A Bull and a Male Donkey Showed a Close Bond, Sex Included

On A GRAZING area at Debre Libanos, on an Ethiopian plateau, cattle and donkeys with different owners share the pasture by day, and in that crowd a bull named Achille and a male donkey named Patroclo kept ending up together.

University of Pisa researchers report in Current Biology that a bull and a donkey stayed close and directed most of their sexual and friendly contact at each other. That makes the sex look like part of a relationship rather than a stray event, though two animals watched in one place can’t show how common such pairings are. The authors say they found no earlier report of it in donkeys, or of any lasting cross-species pairing of this kind among domestic hoofed animals.

Same-sex sexual behavior is widespread in animals: in 2023, a Nature Communications analysis put it in 261 mammal species, about 4 percent of the total, and found it more often in social ones. Two species in one couple is another matter.

The team stumbled on the pair by accident, during an unrelated field study of gelada monkeys. From then on they recorded everything: friendly contact, sex, closeness and aggression, between Achille and Patroclo and between each of them and every other animal. This was an observational field study, so nothing was staged; the researchers watched both animals together on four days and apart on four more. The journal says the researchers counted 51 mountings, 34 with penetration, from March 6 through 19, 2026 (a mounting is one animal climbing onto the back of another, as in mating).

Patroclo Never Pulled Away

Across all 51 mountings, Patroclo never pulled away or showed a threat; he often presented his hindquarters to Achille and nipped him. A second male donkey told a different story: the only two times Achille approached or mounted that animal, it flattened its ears and showed clear avoidance (the study press release counts two mounts, while the paper’s wording reads as one approach and one mount).

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

Achille and Patroclo spent more time near each other than apart, and more of that time with no other animal close by than with company. Friendly contact, meaning licking, rubbing and nuzzling, was likelier between the two than between either one and any other animal. When it did happen, it lasted longer between them than when Achille directed it at others.

Timing is where the friendly contact gets interesting. In a window of about six seconds around each mounting, friendly contact came before the mount in about 25% of cases and after it in nearly 75%. The authors read that surge as more than arousal; they say it appears to prolong the social interaction beyond the sexual act. To check whether Patroclo found any of this threatening, the team scored his facial movements with a catalog of the movements horses can make, the Equine Facial Action Coding System, and compared mounting, affection, closeness and time alone. His face showed no detectable variation across those conditions, which the authors take to mean that Achille’s presence and advances did not register as a threat.

Not a Herd Habit

Achille went through the typical courtship moves with cows but never tried to mount one, possibly because other bulls were around. Nor did the habit spread: no mounting between other cattle and donkeys was recorded, and the one bull that tried to mount Achille was turned away.

None of the usual explanations for same-sex mounting, namely sexual coercion, mistaken identity or a shortage of partners, seems to fully account for the pair, the authors argue, because the sex was selective and mutual and the pasture held plenty of alternatives: three or more bulls, three or more cows, three male donkeys and between two and seven female donkeys on any given day. Whether it brings either animal a direct benefit, or follows from how cattle and donkeys form bonds, is an open question, and the authors name domestication as one possible influence.

Sex, then, may do social work as well as reproductive work. That is the authors’ wider point. Primates hint at the same thing: a 2026 analysis of 491 species found same-sex sexual behavior in 59 of them and linked it to more complex social structures. Whether bulls and donkeys follow the same logic is anyone’s guess.

“For us, a major question for future research is what mechanisms, and possibly what benefits, might underlie these interactions,” says Elisabetta Palagi, who works at the University of Pisa. Comparing species with different social systems, she thinks, could help tell bonding from tension relief from some mix of motives. For now, two animals on one plateau are the whole dataset.

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