The strange bond between a bull and a male donkey in Ethiopia makes biological researchers extremely surprised. Is this interspecies mating behavior just a random mistake or does it reveal the nature of sexuality in nature?

During a survey of gelada monkeys in the highlands of Ethiopia, scientists accidentally witnessed a rare phenomenon that shook familiar notions about animal behavior.

A bull and a male donkey have formed such a strong and strange bond that researchers decided to name them Achille and Patroclo - a reference to two closely linked characters in ancient Greek mythology.

A detailed report just published in the specialized journal Current Biology shows that this is not simply a peaceful coexistence between two species of cattle. During four days of intensive monitoring, the research team recorded up to 51 times the pair climbed on each other, of which 34 times occurred actual intrusive mating behavior.

Notably, the male donkey Patroclo had absolutely no defense or avoidance reaction like another male donkey that had been approached by a cow. In contrast, Patroclo showed initiative by exposing his hindquarters and initiating physical contact. Immediately after each mating session, both animals give each other affectionate licking and rubbing gestures.

This raises a big question for the scientific community: why do two males of two completely different species persist in choosing each other as mates, even when there is no shortage of females of the same species around them?

In fact, homosexual behavior is not unusual in nature as it has been recorded in more than 1,500 species. A study published in 2023 on 261 mammal species also showed that this tendency is common in species with complex herd organizations, which play an important role in strengthening social structures and settling internal conflicts.

Primate researchers have long been familiar with the behavior of bonobos - where females regularly rub their genitals to relieve stress and strengthen solidarity.

Similarly, pods of male bottlenose dolphins often mate in same-sex mating alliances to hunt and approach mates, while females of the Japanese macaque monkey also form short-term emotional relationships to determine group status or relieve needs.

However, sexual behavior between two different species is a much rarer phenomenon. Although scientists have witnessed Japanese macaque monkeys trying to climb on the backs of sika deer, those interactions are often unilateral and lack the consent of the partner. The bond between Achille and Patroclo thus becomes a special case in point: they not only perform physical acts but also express a mutual emotional attachment and a clear preference for the other.

For a long time, evolutionary biology has always considered non-reproductive sexual behavior an intractable paradox. From the perspective of natural selection - which always prioritizes traits that help optimize the ability to maintain the species - the expenditure of energy and time on matings that cannot produce children is often simply explained by confusion of mates, a lack of females, or indiscriminate outbursts when near each other.

However, these arguments all stem from the prejudice that heterosexual behavior is the only norm, and all other behaviors are deviations.

In order to break that way of thinking, a groundbreaking study published in 2019 proposed a completely new hypothesis: the early ancestors of sexually reproducing animals may have had sex-independent sexual behavior. The ability to accurately identify mates and focus on mating with individuals of the opposite sex are actually just adaptive mechanisms that evolved in later stages.

The cases of Achille and Patroclo remind researchers that sex in the natural world does not necessarily have to serve directly for reproductive purposes to have evolutionary value. Strengthening biological alliances can indirectly improve survival chances, support the sharing of life resources, and create a foundation for individual survival.

Four short days of observation cannot summarize the entire history of this special couple's bond, but it is enough to show that the biological world always operates at diverse levels, far beyond the narrow theoretical models that humans have set for themselves.