Researchers studying gelada monkeys in Ethiopia documented a persistent sexual and social bond between a bull named Achille and a male donkey named Patroclo. The unusual interspecies pair engaged in repeated mountings and mutual grooming, challenging traditional assumptions about animal behavior and social relationships.
Fieldwork can take unexpected turns, but few researchers anticipate witnessing an interspecies romance. While students from the University of Pisa traveled to the Ethiopian highlands to observe gelada monkeys, their attention drifted to a nearby grazing area where a bull and a male donkey were interacting in ways that baffled the science team. What was an unexpected observation for the researchers was evidently a familiar sight to local people, with Palagi noting that what surprised the team was apparently routine for those nearby.
The animals belonged to different local owners and only crossed paths when their respective herds grazed on the same plateau. Despite having ample opportunity to seek out mates of their own species and opposite sexes, the two males repeatedly chose each other.
Documenting 51 Mountings Across Two Weeks in Ethiopia
Intrigued by the initial sighting, the research team systematically recorded the interactions between the pair. Between March 6 and March 19, 2026, the observers documented 51 mounting events, with 34 resulting in intromission.
The researchers noted that the donkey played an active and receptive role. Patroclo repeatedly presented his hindquarters to the bull, named Achille, and nipped at him affectionately without showing any defensive aggression or avoidance behavior.
This reception stood in sharp contrast to how other animals reacted. When Achille attempted similar mounting behavior with another male donkey on the plateau, that animal displayed clear avoidance tactics, flattening its ears and showing threatening facial expressions. Achille mounted another male donkey only twice during the observation window, and that individual showed clear avoidance behaviors and threatening expressions, such as flattened ears.
The sexual interactions between Achille, the bull, and Patroclo, the male donkey, were not isolated events. Elisabetta Palagi, evolutionary biologist at the Università di Pisa
Elisabetta Palagi, an evolutionary biologist at the Università di Pisa, noted that what really caught the team’s attention was that these interactions were embedded in a selective and consistent social relationship between the two individuals.
Beyond Reproduction: Grooming, Licking, and Social Bonding
While the mounting behavior initially drew the researchers in, the broader social dynamic proved far more compelling. The two animals spent most of their shared time in close physical proximity, engaging in high frequencies of affiliative behaviors such as licking, rubbing, and nuzzling. Martina Francesconi, lead author of the study and a PhD student at UNIPI, emphasized that the key finding is not simply that a bull mounted a male donkey, but that this interaction formed part of a broader, selective social relationship.
Crucially, the researchers observed that these gentle grooming sessions occurred not just before mounting, but frequently afterward. Compared to their interactions with other animals, Achille and Patroclo engaged in longer and more frequent friendly exchanges, such as nuzzling, rubbing, and licking. This pattern suggests the physical interactions served to prolong their social connection well beyond the sexual act itself.
In one recorded incident, when Achille attempted to mount another donkey, Patroclo approached the bull closely to intervene, prompting Achille to redirect his attention back to the donkey and begin licking him. Researchers published these findings in the journal Current Biology, opening a new window into how animals navigate complex social environments.
Scientific Implications and Questions Left Unanswered
Traditional evolutionary theory often categorized non-reproductive sexual behavior as a costly mistake or evolutionary paradox, but recent studies suggest these behaviors play essential roles in social bonding, tension reduction, and relationship maintenance.
Researchers acknowledge the limits of interpreting animal behavior through human lenses. As the study’s co-authors note, they see this study as a starting point for asking these broader questions.
Whether the bond between Achille and Patroclo survives their current separation remains unknown. Palagi’s students have returned to Ethiopia, intending to check with herders about a possible reunion and to see whether their months apart have weakened their bond.
