Researchers saw the primate attempting to mate with at least two deer in November 2015, during the macaque breeding season.
The macaques regularly bathe in hot springs in snow-covered parts of Japan, and live side-by-side with the deer.
The macaques have previously been observed grooming the deer or riding them in a playful manner.
The curious behaviour is outlined in a study published in the journal Primates.
The monkey seemed to be a low-status male, who guarded his „love interests“ by chasing away other male monkeys who came near.
Co-author Alexandre Bonnefoy commented: „The male mounted the deer and displayed some copulation behaviours, which included about 15 sexual movements over a period of 10 seconds, before dismounting. “
It’s not the first example of inter-species mating. In 2014, reports of an Antarctic fur seal coercing king penguins into sexual relations made headlines worldwide.