Unlikely couple: Liaison between a Sika deer and a Japanese snow monkey

Japanese monkeys (Macaca fuscata yakui) are regularly photographed bathing in hot springs in snow-covered parts of their native Japan. They comfortably live side by side with Sika deer (Cervus nippon yakushimae). The deer often eat food that the monkeys drop from trees, and are also known to feed on the macaques' faeces. Some macaques have been seen grooming the deer or riding them in a playful manner.

The sexual behavior in question was noted in early November 2015 by co-author Alexandre Bonnefoy while filming on Yakushima Island south of Japan during the macaque breeding season. Bonnefoy observed one healthy adult male macaque attempt to copulate with at least two different female Sika deer by performing sexual mounts on the animals' backsides.

"This macaque was a non-troop adult male, in other words low in hierarchy. He was therefore probably either peripheral or belonged to a group of peripheral males, as other males were observed in the vicinity of the deer," says Pelé.

"The male mounted the deer and displayed some copulation behaviors, which included about 15 sexual movements over a period of 10 seconds, before dismounting," said Bonnefoy, explaining what he saw. "Ejaculation seemed to have occurred as the deer licked the seminal liquid after the mount. This might indicate that the sperm could be a good source of protein to the deer."

It is important to note that penetration did not occur as the penis of the macaque was directed at the back and not the genital area of the deer. The researchers guess that it might be explained by the difference in the build and size of the animals involved.

Bonnefoy also saw the male try to mount a second female deer without any success. She attempted to escape, and tried to get rid of the monkey by moving faster, turning around and displaying threatening behavior. The monkey went on to display a form of mate-guarding behavior in that he chased other peripheral males away from the deer under his watch. However, he was never aggressive towards the deer he followed or rode on.

The researchers believe the hormonal surge experienced by the Japanese macaques during breeding season and the close cooperation between these primates and Sika deer culminated in this copulation behavior between two unrelated species.

"It could also be a sexual manifestation of the known play behavior between Japanese macaques and the deer they are known to sometimes ride," says Pelé.