Animal Grouping For Protection From Parasites: Selfish Herd and Encounter-Dilution Effects

Grouping has been widely accepted as a mechanism for protection from predation. Just as has been apparent with predation, there is now ample evidence that parasites (biting flies, warble flies and parasitoids) can impact an animal's individual fitness. Some aspects of grouping, namely an encounter-dilution effect and the selfish herd effect, appear to apply as much to protection of animals from flying parasites as protection from predators. The encounter-dilution effect provides protection when the probability of detection of a group does not increase in proportion to an increase in group size (the encounter effect), provided that the parasites do not offset the encounter effect by attacking more members of the group (the dilution effect). The selfish herd effect provides protection from flying parasites to animals that are in the center of a group or more closely placed to other animals. Most of the quantitative evidence for the protection from flying parasites from grouping comes from studies on ungulates. Further investigation of these effects among a variety of taxa is needed for a full appreciation of the role of parasites in animal grouping and sociality.