The unusual story of a bull and a donkey: Sex, affection and a lasting bond between two males of different species

Achilles is a bull that grazes almost every day in a meadow on the Ethiopian plateau. There, his herd meets another one made up of cattle and the occasional equid. Among them is one animal that is special to him. His name is Patroclus, and he is a donkey.

Whenever the two meet, Achilles seeks out Patroclus. The pair spend almost all their time together. The bull strokes the donkey’s back, licks his muzzle as if kissing him, sniffs his hindquarters and eventually mounts him. The donkey does not reject the bull. In fact, after copulating, they exchange gestures that can only be described as affectionate.

The group of biologists who studied the pair admits they are puzzled. Existing theories about the evolutionary role of homosexuality in the animal kingdom do not fit this case. “The observation of Achilles, a bull, and Patroclus, a male donkey, was totally unexpected,” says Elisabetta Palagi, an ethologist at the University of Pisa in Italy.

Palagi leads a research team that has spent years studying communities of geladas, a species of monkey, on the Ethiopian highlands. “While we were studying the geladas, my team saw Achilles mounting Patroclus,” she explains. “What surprised them most was the donkey’s reaction: he did not avoid the bull nor display the defensive behaviors they had seen in other donkeys.”

The shepherds looking after the two herds were already familiar with the encounters, which had been taking place long before the researchers arrived.

The discovery prompted the scientists to take a break from studying monkeys and investigate the relationship between the bull and the donkey, whom they named Achilles and Patroclus. Although Homer’s Iliad does not describe a sexual relationship between the two characters, it does portray the deep bond that united them. Achilles was inconsolable after the death of his friend, wishing to die himself, refusing to eat and seeking revenge during the Greek siege of Troy. The findings of the study were published Monday in the scientific journal Current Biology.

For eight days spread over two weeks, the biologists observed the two animals. During that brief period, Achilles mounted Patroclus dozens of times. They never saw Patroclus mount Achilles. Before and after the mountings, they recorded numerous affiliative interactions.

“These are friendly physical interactions, such as licking, rubbing against the other animal or nuzzling with the muzzle,” Palagi explains. Ethologists — scientists who study animal behavior — use that term to “describe observable behaviors associated with social closeness,” she adds. To human eyes, they look like caresses and displays of affection.

These affiliative interactions preceded the mountings, but they would resume once copulation had ended. “I would be cautious about calling these contacts foreplay: the term suggests their function was to prepare for sex, while our observations suggest they were part of the pair’s broader social relationship,” the researcher says.

The pattern was always the same: whenever the two herds met, Achilles and Patroclus, especially the former, sought each other out and spent far more time together than apart. This was despite the fact that both had access to males and females of their own species within their respective herds. “During our observations, we recorded 51 mounting events between them, including 34 with penetration,” Palagi says.

Achilles was not always faithful. Although the researchers never saw him with the bulls or cows in his own herd, they did observe him attempting to mount other donkeys. Those encounters were not successful. “The other donkeys showed avoidance or defensive behaviors, such as kicking or threatening facial expressions, whereas we never observed these responses in Patroclus during mounting interactions,” she says.

On one occasion, when Achilles was attempting to mount another donkey, Patroclus approached and interrupted the encounter. Achilles abandoned his efforts and turned his attention back to Patroclus, who, as the researcher notes, “also repeatedly presented his hindquarters to him.”

Palagi and her colleagues from the University of Pisa and Addis Ababa University eventually had to return to their work with the monkeys, leaving behind the story of Achilles and Patroclus and a number of unanswered questions.

Same-sex mounting has been observed in domestic livestock under particular circumstances, such as a shortage of females or the close proximity of males. But the situation involving this bull and this donkey is very different.

“Both had access to other potential partners, including individuals of their own species and of both sexes. Therefore, what distinguished their relationship was its selectivity and consistency,” says the Italian researcher. Simple explanations — sexual coercion, identity confusion or limited access to opposite-sex mates — do not apply to this case.

Homosexual behavior is not unusual in the animal kingdom. In fact, its presence across a wide range of mammalian species suggests an evolutionary role beyond reproduction alone. Its occurrence in all groups of primates, for example, points to deep evolutionary roots. But the story of Achilles and Patroclus does not fit neatly into those explanations either. The authors caution that a single case involving sexual behavior between two males of different species cannot be generalized.

“My interpretation is that their sexual interactions were tightly linked to a selective social bond. Both animals had access to other potential partners, yet repeatedly sought each other’s company,” Palagi says.

The researcher is cautious when asked about emotions. “The term affection provides an understandable description of this pattern, and I would not assume that the capacity for affectionate relationships is exclusively human,” she says.

However, she adds: “From a scientific point of view, we must distinguish between what we observe and what we infer about their emotional experience.” She continues: “We can document their mutual preference, their repeated friendly contacts and their sexual interactions; however, we cannot determine exactly what they felt”

Palagi is now back in Italy, but several of her students remain in Ethiopia. The meadow where Achilles and Patroclus used to meet has dried out during the dry season, prompting the shepherds to move their herds elsewhere in search of pasture.

“My students are trying to organize a meeting between the herders so that Achilles and Patroclus can reunite after several months apart,” the ethologist says.

If they succeed, the cameras will start rolling again. Then the researchers will find out whether the story of Achilles the bull and Patroclus the donkey is set to continue.

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