My sources at Harper's Magazine inform me that "A family of Ugandan mountain gorillas (Gorilla beringei beringei) groomed an American in Bwindi Impenetrable Forest." It seems quite a gentle moment between the gorillas and the tourist:
As you can see from his photo, the fellow did not appear to be in pressing need of such supererogatory grooming, looking much like any American tourist abroad and frankly better than most . . .
. . . so I can only surmise that the act was an aesthetic or stylistic comment on the part of the gorillas. When it was over, the grateful subject emerged with a new look:
After the gorillas departed the encampment, the tourist explained, "When a 900-lb. gorilla decides his kids are going to groom you, then you pretty much have to go with his sense of what suits."