Sexual interactions between distant related species are very rare, with the only known cases being the sexual assaults of king penguins by Atlantic fur seals.
Researchers found that a video that surfaced earlier this year of a monkey seemingly having a sexual interaction with a deer was actually not an isolated case of unusual monkey behavior. Gunst-Leca - who apparently doesn't have much of an imagination - said it wasn't clear at first what exactly was going on.
Researchers found that monkeys' inclination towards mounting deer, while rather surprising, is not an abnormal thing, Newsweek reports.
The female monkeys were primarily observed mounting male bucks.
In total, the team recorded 12 successful interactions between monkeys, involving six adolescent females, between November 2012 and January 2013.
"Then, during the surge of sex steroid hormones characteristic of the adolescence period, they may seek similar sexual reward with deer mates, particularly when sexually deprived of conspecific male mates".
To investigate, the team recorded the behaviour of snow monkeys at Minoo, north of Osaka in Japan.
Unexpectedly, pelvic thrusting was more common when the sexual partner was a deer, as opposed to another monkey.
In study published in the peer-reviewed journal Archives of Sexual Behavior, researchers documented repeated attempts by Japanese snow monkeys, to climb on the backs of deer and thrust their pelvises.
"It is maybe a new/innovative behaviour that can be socially transmitted and will spread", he said.
According to Dr Cédric Sueur of the University of Strasbourg, a co-author of the original study, tells the Guardian that it might just be attributed to females not having enough males to go around during mating season.
Most of the sika deer that the monkeys were having sex with were adult males, although the scientists also observed encounters with two female deer and three young males.
The activity was filmed by during the monkey mating season.
It remains unclear why the snow monkeys engage in sexual interactions with deer, but scientists believe they could be practicing for sex with other monkeys or they could have been rejected by males, leaving them with no sexual partner.
"Future observations at this site will indicate whether this group-specific sexual oddity was a short-lived fad or the beginning of a culturally-maintained phenomenon", the authors note.