Researchers Came for Ethiopia's Monkeys and Documented a Bull-and-Donkey Bond

Researchers from the University of Pisa have documented a selective social and sexual bond between a bull named Achille and a male donkey named Patroclo near Debre Libanos, Ethiopia. In March 2026 they recorded 51 mountings plus licking and nuzzling. The single-case study was published in Current Biology on Nov. 2, 2026.²³

The team was not there for cattle or donkeys. They were in the Ethiopian highlands to study gelada monkeys, sometimes called bleeding-heart monkeys, when they noticed the pair by chance and decided to document what they were seeing systematically.¹⁴

The students on the team named the animals after Achilles and Patroclus from Homer’s Iliad.¹⁴

What the researchers saw

Achille and Patroclo belong to different owners and different herds. They were kept apart at night and met only during daily shared time on pasture.¹⁴

Between March 6 and March 19, 2026, the researchers counted 51 mountings between the two animals.²³ Nautilus reports that the team made eight separate observations in under two weeks. Most of the mountings involved copulation, and the animals also nipped, licked and nuzzled each other.¹

Related: Local Herders Weren’t Surprised by the Bull and Donkey. The Scientists Published a Paper

Patroclo was not a reluctant participant. He pressed his haunches toward the bull and nipped at him, and he never kicked or resisted. Both animals aimed most of their social and sexual interactions at each other.¹³

A second male donkey got a very different reception, or rather gave one. Achille mounted him only twice, and that donkey showed clear avoidance, with flattened ears and threatening expressions.¹²

“We believe the key finding is not simply that a bull mounted a male donkey,” lead author Martina Francesconi, a PhD student at the University of Pisa, said in the Cell Press release. “It is that this unusual sexual behavior was part of a broader, selective social relationship.”²

“The recurring feature was the close association between sexual behaviour and friendly social contact,” Elisabetta Palagi, the evolutionary biologist who leads the team, said in an email to Nautilus.¹ “It is really, really unusual,” she told The Guardian, as relayed by UPI.⁴

Why it matters, and what it can’t tell us

Same-sex behavior is widespread across the animal kingdom, but sexual interactions between different species are much rarer. The researchers describe this as the first documented sustained same-sex sexual relationship between a bull and a donkey.⁵ It is one bull and one donkey, so it is a single case, not a pattern.

The authors say the behavior is not easily explained by a simple lack of other mating partners. They also say they cannot determine from this study what function the sexual interactions served.²

They suggest it may be connected to domestication, or to the social bonding systems of the two species. Palagi has speculated that the bond might help with stress and reward, but the team did not test any of these ideas.¹

As for Achille and Patroclo, they no longer share a field. After the study ended, the pasture where they had grazed together was no longer available, and the two were moved to different grazing areas.¹⁵