Social scratch: Another custom in wild chimpanzees?

Chimpanzees (Pan troglodytes schweinfurthii) in the Mahale Mountains National Park, Tanzania, scratch other individuals' bodies while they groom them. This behavioral pattern of 'social scratch' is another example of locality-specific social behavior, or custom, as it is not found in the Gombe National Park, Tanzania, about 150 km north of Mahale, nor has it been reported from any other sites of chimpanzee study. Frequency of social scratch was correlated with frequency of social grooming, but not with frequency of self-scratch. Frequencies of social scratch per grooming bout among adult and adolescent males, and from lactating females to infants or juveniles, were high, and among males, higher-ranking males especially received more. These facts indicate some social function of the behavior. Social scratch was directed mostly to the dorsal side of the body. However, when lactating females social scratched to infants or juveniles, they scratched other body parts. Social scratch was not lateralized to left or right. We present four hypotheses on the functional origin and on the learning process of this cultural behavioral pattern.