In Japan, adolescent snow monkeys are frequently engaging in sexual activities with sika deer in what may be an entirely new type of behaviour, according to researchers.
A new study, published in the Archives of Sexual Behaviour, further investigates the phenomenon following a report earlier this year of a male snow monkey – otherwise known as the Japanese macaque – mounting a female deer on Yakushima Island - which lies south of the mainland.
After observing adolescent females interacting with deer at Minoo Park near Osaka, researchers are now fairly certain that the behaviour is sexual in nature.
"We observed multiple occurrences of free-ranging adolescent female Japanese macaques performing mounts and sexual solicitations toward sika deer," they said.
Sexual liaisons between distantly related species are extremely rare, with the only other documented example being the sexual harassment of king penguins by Antarctic fur seals.
Noëlle Gunst-Leca, co-author of the study from the University of Lethbridge in Canada, told the Guardian it was not clear if the behaviour recorded from the report earlier in the year was sexual in nature. "The sexual nature of this interaction was not clearly demonstrated," she said.
So Gunst-Leca and her team began investigating a different set of relationships – between adolescent female snow monkeys and sika deer. Her team recorded 12 successful interactions, which were sexual in nature, between November 2012 and 2013, and a further 13 between November 2014 and January 2015.
The monkeys were seen climbing onto the backs of deer, thrusting their pelvises and rubbing their genitals on them. They were also observed having body spasms, screaming and making high pitch calls while engaging in the behaviour – in a similar manner to when they have sexual interactions with other monkeys.
Most of the deer that the monkeys were seen cavorting with were adult males, although the researchers also recorded encounters with two female deer and three young males.
Because no previous monkey-deer interactions have been observed at Minoo before, the researchers think it could be the beginning of a new custom, which the female monkeys learn by watching others take part.
A possible explanation for this behaviour could be that the adolescent females in question are rejected by adult males during the mating season so they take part in sexual behaviours with other females or deer.
Another possibility could be that they the females are simply practising sex in safety, rather than risking an interaction with aggressive male monkeys.
The researchers also suggest that, because snow monkeys are known to ride sika deer for non-sexual reasons, the adolescents may have simply acquired a liking for the genital stimulation.
Future studies will need to be conducted at the site to determine whether the phenomenon is a short-lived fad or the beginning of something more long-term.