Out 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 unanticipated tryst: 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 encountered each other when their separate owners brought them and other animals to a shared field to graze, the scientists reported November 2 in the journal Current Biology. “An unexpected encounter led us to an entirely new research question,” said the study’s senior author Dr. Elisabetta Palagi, an associate professor of biology at the University of Pisa in Italy. When the bull and donkey did meet, sparks flew. The bull would mount the donkey over and over again, and the donkey “never avoided these attempts,” the researchers wrote. In fact, the donkey frequently presented his hindquarters to the bull. The bull also licked the donkey’s body and gently butted him with his head, both common mating behaviors between bulls and cows. “Reports like this are the first step towards increasing our understanding of the true breadth of sexual behaviours 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 U.K., told CNN in an email. “We know so little about non-reproductive sexual behaviours across the animal kingdom,” said Brindle, who was not involved in the new research. “These behaviours are often dismissed as unimportant, or even aberrant,” and therefore are not recorded. Such observational field reports of same-sex interactions between different species, she added, are critical for charting this understudied behavioral spectrum. A deep attachment Researchers named the bull Achille and the donkey Patroclo as 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 central to the story,” Palagi told CNN in an email. “Homer does not explicitly describe them as lovers, although later ancient authors interpreted their relationship in that way.” Achille the bull and Patroclo the donkey, however, were definitely engaging in sexual activity. Of the 51 mounting episodes, the researchers observed 34 instances of penetration. It’s unclear when the animals’ relationship began, but local herders were already well acquainted with Achille and Patroclo’s actions by the time the scientists arrived and began their observations. “When they saw our researchers pointing cameras towards the pair, it prompted quite a bit of laughter on the plateau,” Palagi recalled. “What was unexpected for us was evidently a familiar sight to them.” Preferential association Scientists have previously documented many other examples of cooperative behaviour between species, including sexual encounters. And it’s not just males that cross species lines for sexual gratification, Brindle said. “Female Japanese macaques have been known to mount sika deer and masturbate onto their backs,” she said. “Although these aren’t reciprocal relationships, the deer are somewhat indifferent to the female macaques’ antics.” Males mounting other males also has been recorded in different types of animals. There’s even one famous case in the Netherlands of a male mallard mounting another male duck that was dead. What struck the researchers about Achille and Patroclo’s social relationship — and what is less understood — was their repeated sexual activity and their exclusivity, Palagi said. Achille and Patroclo were often seen close together, and were more likely to lick, rub and nuzzle each other than the rest of the animals in their shared grazing area. “Both animals had access to other potential partners, including individuals of their own species and of both sexes, yet they preferentially associated with each other,” she said. That’s not to say that Achille didn’t occasionally try mounting other male donkeys. There were two attempts, during which the other donkeys flattened their ears, kicked at the bull and sidestepped away. Patroclo, by comparison, never showed aggression or annoyance while Achille was mounting him. And when Patroclo approached Achille during one of these outlier episodes, the bull would instantly abandon his unwilling partner and redirect his attention toward Patroclo. On two occasions when a different bull approached Patroclo, the donkey bared his teeth, bit the bull’s flanks and chased it away. ‘Perhaps they just liked each other’ Mounting might be an act associated with creating and maintaining the pair’s social bond, Palagi suggested, but it’s unclear if the behavior initiated their relationship or if it happened later. Domestication explains in part what brought them together in that field, but it only tells a small portion of their story, she added. “This study concerns one pair, so it cannot establish how common such relationships are or what benefits they provide,” Palagi said. “It does, however, offer a systematically documented case that raises broader questions about animal social flexibility.” Brindle agrees that Achille’s and Patroclo’s behaviors don’t reveal an obvious evolutionary strategy or benefit — but that doesn’t mean these behaviors are abnormal. “Perhaps they just liked each other,” she said. Palagi and her coauthors have since focused their attention back on gelada baboons, but they nonetheless wondered if the union of Achille and Patroclo would stand the test of time. Domesticated animals’ living situations are at the whim of their owners, who may thwart even the strongest bonds. After the researchers left Ethiopia, they heard that Achille and Patroclo had been separated. Their herders moved the unlikely couple to different grazing areas because the field they once shared no longer had enough grass. However, all may not be lost for these friends with benefits. “My students are now back in Ethiopia and are trying to arrange for the animals to be reunited after several months apart,” Palagi said. “If this becomes possible, we will record their reunion to see whether they resume their previous interactions. For now, we cannot say whether their relationship has persisted through the separation.” Mindy Weisberger, CNN
