The heart wants what the heart wants.
At least, that's one way of looking at an extraordinary relationship scientists discovered by chance during fieldwork in Ethiopia.
Over a series of observations, researchers documented an unusual sexual bond between a male donkey and a bull.
The observations, reported in Current Biology, show animal sexual behavior can be a more complex social phenomenon than we might realize, the team says.
"Nature does not always respect the boundaries we draw around it," ethologist Elisabetta Palagi from the University of Pisa in Italy told ScienceAlert.
"Animal behavior is not a picture painted in black and white: It uses the whole palette, with all the colors of the rainbow."
While same-sex sexual behavior (SSSB) is a widespread behavior among animals, observations of sex between animals of different species are much rarer.
Editor's note: The video below shows footage of the bull mounting the donkey.
Yet the interactions observed by Palagi's team between Achille (a bull) and Patroclo (a male donkey) might be something even rarer still – a bond that possibly transcended the sexual encounters they shared.
"Both animals had access to other potential partners, including individuals of their own species and of both sexes," Palagi said.
"Yet they repeatedly sought each other out."
The research team had traveled to the Ethiopian highlands to study gelada monkeys when they noticed by accident the interactions between Achille and Patroclo in a nearby grazing area.
Over a number of days, Achille was seen mounting Patroclo. Mounting was documented 51 times, with 34 of the events resulting in intromission (penile penetration).
Editor's note: Graphic image below of the bull mounting the donkey.
When Achille tried the same thing with another male donkey in the pasture, the donkey showed clear avoidance behaviors, including a display of threatening facial expressions (ear flattening).
In contrast, Patroclo never avoided or kicked away Achille, but instead appeared to present his hindquarters to the bull, and both animals engaged in other acts that caught the researchers' attention.
"Achille and Patroclo were more likely to exchange affiliative behaviors, such as licking, rubbing and nuzzling, with each other than with other animals, and these interactions lasted longer. They also spent most of their observed time close together," Palagi said.
"The sexual interactions were one component of a much broader, selective relationship."
According to the researchers, pre-mounting behaviors (involving acts such as licking and nuzzling) are not unexpected between animals, as they may help facilitate mounting.
But Patroclo and Achille's behavior was perhaps unusual, in that they engaged in post-mounting interactions much more often than any foreplay, suggesting there was more to their relationship than sex alone.
"Although interpreting this behavior is challenging, the interactions observed between Achille and Patroclo, embedded in repeated, reciprocal, and mutual affiliative interactions, may also function as a signal of relationship quality," the researchers write in their paper.
"Rather than being solely attributable to sexual arousal, these interactions appear to play a role in prolonging the social interaction beyond the sexual act."
On another occasion, when Achille mounted another donkey, Patroclo moved in close, as if to prevent further interactions between them, and Achille ended up interacting with Patroclo instead, including body licking.
According to Palagi, this apparent selectivity and the consistency of their association were the most surprising part of the research, especially since the animals belonged to different owners and spent their nights apart, only encountering each other during daily grazing.
"The mounting initially attracted our attention, but their repeated preference for each other's company became the most compelling part of the story," Palagi told ScienceAlert.
"Patroclo's behavior was particularly striking… His participation made the relationship especially intriguing."
According to the researchers, Achille and Patroclo's interactions appear to be the first documented case of a sustained same-sex sexual relationship between a bull and a donkey.
While we can't know exactly what the relationship meant to them – and need to be careful not to interpret their interactions through an anthropomorphic lens – it's clear something was going on here, suggestive of a social bond that crossed a boundary between species.
Whether that bond still exists is uncertain. After the study, Achille and Patroclo were moved to different grazing areas.
The researchers hope to return to see if the animals resume their activities, and to investigate whether similar relationships are occurring between species more often than we realize.
"Unusual behavior can remain anecdotal simply because nobody has the opportunity to document it systematically," Palagi said.
"Nature's full palette becomes visible only when we are willing to notice the unexpected."
The findings are reported in Current Biology.
This article was fact-checked by Fiona MacDonald and edited by Fiona MacDonald. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know.