An investigation published in the Current Biology journal documented a persistent sexual and social relationship between a bull and a male donkey, both males, living on an Ethiopian plateau. The researchers observed 51 mounting episodes over two weeks, accompanied by proximity and interaction behaviors that suggest a bond between the two animals.
An unusual relationship between a bull and a donkey is helping scientists better understand the social dimension of animal sexual behavior. The investigation, conducted by an international team and published on November 2 in the scientific journal Current Biology, describes the relationship between Achille, a bull, and Patroclo, a male donkey, who live on the same Ethiopian plateau.
The two animals belong to different owners and meet only when they are grazing. Although they have access to other potential partners, including females and animals of their own species, they maintained a particularly close relationship, with sexual interactions and affiliative behaviors, such as licking, rubbing bodies against each other, and touching noses.
The initial observations emerged when the researchers were conducting fieldwork as part of another study, initially devoted to Ethiopian baboons. The team witnessed Achille mounting Patroclo and was surprised by the donkey’s lack of defensive reaction, which did not attempt to move away from the bull nor avoid contact.
Following this observation, the researchers decided to systematically monitor the relationship between the two animals.
51 mounting episodes in two weeks
Between March 6 and March 19, 2026, the team recorded 51 mounting episodes between Achille and Patroclo. During this period, the two animals repeatedly sought closeness to one another and focused most of their social and sexual interactions on each other.
Patroclo even frequently presented the hindquarters to Achille and bit him, behaviors that the researchers recorded in the context of their relationship.
The contrast with other interactions was particularly notable. Achille mounted another male donkey only twice, but in those episodes, the animal reacted in a clearly different way, seeking to avoid contact and displaying threat cues, such as flattening the ears.
“The sexual interactions between Achille, the bull, and Patroclo, the donkey, were not isolated episodes,” explains Elisabetta Palagi, a researcher at the University of Pisa in Italy. “What really drew our attention was that these interactions were integrated into a highly selective and consistent social relationship between the two individuals.”
Sexual behavior may also play a social role
For the researchers, the main interest of the discovery is not only the sexual behavior between animals of different species, but the existence of a continued relationship, marked by proximity and reciprocity.
“We believe that the main finding is not simply the fact that a bull mounts a male donkey. It is that this unusual sexual behavior was part of a broader and more selective social relationship,” says Martina Francesconi, the study’s first author and a PhD researcher at the University of Pisa.
The researcher adds that the observations do not allow a precise explanation of the function of these interactions, but they make it unlikely that they are simply a consequence of a lack of mating alternatives.
The team suggests that the behavior may be related to the creation and maintenance of social bonds, the reduction of tensions, or a combination of social and sexual motivations.
“One of the main questions for future investigations is to understand what mechanisms, and potentially what benefits, may underlie these interactions,” says Elisabetta Palagi. “Will they be primarily related to the creation of social bonds? Could they play a role in maintaining relationships or reducing tension? Or will they result from a combination of social and sexual motivations?”
The researchers argue that the study could pave the way for new cross-species comparisons among species with different social structures, helping to understand how relationships between animals influence sexual behavior.
Thus, the investigation reinforces that sexuality in the animal world should not be analyzed exclusively from a reproductive perspective, but may also incorporate complex social dynamics.
