Bull and Male Donkey Formed Unusual Sexual Bond, Scientists Find

A bull named Achille could have had his pick of the pasture. Cows, female donkeys, other bulls and jacks all grazed within reach. Instead, he kept returning to a single male donkey named Patroclo, mounting him 51 times and grooming him in between. Scientists in Ethiopia studying monkeys happened to catch the pair and documented something they say has never been reported before: a lasting, two-way sexual and affectionate bond between two completely different species of farm animal.

Achille and Patroclo belonged to different owners and lived in separate housing at night. Achille bunked with another bull, a cow and a calf. Patroclo stayed with two other male donkeys. Yet whenever their herds mixed during the day, the two sought each other out. Achille repeatedly mounted Patroclo, who repeatedly presented his hindquarters and never avoided the attempts, and the two groomed each other by licking and nipping, even when other animals were nearby and available.

Researchers from the University of Pisa and other institutions spent days watching the pair and recording their behavior on video. Their findings, published in the journal Current Biology, push back against the usual explanations scientists reach for when animals of the same sex, let alone two completely different species, engage in sexual behavior with each other. The pattern was too selective, too consistent and too affectionate to fit neatly into old theories about mistaken identity or simply settling for whatever partner happens to be around.

How Scientists Documented the Bull and Donkey Bond

Scientists were not initially setting out to study a bull and a donkey. The fieldwork centered on a monkey called a gelada, but researchers noticed the odd pairing in the same pasture and decided to track it. Over their observation period, the two animals were seen together on four days and apart on four others, giving researchers a window into how their behavior shifted depending on whether they shared the same space.

Their grazing herd was not small. At any given time, the pasture held between three and six bulls, three and seven cows, three male donkeys, and two to seven female donkeys. That matters, because it means Achille and Patroclo weren’t simply making do with the only animals around. They had real options and kept picking each other anyway.

Researchers counted 51 times that Achille mounted Patroclo, 34 of them involving full sexual contact. What stood out even more was what happened around those moments. Before a mount, affectionate behavior like licking and rubbing showed up in about a quarter of interactions. After a mount, that number jumped to nearly 75 percent. Physical closeness didn’t end when the sex did. If anything, it grew afterward, which researchers say is hard to explain as simple arousal and instead suggests the animals were stretching their bond well beyond the act itself.

A Donkey That Never Flinched From the Bond

One of the more telling details involves what didn’t happen. On two separate occasions, Achille approached and mounted a different male donkey instead of Patroclo. That donkey reacted like a frightened or unwilling animal, pulling away and flattening its ears in what researchers identified as a threat display. Patroclo never once reacted that way to Achille, not during mounting, not during affectionate moments, not even when the two were simply standing near each other.

To check whether Patroclo might be hiding distress, researchers analyzed his facial expressions using a specialized scoring system designed to read the faces of horses and donkeys. They compared his expressions during mounting, friendly interactions, quiet closeness and moments alone, and found no meaningful difference across any of them. That suggested Patroclo did not perceive Achille, or his advances, as threatening.

There was also a moment caught on video that reads almost like jealousy. When Achille tried mounting the other male donkey, Patroclo moved toward him. Achille immediately broke off the interaction with the other donkey and turned his attention back to Patroclo.

Numbers backed up what researchers were seeing with their own eyes. Achille and Patroclo spent far more time near each other than apart, and more time alone together than they did when other animals were nearby. Friendly behavior between the two happened far more often and lasted longer than anything either showed toward any other animal in the herd. None of their interactions ever turned aggressive. Meanwhile, Patroclo did show aggression, chasing and biting, toward a different bull that twice tried to approach him, a sign he may have been unwilling to tolerate rivals getting close.

Why the Bull and Donkey Bond Defies the Usual Explanations

Scientists have a handful of go-to theories for why animals engage in same-sex sexual behavior. Sometimes it’s coercion. Sometimes an animal mistakes a potential partner’s sex. Sometimes there simply aren’t enough opposite-sex partners around, so animals make do with whoever’s available. Researchers say none of these explanations hold up here. Achille showed normal courtship behavior toward female cattle, he just didn’t mount them, possibly because of competition from other bulls in the group. But with Patroclo, he showed similar courtship behavior and followed through.

Same-sex mounting among cattle has mostly been documented in captive settings, where males are kept apart from females or have limited access to them. Achille and Patroclo were free to roam a mixed herd with plenty of females from both species present. Same-sex sexual behavior has shown up before in other members of the horse and donkey family, but researchers say they could find no published case of it happening specifically in donkeys, and no documented case anywhere of a lasting, two-way relationship like this one forming between domesticated farm animals.

Researchers raised one more possibility: domestication itself might play a role, since living alongside humans and other species can reshape an animal’s social world and open up chances to form bonds that wouldn’t happen in the wild.

What happened between Achille and Patroclo isn’t fully explained by a lack of options, confusion or coercion. They had choices, and over and over, they chose each other. That doesn’t answer every question about why animals form same-sex bonds across species, but it does show the question is far harder to pin down than scientists assumed.