Researchers from the University of Pisa and collaborators described a same-sex, cross-species pairing between a domestic bull and a male donkey on shared pasture at Debre Libanos, Ethiopia, in a Current Biology correspondence³. Stuff and The Guardian both reported the paper on Monday^1,2.

The animals, named Achille (the bull) and Patroclo (the donkey), belonged to different local herders. They met only when mixed herds used the same daytime grazing area and spent nights in separate compounds^1,3.

The paper records 51 mountings of Patroclo by Achille, 34 with intromission. Patroclo did not avoid those attempts and often presented his hindquarters or nipped the bull^1,3. Affiliative licking and rubbing often occurred around mounts, more often after than before³.

The first sexual interaction was noticed during fieldwork on gelada monkeys^2,3. Both animals were watched together on four days and separately on four more^2,3. Other donkeys showed avoidance when Achille approached them, according to the paper and The Guardian^2,3.

Stuff framed the case as confounding traditional evolutionary assumptions about animal behaviour¹. The Guardian highlighted comments from ethologist Elisabetta Palagi that the pairing is unusual in the published literature².

The correspondence is a short observational report on two named animals, not a survey of how common such pairings are. The authors write that classical explanations such as coercion, mistaken identity, or lack of opposite-sex partners do not fully account for the dyad, and that whether the behaviour provides benefits remains an open question³.

What remains unknown includes how long the association lasted beyond the brief field window, whether similar pairings occur elsewhere among domestic ungulates, and what function, if any, the sexual contact served³.