Of all the things you might see on you lunch break, a bull having sex with a male donkey is likely low on the list. But ecologists “always have an eye on the horizon,” says André Beirão da Cruz. And when he and fellow Ph.D. candidate Martina Francesconi stopped for a bite during a field research trip in Ethiopia, that is exactly what they saw.

“We were not sure if we had actually seen it or if it was some type of mirage,” says da Cruz, who’s completing his Ph.D. in ecology at the University of Pisa in Italy and the NOVA University of Lisbon in Portugal.

This is the story of Achille and Patroclo, a domesticated bull and a male donkey, respectively, that live on an Ethiopian plateau and often graze together. In a new paper, da Cruz and his colleagues describe the deep bond between the two animals, who were named for the ancient Greek hero Achilles and his lover Patroclus. During the ecologists’ observations, Achille and Patroclo’s relationship involved dozens of sexual interactions and indicated a strong social connection.

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The case study, the researchers say, may offer a window into the complexities of animals’ same-sex behaviors.

“Most of the time, people and researchers start with an idea in the field, and they are focused on that in a very strict and tight way,” says Elisabetta Palagi, senior author of the paper and an ecologist at the University of Pisa. “But this prevents people from looking around, and it is possible that this kind of interaction is much more frequent than has been reported.”

Achille and Patroclo have different owners but often graze in the same field; the researchers observed them for four days together and four days apart. In the four days together, Achille mounted Patroclo 51 times.

Same-sex sexual behavior among animals is well studied, but ecologists know far less about interspecies sexual behavior. The reciprocity in Achille and Patroclo’s behavior was most notable, Francesconi says: Patroclo was receptive when Achille mounted him but aggressive when any other bull was nearby. And the two animals’ interactions often went beyond just mounting, with the pair licking or rubbing against each other after sexual interactions. This suggests that sexual relationships between animals are far more complex than our current understanding, Francesconi adds.

Vincent Savolainen, an ecologist at Imperial College London, who was not involved in the work, says the authors “show quite nicely that these two are not just a random dyad.”

He adds, however, that Achille and Patroclo’s domestication may mean that their behavior does not reflect social traits or patterns among wild animals.

“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 says. “If we want to make massive steps in the field of behavior ecology, you would want to study a larger group of wild animals.”

“What we know about interspecies sexual interactions is scarce,” Francesconi says. “And it’s mostly connected with a subject forcing the other or mistaken identity or something randomly going on by chance. But what’s not present in the literature is something recurrent, selective, and somehow voluntary and reciprocal.”

The research was published today in Current Biology.

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