Researchers in the Ethiopian highlands went looking for monkeys and found something no scientific paper had described before. On a shared pasture, a domestic bull and a male donkey repeatedly sought each other out, mated, groomed, and stayed close, in what the team calls the first documented sustained same-sex sexual relationship between these two species.

The case study, published in Current Biology on Nov. 2, comes from scientists at the University of Pisa working with Addis Ababa University. They named the animals Achille and Patroclo, after Achilles and Patroclus from Greek mythology.

An Accidental Discovery During Monkey Fieldwork

The team was in Debre Libanos, in Ethiopia's Oromia region, to study gelada monkeys, Stuff reported. During a lunch break, doctoral student André da Cruz noticed the bull mounting the donkey.

"I told colleagues: 'I think I saw a bull mounting a donkey.' And they didn't believe me," da Cruz told The Guardian. The researchers set their primate work aside and began recording the pair systematically.

Between March 6 and March 19, 2026, they logged 51 mountings, 34 of which involved full sexual contact. The two animals belonged to different herders, spent nights in separate compounds, and met only when their herds shared the same grazing land during the day.

Signs of a Bond Rather Than a One-Off

The researchers argue the relationship went beyond sex — and they point to several lines of evidence. The clearest contrast came when Achille tried to mount a different male donkey. That animal flattened its ears and showed avoidance and threat behavior.

Patroclo did the opposite. He never avoided or kicked away the bull, and he often presented his hindquarters or gently nipped Achille to start contact, ScienceAlert reported.

The pair also licked, rubbed and nuzzled each other more often, and for longer, than they did with other animals. They spent most of their observed time near each other, even though both had access to other partners. Friendly contact happened much more often after mating than before it, which the researchers say suggests the interactions extended the social connection rather than simply serving a sexual purpose.

One moment stood out. When Achille mounted another donkey, Patroclo moved in to interrupt, and Achille then groomed him. Lead author Elisabetta Palagi, an ethologist at the University of Pisa, said there was no sign that the relationship involved force.

Same-Sex Behavior Is Common, Cross-Species Bonds Are Not

Same-sex sexual behavior has been recorded in more than 1,500 animal species, and many scientists believe it is underreported. A 2024 survey of animal behavior researchers found many had seen it in the field but rarely published their observations, as Nature World News previously reported.

Sexual behavior between different species is much rarer, and a sustained, selective relationship between two species is rarer still. In many social mammals, grooming and close contact help build and maintain bonds, reduce tension and provide support. The researchers suggest Achille and Patroclo may have gained attention, and possibly protection, from each other.

Broader research offers context. A 2023 analysis of mammals found that same-sex sexual behavior was more likely in social species and may help animals manage tension and aggression within groups, as Nature World News also reported. Cattle and donkeys are both herd animals that rely on social ties, which may help explain why two individuals of different species could form a lasting connection when they share space every day.

"Nature does not always respect the boundaries we draw around it," Palagi said. She noted that unusual behavior can stay anecdotal simply because nobody has the chance to document it carefully.

The Limits of a Single Case

This is a short observational case study of two animals over about two weeks. It cannot explain why the bond formed, whether it is common, or what each animal experienced. The researchers caution against reading human emotions or relationships into the behavior, even though the mythological names make that tempting.

The observations also cover only daytime grazing hours, so the team could not see how the animals behaved at night, when they were kept apart. Both animals are domesticated and lived in a human-managed setting, which may shape their social choices in ways that differ from wild animals. After the study, they were moved to different grazing areas, so it is unknown whether the bond continues.

The team hopes to return to see if the two reunite and to look for similar cross-species bonds elsewhere. Their broader point is about method. Behaviors that seem rare may simply be the ones scientists have not had the chance, or the willingness, to record systematically.

Lessons for Farms and Field Science

For readers who keep livestock, the study is a reminder that farm animals have complex social lives. Cattle, donkeys, goats and horses kept together can form strong preferences for particular companions, and separating bonded animals can cause stress. Farmers and animal welfare specialists already use companion animals, such as donkeys with horses, for this reason.

For science, the case adds to a growing body of work showing that animal social behavior is more flexible than once assumed. It also highlights how much can be learned by paying attention to the unexpected during routine fieldwork.

The next step is simple and uncertain — if the researchers find Achille and Patroclo again, they will learn whether a bond that crossed a species line can also survive time apart.

What Readers Want to Know

What did the researchers observe? A bull and a male donkey in Ethiopia repeatedly mated, groomed, and stayed close to each other over about two weeks.

Why do scientists think it was a bond? The donkey never avoided the bull, the pair groomed far more than with others, and friendly contact often followed mating.

Is same-sex behavior common in animals? Yes. It has been recorded in more than 1,500 species, though cross-species relationships are much rarer.

Where did this happen? In Debre Libanos, in Ethiopia's Oromia region, near a site where researchers were studying gelada monkeys.

Are the animals still together? Unknown. They were moved to different grazing areas after the study.

Does this mean animals have human-like relationships? The researchers caution against that interpretation and say the behavior should not be anthropomorphized.