Are animals in herds selfish?

What draws animals together in schools, flocks, or herds? One possibility is shared vigilance; perhaps all animals in the group gain mutual benefit from having others in the group spend time watching for predators. On the other hand, an animal can simply take advantage of another's efforts without returning any effort of its own. If one bird is looking for food, then others may gather around, ready to fight for it. Or, there may be an advantage to being in a group when a predator approaches; the flurry of motion when a flock takes wing may make it much more difficult for the predator to catch any one animal. Not all positions within the flock are equally protected, so the animals may selfishly compete for position within the group.
The trailing fish in this school may be more vulnerable to predation; if the predator focusses on the last fish in the group, the others will scatter forward and to the sides. In this scenario, the scattering fish do not interfere with the predator's visual fix on the last fish, making the chase easier for the predator.

Flock of juvenile herring gulls, Oregon coast

Herring gulls, Larus argentatus, flock at the seashore. Juveniles with dull brown coloration are noticably different from adult; adults of both sexes are mostly white, with grey/brown wings and darker coloration on their tails (see picture of adult, below). Juveniles are segregated from the adults, which live in much less cohesive flocks. Gulls within flocks watch each other and intensely compete for food items, such as crabs. Using other individiduals as cues for locating food is a key "selfish" element in holding the flock together. Also, by staying near others, the gulls can observe their responses to approaching theats, such as hawks or foxes, and can take advantage of the visual confusion of many birds taking simultaneously taking flight it a predator comes near.

Adult herring gull
A school of marine fish. These will scatter if a predator darts in their midst, creating visual confusion. Some positions in the school, particularly the rear, are more vulnerable than others, creating the possibility of competition for locations within the school.

Elgar, M. A. 1986. House sparrows establish foraging flocks by giving chirrup calls if the resources are divisible. Anim. Behav. 34:169-174.

Hamilton, W. D. 1971. Geometry for the selfish herd. J. theor. Biol. 31:295-311.

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copyright ©2001, 2002 Michael D. Breed, all rights reserved