On the plateaus of the Ethiopian highlands, a team of biologists set out to study high-altitude gelada baboons, a type of grazing monkey. Instead, they got a front row seat to an unscheduled meeting: a bull and a male donkey engaged in a display of interspecies sex.
Researchers observed the bull (Bos indicus) repeatedly mount the same male donkey (Equus asinus) a total of 51 times over four days in March. The animals did not live together and they only met when their separate owners brought them and other animals to a common field to graze, the scientists reported Nov. 2 in the journal Current Biology.
“An unexpected encounter led us to a completely new research question,” said the study’s lead author, Dr. Elisabetta Palagi, associate professor of biology at the University of Pisa in Italy.
When the bull and the donkey met, sparks flew.
The bull rode the donkey again and again, and the donkey “never avoided these attempts,” the researchers wrote. In fact, the donkey frequently presented its hindquarters to the bull. The bull also licked the donkey’s body and gently nudged it with its head, two common mating behaviors between bulls and cows.
“Reports like this are the first step toward better understanding the true extent of sexual behaviors that occur in non-human animals,” evolutionary biologist Dr. Matilda Brindle, a postdoctoral researcher in the department of biology at the University of Oxford in the United Kingdom, told CNN.
“We know so little about non-reproductive sexual behavior in the animal kingdom,” said Brindle, who was not involved in the new research. “These behaviors are often considered unimportant, or even aberrant,” and are therefore not recorded. Such field observational reports of same-sex interactions across different species, she added, are essential for mapping this understudied behavioral spectrum.
Researchers named the bull Achilles and the donkey Patroclus in a nod to the ancient Greek heroes Achilles and Patroclus, whose close and openly affectionate relationship was described in Homer’s “Iliad.”
“Their deep attachment is at the heart of the story,” Palagi told CNN in an email. “Homer does not explicitly describe them as lovers, although later ancient authors interpreted their relationship that way.”
Achilles the bull and Patroclus the donkey, however, were definitely engaging in sexual activity. Of the 51 escalating episodes, researchers observed 34 cases of penetration. It is unclear when the relationship between the animals began, but local shepherds were already well aware of the actions of Achilles and Patroclo by the time the scientists arrived and began their observations.
“When they saw our researchers pointing the cameras at the two men, it caused quite a bit of laughter on set,” Palagi recalls. “What was unexpected for us was obviously a familiar sight for them. »
Scientists have already documented many other examples of cooperative behavior between species, including sexual encounters. And it’s not just males who cross species for sexual gratification, Brindle said.
“Female Japanese macaques have been known to ride sika deer and masturbate on their backs,” she said. “Although these are not reciprocal relationships, the deer are somewhat indifferent to the antics of the female macaques.”
Males mounting other males have also been recorded in different types of animals. There is even a famous case in the Netherlands where a male mallard duck rode another dead male duck.
What struck researchers about the social relationship between Achilles and Patroclo — and what is less well understood — was their repeated sexual activity and exclusivity, Palagi said. Achilles and Patroclo were often seen in close proximity to each other and were more likely to lick, rub, and snuggle than the rest of the animals in their shared grazing area.
“Both animals had access to other potential mates, including individuals of their own species and both sexes, but they associated preferentially with each other,” she said.
This is not to say that Achilles did not occasionally try to ride other male donkeys.
There were two attempts in which the other donkeys flattened their ears, kicked the bull and walked away. Patroclus, in comparison, never showed any aggression or annoyance while Achilles was riding him.
And when Patroclus approached Achilles during one of these aberrant episodes, the bull would instantly abandon his reluctant partner and redirect his attention to Patroclus. Twice, when another bull approached Patroclus, the donkey bared its teeth, bit the bull’s flanks and chased it away.
Mounting might be an act associated with creating and maintaining the couple’s social bond, Palagi suggested, but it’s unclear whether the behavior sparked their relationship or happened later. Domestication is part of what brought them together in this area, but it only tells a small part of their story, she added.
“This study only involves one couple, so it cannot establish how common these relationships are or what benefits they provide,” Palagi said. “It does, however, offer a systematically documented case that raises broader questions about the social flexibility of animals.”
Brindle agrees that the behaviors of Achilles and Patroclo do not reveal an obvious evolutionary strategy or advantage – but that does not mean these behaviors are abnormal.
“Maybe they just loved each other,” she said.
Palagi and his coauthors have since refocused their attention on gelada baboons, but they nonetheless wonder whether the union of Achilles and Patroclo would stand the test of time. The living conditions of pets depend on the whims of their owners, which can thwart even the strongest bonds. After the researchers left Ethiopia, they learned that Achilles and Patroclus had been separated. Their shepherds moved this unlikely pair to other grazing areas because the field they once shared no longer had enough grass.
But all may not be lost for these friends with benefits.
“My students are now back in Ethiopia and trying to organize the reunion of the animals after several months apart,” Palagi said. “If it becomes possible, we will record their reunion to see if they resume their previous interactions. At this time, we cannot say whether their relationship persisted during the separation.”
Mindy Weisberger is a science writer and media producer whose work has appeared in Live Science, Scientific American, and How It Works magazines. She is the author of “Rise of the Zombie Bugs: The Surprising Science of Parasitic Mind-Control” (Hopkins Press).
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