Elisabetta Palagi, an evolutionary biologist at the University of Pisa, had sent her team of student researchers to Ethiopia to study geladas, monkeys whose hearts seem to glow out of their chests. But as his students watched geladas snack on a grassy plateau alongside other grazing animals, they noticed a peculiar interaction between two other animals. A bull approached a male donkey and mounted him. This behavior does not seem to bother the donkey. The donkey did not kick him or try to escape the interaction. The researchers contacted Palagi and asked if she thought such interspecies sex was worth studying. “I said yes,” Palagi wrote in an email.
After careful observation, researchers discovered that the bull, nicknamed Achilles, and the donkey, nicknamed Patroclo, had a persistent mutual sexual relationship, described in a new article in Current biology. The relationship between Achilles and Patroclo was nothing new to local shepherds, who laughed when they saw researchers filming the unexpected couple. “What was an unexpected sighting for us was obviously a familiar sight for them,” Palagi said. (The document includes explicit images of the couple copulating which we have omitted out of respect for their privacy.)
Achilles and Patroclus are of course far from the only two male animals we have observed making love. Although scientists have observed same-sex sexual behavior in animals for centuries, they were often reluctant to report this behavior for reasons you might expect. This underreporting created the impression that same-sex sexual behavior was rare and that the animals that engaged in it were outliers, according to a paper published in 2025. PLOS One. Scientists also generally viewed this behavior as a kind of evolutionary paradox, assuming that engaging in any type of sex that doesn’t lead to reproduction must be a waste of energy. But a recent wave of research has taken same-sex sexual behavior seriously, studying its prevalence among certain groups of animals and even suggesting that the behavior has been around for as long as animals have had sex.
Achilles and Patroclus are also not the only animals of different species to be observed making love. This type of behavior is often described as an example of “reproductive interference”, in which individuals of distinct species engage in sexual or courtship behaviors that presumably waste time and energy that could be spent directing these behaviors toward a suitable partner. This behavior is often explained as a simple error. Frogs are famous for this, seen attempting to mate with shoes, tennis balls, coconuts, turtles, mangoes and a literal piece of shit. Reproductive interference is most often observed between closely related species, which may even have the capacity to hybridize, or between introduced species. But it also happens between surprising couples, often as an apparent act of aggression or harassment, as in the case of an Antarctic fur seal attempting to penetrate a king penguin or a Japanese macaque riding a sika deer.
Scientists have far fewer examples of same-sex sexual behavior between different species, especially between species as distantly related as donkeys and cattle. Most male-male montages have been observed in domestic cattle, often those kept in pens containing only bulls, not cows. Researchers have found no examples of male-male mounting in donkeys. “I would be cautious about claiming that this is an absolute first,” Palagi said, noting that similar relationships may have been observed in the past but not reported. But the new study goes beyond just a one-off observation, quantifying the couple’s interactions over a longer period of time.
If researchers were first attracted by the montage itself, they were even more surprised by the nature of the bond that united Achilles and Patroclus. The two never spend nights together, as they belong to different owners. But on set, the two licked, rubbed, and spent most of their time in close proximity. Over four days, researchers observed Achilles mount Patroclo 51 times, 34 of which resulted in penetration. In none of these interactions did the donkey react with avoidance or threat. “Patroclo also repeatedly presented his hindquarters to the bull,” Palagi said. And their pairing wasn’t simply the result of a lack of options; there were other cattle and donkeys on the plateau, female and male. Rather, it seems that, when given the opportunity, Achilles and Patroclus choose to be together.
In one intriguing encounter, when Patroclus approached Achilles while the bull was riding another donkey, the bull redirected its attention to Patroclus, licking the donkey. This encounter illustrates the active role of the donkey in maintaining its association with Achilles. When I asked Palagi if this could be understood as a kind of jealousy or possessiveness, she replied that these terms imply motivations or emotional experiences that we cannot discern from the interaction. Palagi added that there is not enough information to know whether this interaction is equivalent to those seen in mating relationships between donkeys or cattle. But she noted that relationships are valuable resources for social animals. “We can’t say exactly what this relationship meant for Achilles and Patroclo, but we can document how consistently they sought each other out and interacted,” Palagi said.
Researchers have more questions than answers about the bond between the couple. Palagi wants to know more about how the association between Achilles and Patroclo developed, such as whether the two had a social bond before a sexual bond. But the paper still contains important takeaways, such as the fact that social relationships between animals are often more complex and flexible than researchers think, she added. Palagi’s students named the bull after Achilles, hero of the Iliadand the donkey after Achilles’ dear companion, Patroclus. Homer “does not explicitly describe them as lovers, although later ancient authors interpreted their relationship that way,” Palagi said. In the Iliadafter Patroclus dies in battle, Achilles falls into deep despair which eventually turns into deep rage. “Their deep attachment is at the heart of the story,” she added.
For now, their ungulate counterparts have also been separated, but under less tragic conditions. The enclosure where Achille and Patroclo once grazed together no longer has grass, and now each of their herds are taken to separate areas. Palagi’s students are back in Ethiopia and plan to ask herders if the two can be brought together. Palagi wonders how this encounter might go: whether the two would resume their licking and riding, or whether their interest would have waned after months apart. But for now, their epic affection for each other is recorded as a story in the annals of scientific literature, a cock story that’s actually true.
Gn world
