sheep doesn't have to outrun the wolf, just the other sheep in its flock. Many researchers think that such selfish behavior, not cooperation for the benefit of the whole crowd, shapes the movements of groups of animals. But the decades-old "selfish herd theory" has been hard to back up with data. Now, a detailed analysis of how a flock of sheep moves to avoid a sheepdog has found that the theory holds true. Each sheep heads to safety in the center of the flock, rather than running directly away from the dog.

"It's really difficult to measure 2D spatial information on large animals in the wild," says biologist Theodore Stankowich of the University of Massachusetts, Amherst, who was not involved in the new work. "They've taken advantage of a unique opportunity to work with the sheep to answer these types of questions in a controlled environment."

Studies on seals, crabs, and pigeons have shown that those animals seem to herd for selfish reasons, but the data have often been crude. Biologist Andrew King and colleagues at the Royal Veterinary College of the University of London attached GPS backpacks to 46 sheep and to a trained Australian Kelpie dog. When they released the dog to herd the sheep, they recorded the location of each animal every second. Then, they analyzed the data to determine what factors influenced each sheep's path. The movements of the sheep, the researchers reveal today online in Current Biology, could be best predicted by the center of the flock. Rather than run in a line away from the dog, scatter in all directions, or follow their nearest neighbors, the sheep all hurried toward the flock's center. The sheep began to converge when the dog was 70 meters away. Even as the flock as a whole moved, each sheep continuously competed to be as near the middle as possible.

"The fact that they're running toward the center reduces the chances of their being on the edge and being picked off by a predator," says King. It's a selfish behavior since each sheep puts the animals at the fringes of the flock at risk in order to save itself.