According to researchers from the University of Strasbourg, the monkey was displaying an example of interspecies sexual behavior, the first seen between the two species.
The incident occurred on Yakushima Island, south of Japan, in November 2015, during macaque breeding season, and it’s now the subject of a study published in the journal Primates. A male Japanese macaque has been caught trying to mate with a female Sika deer.
The scientists are quick to note there was no actual penetration, as the monkey’s attention was directed at the deer’s back and not her genital area, something the researchers guess can be explained by the different sizes and builds of the animals.
In total, the monkey – one low in his troop’s social hierarchy, say the scientists – was observed trying to mount two female deer, neither of which seemed interested in reciprocating. One of his objects of desire tried to ditch the monkey, as can be seen in the video below. That did not stop the primate from displaying so-called mate-guarding behaviour, chasing other males of his troop away.
The two creatures live equitably alongside each other in Japan. The deer eat food the monkeys drop from trees and even eat the primates’ waste. Meanwhile, the macaques have been known to groom the deer and even ride them, in a manner more playful than the one witnessed here.
The researchers have not been able to learn the exact cause of the event.
In the end, it may have been a lack of ladies that saw the monkey take his chances with another species. “The most realistic hypothesis would be that of mate deprivation,” the scientists wrote, “which states that males with limited access to females are more likely to display this behaviour.”