Elimination of External Parasites (Lice) Is the Primary Function of Grooming in Free-ranging Japanese Macaques

Grooming behavior was studied from September 1990 to May 1992 in the provisioned troop of free-ranging Japanese macaques (Macaca fuscata) at Jigokudani Monkey Park, Japan. By confiscating what macaques picked up during grooming, we found that macaques pick up lice (Pedicinus obtusus and Pedicinus eurygaster) and the eggs of lice and eat them. Slow motion video replays were used to discriminate the prey of grooming. Most (98.9%) of what Japanese macaques picked up and ate during grooming were the eggs of lice. Hence, allogrooming has an altruistic function to eliminate the external parasites on others. In particular, grooming of infants by mothers appears to be important maternal care. On the other hand, in Japanese macaques, grooming is also insect-feeding behavior, and such selfish benefit may favor grooming under individual natural selection.