A team of primate researchers in Ethiopia went looking for data on geladas, the country's distinctive "bleeding-heart" monkeys. What they found instead, grazing in a nearby pasture, was a bull and a donkey with a relationship unlike anything in the scientific literature.
Ph.D. student André da Cruz first noticed the pair, a bull named Achille and a donkey named Patroclo, during a lunch break in the field. The animals belong to different owners but meet daily in a shared pasture, and the research team — led by Dr. Elisabetta Palagi of the University of Pisa — decided the behavior was worth documenting formally. Their case study appears in the journal Current Biology.
Reciprocal, selective and recurring
Over eight days of observation — four with both animals present, four with only one — the researchers recorded Achille mounting Patroclo 51 times. Just as notable to the team was what surrounded those encounters: the pair licked and rubbed each other both before and after, behavior the researchers say suggests the bond extends beyond sex into broader social affection.
Neither animal showed comparable interest in other individuals of its own species and sex, though Achille did direct ordinary mating behavior toward female cattle when they were present. When Achille approached other animals in the pasture, Patroclo would move toward him and attempt to redirect his attention — a pattern the team read as active, mutual seeking-out rather than coincidence.
Palagi called the relationship "really, really unusual" and said it was the first such case reported in scientific literature. Fellow researcher Martina Francesconi noted that most documented instances of interspecies sexual behavior in animals involve coercion, mistaken identity or chance encounters; Achille and Patroclo's bond, by contrast, was recurring, selective and reciprocal — a combination she said has not been described elsewhere.
Palagi added that the discovery says as much about scientific habits as about cattle and donkeys: researchers often train their attention narrowly on the hypothesis they set out to test, she said, which may mean behavior like this is more common in the animal world than the published record suggests.
