An astonishing new study claims that adolescent female monkeys in Japan are regularly having sexual intercourse with sika deer, and scientists aren’t sure why it’s happening. The findings were published in the journal Archives of Sexual Behavior and are a follow-up to an earlier report that found a male macaque mounting a female sikda deer.

Although the report was anecdotal, scientists wanted to look into it deeper to find out if this was more common than people realized. While studying adolescent female monkeys and deer in Minoo, Japan, researchers were surprised to find that there were definitely sexual interactions between the two species, with female monkeys observed climbing onto the deer and grinding against their backs.

While it’s not uncommon to see Japanese macaques riding deer like horses, the study shows that the monkeys are going a bit further in this relationship than that. The reactions of the deer were interesting as well, as sometimes the deer shook the monkeys off and fled, but many just stood their calmly and let the female monkeys do their thing, sometimes even continuing to eat.

“This is the first quantitative study of heterospecific sexual behavior between a non-human primate and a non-primate species,” reads the abstract from the paper. “We observed multiple occurrences of free-ranging adolescent female Japanese macaques (Macaca fuscata) performing mounts and sexual solicitations toward sika deer (Cervus nippon) at Minoo, central Japan. Our comparative description of monkey-deer versus monkey-monkey interactions supported the “heterospecific sexual behavior” hypothesis: the mounts and demonstrative solicitations performed by adolescent female Japanese macaques toward sika deer were sexual in nature.”