On the Japanese island of Yakushima, a young male snow monkey (Macaca fuscata) repeatedly tried to mate with female sikas, a small local deer (Cervus nippon). The strange behavior was observed in 2015 and is described in an article published in the scientific journal Primates.
Stay away. The macaque tried sexual approaches with two different females, and the only contact was with the animal's back. In one case, the deer shook the monkey from the back; in the other, the female did not seem to oppose. The male was also seen to "defend" the potential partner from other macaques.One of the attempts to mate the Japanese macaque with a female sika deer. | Alexandre Bonnefoy - Éditions Issekinicho
We remain friends. These two species generally have a collaborative relationship. The deer feed on the fruits that the macaques drop from the trees, and occasionally tolerate having them on the back for grooming. The filmed approaches could be an evolution of this behavior on the part of a male who is still inexperienced, marginalized from the rest of the group precisely in the mating season and struck by the lack of partners.
Not the first case. Just the deprivation of companions would be the most probable reason of the attempts of the primate, that however, for inexperience, could also have been wrong "target". Other examples of interspecies sex have been filmed, in the past, between sea lions and penguins (we had written here), between orphan elephants and rhinos, and between a chimpanzee and a toad.