A bull and a male donkey living on an Ethiopian plateau have formed an unusual social and sexual relationship that scientists say challenges common explanations for interactions between different animal species. The two domesticated animals repeatedly seek each other out, spend time together and engage in behavior that researchers believe extends beyond sexual activity alone.
The animals, named Achille and Patroclo after two heroes of Greek mythology, were the subjects of a study published November 2, 2026, in the scientific journal Current Biology. Researchers from the University of Pisa in Italy documented 51 mounting incidents between the pair, along with repeated licking, rubbing and nuzzling.
Their findings suggest that sexual interactions between animals may sometimes be closely connected to selective social relationships rather than reproduction or dominance. However, the scientists caution that observing one unusual pair cannot establish how frequently similar relationships occur or explain the animals' emotional experiences.
An Unexpected Discovery in the Ethiopian Highlands
The research began by accident when scientists studying gelada monkeys in Ethiopia noticed something unusual in a nearby grazing area. André da Cruz, a doctoral researcher at the University of Pisa, first spotted a bull mounting a donkey while taking a lunch break with colleagues.
The observation attracted attention because both animals were male and belonged to different species. More surprisingly, the donkey did not attempt to escape, kick or otherwise resist the bull.
The researchers initially questioned what they had witnessed. After observing further interactions, they decided to investigate whether the encounter was an isolated event or part of a continuing relationship.
The bull and donkey belonged to different owners whose herds used the same grazing area during the day. They were separated when their respective herds returned home, making their repeated interactions particularly interesting.
The researchers named the bull Achille and the donkey Patroclo, references to Achilles and Patroclus from Greek mythology. The legendary warriors are remembered for their close relationship, which has been interpreted in different ways throughout history.
Rather than simply describing an unusual sexual encounter, the scientists began documenting how the animals behaved whenever they encountered each other.
Researchers Record 51 Mounting Incidents
The team conducted observations between March 6 and March 19, 2026. Researchers systematically monitored the animals during periods when they shared the pasture and when only one was present.
Across the observation period, they recorded 51 occasions when Achille mounted Patroclo. The researchers also documented numerous nonsexual interactions that helped distinguish the relationship from a series of random encounters.
The pair frequently remained close together and engaged in licking, rubbing and nuzzling. These behaviors occurred before and after mounting, suggesting that their interactions were not limited to sexual activity.
Patroclo also displayed behavior that researchers interpreted as active participation. He sometimes presented his hindquarters to Achille and did not show the defensive reactions seen in other donkeys approached by the bull.
That distinction became particularly important when Achille attempted to mount another male donkey. The other animal responded with avoidance and threatening facial expressions, including flattening its ears.
Patroclo behaved differently. He repeatedly remained near Achille and appeared willing to continue their interactions.
The researchers also observed occasions when Patroclo approached Achille while the bull was interacting with another animal. In one instance, the donkey appeared to interrupt Achille's attention toward another donkey.
Such observations helped the team identify a consistent preference between the two animals. Their relationship appeared selective even though other potential partners were available.
Why Scientists Consider the Relationship Unusual
Same-sex sexual behavior has been documented in numerous animal species, including mammals, birds and other groups. Scientists have identified several possible explanations, ranging from social bonding and conflict reduction to dominance and sexual behavior unrelated to reproduction.
Sexual interactions between different species are also known, but many documented cases involve coercion, mistaken identification or limited access to appropriate mates. Researchers say the relationship between Achille and Patroclo does not fit those explanations easily.
Both animals had access to members of their own species and to potential opposite-sex partners. Nevertheless, they repeatedly directed much of their social and sexual attention toward each other.
Elisabetta Palagi, an animal behavior researcher at the University of Pisa and co-author of the study, described the animals' repeated preference as a particularly important finding.
The researchers observed no clear signs that Patroclo was being forced into the interactions. His responses contrasted with those of other donkeys that reacted defensively when approached by Achille.
That does not establish what either animal felt or intended. Animal behavior researchers must distinguish measurable actions from interpretations involving emotions, affection or attraction.
The study therefore describes the relationship as a selective social and sexual association. This wording recognizes the repeated interactions without claiming that scientists can determine the animals' internal experiences.
Social Contact Continues After Sexual Activity
One of the study's most interesting observations involved what happened after mounting. Achille and Patroclo frequently continued interacting through licking, rubbing and other friendly contact.
These behaviors are known as affiliative interactions, meaning actions that help establish or maintain positive social contact. They can include grooming, touching and spending time close together.
The fact that such interactions continued after mounting led the researchers to question whether sexual behavior was serving a broader social function. The activity may have been part of an ongoing relationship rather than a separate event.
Martina Francesconi, the study's lead author and another doctoral researcher at the University of Pisa, emphasized that the unusual behavior was connected to a broader pattern of repeated and selective contact.
The researchers consider that pattern more scientifically significant than the mounting itself. It provides evidence that the animals' interactions were not simply accidental encounters between two species sharing a pasture.
However, the researchers could not determine whether the social relationship developed before the sexual behavior or whether repeated sexual interactions strengthened the association.
They also could not establish exactly what biological or social benefits the animals might have gained. Further observations would be necessary to investigate those possibilities.
What the Findings Mean for Animal Behavior Research
The study adds to growing scientific interest in the social functions of sexual behavior among animals. Researchers have increasingly examined how such behavior may contribute to relationships beyond its reproductive role.
Some species use sexual contact in social situations involving alliances, reconciliation or tension reduction. However, those findings cannot automatically explain the relationship between a bull and a donkey.
Achille and Patroclo present a different research question because they belong to separate domesticated species with distinct social behaviors. Their repeated preference suggests that meaningful social associations can sometimes form outside ordinary species boundaries.
The case also demonstrates the importance of observing animals without assuming that every interaction must fit established categories. The researchers had traveled to Ethiopia to study primates, yet an unexpected encounter among livestock produced a separate scientific investigation.
Still, the study has significant limitations. It describes only two animals observed for a relatively short period, so its findings cannot establish how common similar relationships are.
Scientists also cannot use the observations to assign human sexual identities to the animals. The research documents specific behaviors and social preferences rather than defining either animal through human concepts of identity.
Understanding whether the pattern represents a rare exception or an overlooked form of animal interaction will require additional studies.
Researchers Hope to Observe the Pair Again
The researchers are interested in determining whether Achille and Patroclo maintain their relationship over time. Changes in grazing conditions have complicated that effort.
The pasture where the animals previously met dried out during the dry season, prompting their owners to move the herds to different grazing areas. As a result, the bull and donkey were separated after the original observations.
Researchers working in Ethiopia have been exploring whether the owners can arrange another meeting. Such an opportunity could help determine whether the animals recognize each other and resume their previous patterns of interaction.
A reunion would also allow scientists to study how separation affects a relationship that appeared unusually selective. Repeated observations over a longer period could provide stronger evidence about its stability.
For now, the published findings document a rare case in which two domesticated males from different species repeatedly preferred each other's company and engaged in both sexual and friendly behavior.
The most important unanswered questions concern why the relationship formed, what function the interactions served and whether comparable associations occur elsewhere. Those questions remain open as researchers seek another opportunity to observe Achille and Patroclo together.