It was a "single anecdotal event" that surprised scientists and the internet: A male snow monkey was documented trying to have sexual relations with a sika deer in Japan. It turns out it wasn't so singular. A study published Dec. 11 in the Archives of Sexual Behavior establishes that "heterospecific sexual behavior" is occurring between the two species, a conclusion born out of the study of adolescent female Japanese macaques, or snow monkeys, "performing mounts and sexual solicitations" on male sika deer at Minoo in Japan. "The monkey-deer sexual interactions reported in our paper may reflect the early stage development of a new behavioral tradition," says co-author Noëlle Gunst-Leca.
The Guardian explains that to establish the behavior was sexual, the researchers compared sexual behavior between adolescent female monkeys (they were not seen mounting males during the study period) and between those monkey and deer. Over three months beginning in November 2014, they logged 258 mounts that produced 13 successful interactions, meaning the monkey has a "temporary, but exclusive, sexual association" with the deer. The Washington Post reports that as the females thrust their pelvises, bit, and yanked on the deer's antlers, the deer "stood nonchalantly and continued foraging for food." The researcher also found that if the deer walked away, the monkeys "often displayed sexually motivated tantrums ... while gazing at the deer." The authors plan to continue observing the site to determine if the behavior is a "short-lived fad or the beginning of a culturally-maintained phenomenon."