Hooking up with members of other species is generally frowned upon. But maybe we’re just prudes. As grolar bears, mules, and even Homo sapiens have shown, cross-species hookups aren’t as unnatural as we think they are. The history of these freaky orgies got a new chapter on Monday, as scientists publishing in the International Journal of Primatology showed genetic proof that some monkeys in Tanzania’s Gombe National Park don’t give a damn about our narrow-minded sexual taboos.

Florida Atlantic University anthropologist Kate Detwiler, Ph.D., set out to prove that guenon monkeys, in particular, had been screwing around — and successfully breeding — with members of different species throughout their evolution. What sets these monkeys apart are the extraordinarily distinct features associated with different species: some of them have bushy beards, some have brightly colored tufts of hair, some have huge cheeks. Previous work had suggested these features evolved to be especially distinct so that individuals wouldn’t mate across different species, but Detwiler’s research shows the opposite. “There’s a lot of promiscuity taking place in Gombe National Park,” she said in a statement on Monday.

guenon monkey hybrid
"Jimmy" is the hybrid offspring of two guenon monkey individuals of different species. He seems fine!

By biological definition, species are genetically distinct from one another — so distinct that they shouldn’t be able to mate. If you, for example, tried to have a baby with a horse, doing so would not make a centaur. But some species are closer on the evolutionary family tree than others and as such seem to have a better chance at popping out kids. Because the guenon monkeys have lived in relative isolation in Gombe National Park for so long, it’s not unreasonable to think that their original population eventually split into multiple species with different characteristics. In Detwiler’s attempt to showcase the sexual adventurousness of the guenon monkeys, she chose to focus on two clearly defined species: the red-tailed and blue-tailed monkeys that colonized the region, and the hybrid offspring they seemed to produce.

Digging through the poop of 144 monkeys, Detwiler extracted DNA to trace who exactly had been hooking up. Mitochondrial DNA, which is passed down maternally, revealed a beautiful history of curiosity and tolerance. The data suggest that the park, originally colonized by red-tailed monkeys, was at one point infiltrated by blue-tailed male monkeys looking for mates. When they encountered red-tailed females, the blue-tailed males weren’t put off by their different facial features. Both parties gladly hooked up and somehow managed to make babies, thus leading to the present-day population, which Detwiler found is 15 percent hybrid.

These boundary-pushing monkeys shed light on the importance of cross-species hybridization in nature, which tends to happen when there are big shifts in an animal’s environment. Grolar bears, for example, are thought to be the result of polar bears being forced south toward grizzlies as climate change heats the Arctic. While it’s not likely that humans will be hooking up with our neighbors on the evolutionary tree anytime soon, our history reveals the hypocrisy of our social taboos: DNA analyses have shown that migrating Homo sapiens, responding to their self-imposed changes in the environment, occasionally hooked up with Neanderthals, Denisovans, and other hominins, leading to our patchwork genomes.

As the world around us rapidly shifts, perhaps it’s time to rethink our uptight sexual norms. “This research is very timely because hybridization often occurs in response to environmental changes, as we are seeing with climate change and modified landscapes,” said Detwiler of the monkey study. “It is nature’s way to respond.”