An unusual sexual bond between a bull and a male donkey

Sex between animals of the same sex is widespread in nature. But researchers reporting in the Cell Press journal Current Biology on November 2 now describe an unusual partnership involving an ongoing sexual relationship between a bull and male donkey living on an Ethiopian plateau. While the two males could have easily chosen partners of the same species and opposite sex, they continued to choose each other, even though they belonged to different owners and only saw one another during time in the pasture.  

“The sexual interactions between Achille, the bull, and Patroclo, the male donkey, were not isolated events,” says Elisabetta Palagi of the Università di Pisa (UNIPI) in Toscana, Italy. “What really caught our attention was that these interactions were embedded in a very selective and consistent social relationship between the two individuals.” 

The researchers first witnessed the interactions between the two animals while in the field conducting another study. They observed Achille mounting Patroclo and were surprised to see that Patroclo didn’t try to avoid Achille, kick him, or show any defensive reaction at all. It prompted them to look at the relationship systematically. 

“They repeatedly chose to stay close to each other and engaged in many different affiliative behaviors, such as licking, rubbing, and nuzzling, alongside the repeated mounting behavior,” added author André da Cruz, a PhD student at UNIPI. “Importantly, this was happening in a social environment where both animals had access to other potential partners, including individuals of both sexes and of their own species.” 

The researchers recorded all interactions they witnessed involving either Achille or Patroclo. Altogether, they saw 51 mountings between the male bull and donkey from March 6 to March 19, 2026. During that time, Patroclo often presented his hindquarters to Achille and nipped him. The animals repeatedly directed most of their social and sexual interactions toward each other, with no indication from either that they wished to discourage those advances, according to the researchers. Achille mounted another male donkey only twice, and that other male showed clear avoidance behaviors and threatening expressions, such as flattened ears. 

“We believe the key finding is not simply that a bull mounted a male donkey,” says Martina Francesconi, lead author of the study and PhD student at UNIPI. “It is that this unusual sexual behavior was part of a broader, selective social relationship. Our observations suggest that it cannot be easily explained simply by a lack of alternative mating opportunities, although we cannot say from this study exactly what function these sexual interactions may have had.” 

The researchers say that these observations have broader implications for understanding sexual behavior as a complex social trait beyond its role in reproduction. They also raise important questions for future studies.  

“For us, a major question for future research is what mechanisms, and possibly what benefits, might underlie these interactions,” Palagi says. “Are they mainly related to social bonding? Could they play a role in maintaining relationships or reducing tension? Or do they emerge from a combination of social and sexual motivations? We think that comparative research across species with different social systems could help us distinguish between these possibilities.” 

“More broadly, we would like to better understand how social relationships shape sexual behavior and how behavioral flexibility allows animals to navigate complex social environments,” says Palagi. “We see this study as a starting point for asking these broader questions.” 

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The study was supported by the Leakey Foundation (S202310431), Dudley Zoo (UK), Fondazione ARCA (IT), Giardino Zoologico di Pistoia (IT), NaturZoo Rheine (DE), Parc des Félins (FR), Parco Natura Viva (IT), Parco Zoo Falconara (IT), Rotterdam Zoo (NL), Saint-Félicien (CA), Wilhelma Zoo (DE), Zoo de Cerza (FR), Zoo de La Boissière du Doré (FR) (AOO “BIO” 0005878/2022), and Fundação para a Ciência e a Tecnologia, I.P., under the project UID/4038/2025, PhD scholarship 2025.01273.BD.  

Current Biology, da Cruz et al., “A socio-sexual bond between a bull and a male donkey” https://www.cell.com/current-biology/fulltext/S0960-9822(26)01230-3

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