Japanese macaques and sika deer live comfortably together on Japan’s Yakushima Island: The deer eat fruit the monkeys drop from the trees, and the monkeys groom and sometimes hitch a ride on the deer. But a couple years ago, one of the macaques took this relationship to a new level. Unable to get a mate of his own kind, this low-ranking snow monkey used the deer’s back for his pleasure .
He did not penetrate her, but did ejaculate, and the deer then licked her back clean, researchers report in the current issue of Primates. The monkey was later seen attempting to mount another deer, but she objected and threatened him. He also guarded his unlikely love interests, chasing away any other male monkeys who came near. Scientists have only reported one other case of sexual relations in the wild between unrelated species. That one involved male Antarctic fur seals coercing king penguins; one, after sating his lust, ate the bird. In both cases, scientists suspect that the males were unable to acquire a mate of their own kind, and seasonal hormonal surges led them to seek love elsewhere.