This Bull and Male Donkey Have a Surprisingly Sexual Relationship, Providing Insights Into Interspecies and Same-Sex Bonds

The ungulates, dubbed Achille and Patroclo, chose each other repeatedly and showed affection before and after each rendezvous. Same-sex sexual encounters are common among animals of the same species, but less so between different ones

Sara Hashemi | Daily Correspondent

November 5, 2026 1:47 p.m.

Earlier this year, researchers were studying monkeys on an Ethiopian plateau when members of two other species caught their eye. They spotted something unexpected: a bull mounting a male donkey.

“We were not sure if we had actually seen it or if it was some type of mirage,” recalls André Beirão da Cruz, an ecologist at the University of Pisa in Italy and the NOVA University of Lisbon in Portugal, to Mary Randolph at Scientific American.

It wasn’t. Over the course of several days, da Cruz and colleagues observed the two ungulates licking and rubbing each other and having sex despite the availability of members of their own species. Their findings, described in the journal Current Biology on November 2, suggest that scientists still have a lot to learn about social and sexual interactions between members of different species.

“It is really, really unusual,” says study co-author Elisabetta Palagi, an ecologist at the University of Pisa, to Nicola Davis at the Guardian. “And it is the first time it is reported in literature.”

Achille and Patroclo, the bull and donkey’s respective nicknames, belong to different owners but spend time together on the pasture. So, the researchers observed the two animals on four days when they were together and four days when they were separated this past March.

Fun fact: Name origins

The animals’ nicknames were inspired by the Greek mythological figures Achilles and Patroclus, two men who may have been more than just friends.

Overall, the team recorded 51 mountings between Patroclo and Achille in that window, 34 of which involved penetration. The donkey often initiated interactions by presenting his hindquarters to the bull and nibbling him. They displayed affectionate behaviors before and after each rendezvous.

The pair specifically picked each other for these intimate encounters. When another bull tried to approach Patroclo, the donkey displayed aggressive behaviors like chasing and biting. Achille, meanwhile, tried to mount another male donkey, but the animal showed avoidance and threatening behaviors, like flattened ears—something Patroclo never did to him.

The two ungulates don’t seem to have chosen each other out of convenience or because they had no other options. Numerous male and female cattle and donkeys grazed alongside Achille and Patroclo, yet the duo continued to spend time together when the opportunity arose.

“What really caught our attention was that these interactions were embedded in a very selective and consistent social relationship between the two individuals,” Palagi says in a statement.

The bond between the two animals—which the researchers discovered was common knowledge among local herders—raises further questions. While same-sex sexual behaviors are widespread in animals, they usually involve members of the same species, like penguins and bonobos. In nonhuman primates, research suggests these behaviors might reinforce social bonds and help the animals live longer. However, it’s unclear why the male ungulates of different species would pick their trysts.

“Achille and Patroclo cannot tell us why they choose one another. This short study can’t tell us what evolutionary purpose, if any, their relationship serves,” writes Jo Setchell, a biological anthropologist at Durham University in England who wasn’t involved in the study, for the Conversation. “But it does remind us that animal behavior can be more diverse than our theories suggest.”

Vincent Savolainen, an ecologist at Imperial College London who wasn’t involved in the work, tells Scientific American that Achille and Patroclo’s behavior could be influenced by their domestication, meaning wild animals might not exhibit the same social traits or patterns.

“It’s important to publish and publicize this example just to show the diversity of what can happen, but it is obviously limited to two animals from two species that are domesticated,” Savolainen adds. “If we want to make massive steps in the field of behavior ecology, you would want to study a larger group of wild animals.”

Still, Josh Luke Davis, a science writer at the Natural History Museum in London and author of A Little Gay Natural History, cautions against “reading too much into the significance” of Achille and Patroclo’s relationship.

Given that queer behaviors are common in the natural world and interspecies connections aren’t that unusual, it’s “probably not that surprising” that the two have overlapped, he tells the Guardian.

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