In only the second recorded instance of interspecies sex, a male snow monkey has been filmed attempting to have intercourse with female sika deer on Yukushima Island in Japan, with researchers citing mate deprivation as the most likely reason for the unusual behavior.

The two animals typically have a symbiotic partnership. The deer eat fruits dropped from trees by the monkeys, while the latter groom the former and ride on their backs at times.

Likely A Deprived Monkey

In a new study discussed in the journal Primates, researchers described a low-hierarchy male monkey repeatedly imposing himself on at least two female deer, with one seen to get away while another appearing to stay put and lick her side as the monkey got off.

“No ambiguity is possible, it is clearly sexual behavior,” said lead study author Marie Pelé of the University of Strasbourg in France, who added that the monkey can be witnessed guarding his deer target and chasing away other male monkeys who attempted to come near.

The rare event is likely a result of “mate deprivation,” the team said. They had also already considered the likelihood that the male only began to learn to copulate and failed to recognize that the deer was of a different species.

“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,” the researchers wrote.

Mixed-species interaction exists for different reasons, including heightened vigilance against predators.

“Whatever the reason for this particular event, the observation of highly unusual animal behavior could be a key element in understanding the evolution of interspecific mating behavior in the animal kingdom, and specifically in understanding zoophilia in humans,” the team added.

Heating Up The Sub-Antarctic Climes

The only other documented case of interspecies sexual interaction was one between an Antarctic fur seal and the king penguins it harassed back in 2014. More than one fur seal was caught in the act on more than one occasion, with one even seen to consume a bird after copulation.

In 2006, the researchers detailing their findings in the journal Polar Biology saw, for the first time, a fur seal commit such sexual advances on a king penguin on the sub-Antarctic Marion Island, which served as a home to both animals.

In their research, the team speculated that the unusual act could be the reflection of a sexually inexperienced and frustrated seal’s behavior, a show of aggression or predation, or simply animal play that became sexual.

The sexual coercion was displayed on four more occasions on Goodhope Bay, Funk beach and Trypot beach. All incidents followed a similar patent of seal chasing, and then capturing and mounting the penguin. There were also multiple attempts lasting around five minutes each.

Biologists have also long wondered why animals commonly engage in sexual behavior with others of the same sex despite the act not leading to actual offspring production and continuation of the species. A 2016 Swedish study, where male beetles were selectively bred for same-sex sexual behavior, found that the gene encouraging this behavior can actually offer evolutionary benefit to the opposite sex.

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