Scientists may have caught two animals from completely different species having consensual sex for the first time.

A new paper, titled ‘Interspecies sexual behaviour between a male Japanese macaque and female sika deer’, describes what is thought to be one of the first ever recorded instances of “reproductive interference” between two very different animals.

Sex between animals of different kinds has been reported across a wide range of the animal kingdom.

But those tend to be seen only between animals that are closely related and look similar, and are understood in the context of their relationship to how they allow the animals’ species to survive. Those reports of very different animals are usually seen between animals that are born and bred in captivity.

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Most of those examples also constitute a kind of sexual harassment. For example, scientists had previously observed Antarctic fur seals harassing king penguins.

But a new paper reports mating behaviour between two wild animals – a male Japanese macaque and a female sika deer in Japan. In this case there appeared to be no coercion and both of the animals appears to behave as if the approach was consensual.

Japanese macaques have in the past been known to ride along on deer. But the researchers say that the monkey in the new study “showed clear sexual behaviour towards several female deer”. Some of those deer tried to escape – but others apparently consented to the behaviour and “accepted the mount”, the researchers write in the journal Primates.

The researchers wrote that one deer "seemed to accept to be ridden by the male macaque", and that it was apparently licking sperm that had been deposited on its back by the monkey. Another deer refused the mount and threw the macaque off its back.

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The scientists say that the most likely explanation for the strange behaviour is “mate deprivation”, a theory that suggests that animals that don’t have access to females are more likely to show such behaviour.

That may have been encouraged by the fact that the two animals already play together and co-operate, and that the macaques were entering breeding season, the scientists speculated. As such, it might just be a sexual manifestation of the play already seen.

Other explanations include the argument that the behaviour is a way of learning to copulate, the scientists said, but that is unlikely given that they are social animals and can learn from one another. It’s also unlikely that they would have sex with them because they were short of a mate – since in that case they tend either to masturbate or show homosexual behaviour.

Usually, reproductive interference is thought to be the result of one animal not properly recognising what species another animal is. But that also explains why it mostly happens in animals that are closely related and look similar – and so the explanation doesn’t apply in the new case.