A video showing adolescent female monkeys in Japan repeatedly climbing onto the backs of sika deer and engaging in sexual behaviors were being shared widely in the internet recently. Since then, several reports explaining this weird animal behavior were released.

Researchers decided to study this heterospecific sexual behavior between a primate and a non- primate species. Their findings published earlier in December in peer-reviewed journal Archives of Sexual Behavior said these interactions were definitely sexual in nature.

The footage captured in Minoo, Japan, showed female macaques’ jumping onto the backs of large sika deer and thrusting their pelvis in a clearly sexual motion. They were also seen biting, scratching and pulling on the antlers of deer.

The female monkeys were seen preferring male bucks. The study could not say if this was because female deer and juveniles would buck them off, or because these females actually had a sexual preference for stags.

The study said five of the female monkeys observed had "successful heterospecific consortships" with deer. That is, they had "a temporary, but exclusive, sexual association" with an individual from another species, with "three or more mounts within a 10-minute period." There were 13 successful pairings, and 258 separate instances of mounting documented by the team.

In January, a study was published, which reviewed the footage of a male Japanese macaque (Macaca fuscata yakui) and female sika deer (Cervus nippon yakushimae) on Yakushima Island in Japan.

The study said Japanese macaques were known to ride deer but the individual under scrutiny showed clearly sexual behavior towards several female deer. While some deer tried to escape others, accepted the mount.

Monkeys are often seen sitting on sika deer backs and riding along as they graze and the deer don’t shrug them off because of the grooming and food droppings they got, forming a kind of symbiotic relationship. But this was the first time sexual behavior between these two species was documented.

“Although this phenomenon may be explained as copulation learning, this is highly unlikely. The most realistic hypothesis would be that of mate deprivation, which states that males with limited access to females are more likely to display this behavior,” said the study published in journal Primates on male macaque mounting a female sika deer on Yakushima Island.

Sexual interactions between animals of different species are not unheard of. But it’s often similar, related species, and the mating is explained by an identity mix-up.

The study said "this interpretation is implausible in our case because the anatomical differences between Japanese macaques and sika deer are profound."

Scientists had five theories about why young monkeys sought out sexual relationships with deer, which include a practice for future sexual encounters with other primates. The second theory was the “safe sex” theory, which proposed female macaques might find deer less dangerous than male macaques to have sexual interaction.

These young macaques could also be sexually frustrated due to the lack of mating partners, which could have led to this kind of behavior. This is what the researchers call the "best of a bad job" hypothesis.

Female macaques are often shunned by males in the group. They are left increasingly frustrated as they can go their entire adolescent life searching for a mate. The best of a bad situation route taken by the female macaques of the group shows how sexual frustration can drive even animals to behave abnormally.

Due to the danger of aggression, general lack of partners and a host of other reasons, the team said there could be a simpler explanation to this monkey madness. Since the lives of the two species are intertwined in the jungles of Japan, normal behavior is already out of questions. These monkeys ride the deer as they graze, much like Caesar riding a horse in movie “Rise of the Planet of the Apes.”

Accidental discovery and learning from other monkeys who discovered this while riding deer could be a very plausible explanation too, similar to monkey masturbation.