A bull and a male donkey from different owners in Ethiopia have formed what researchers are calling a "special relationship" — a repeated, mutual bond that included daily meetings, grooming and sustained physical contact over several days, according to a case study published this week in the journal Current Biology.
The animals were first noticed by André da Cruz, a Ph.D. student with a University of Pisa team that was in Ethiopia studying geladas, the primates sometimes called bleeding-heart monkeys. During a break from fieldwork, the researchers spotted the bull and donkey together in a shared pasture and began tracking their behavior.
Fifty-one encounters in four days
Over four days of observation, the bull mounted the donkey 51 times. Crucially, the researchers say, the behavior did not look like the coercive, accidental or mistaken-identity interactions that have occasionally been documented between different species before. The bull and donkey sought each other out specifically, the donkey intervened when the bull approached other animals, and the pair groomed — licking and rubbing against each other — both before and after mounting. Neither animal showed comparable interest in members of its own species at the pasture, though the bull did mount female cattle separately.
Elisabetta Palagi, who led the research team, said the case stood out precisely because of its consistency. "We interpret our data as a special relationship," she said, adding that "it is the first time it is reported in literature" — a reciprocal, selective bond between two large domestic species that belong to different owners and otherwise have no reason to interact.
Why researchers are paying attention
Co-author Arianna Francesconi noted that most previously documented cross-species sexual behavior has been explained away as forced, accidental or driven by mistaken identity — a male animal responding to unfamiliar cues rather than forming any lasting preference. This case, by contrast, repeated itself daily and appeared to be chosen by both animals.
Palagi argues the finding is also a methodological reminder: researchers who go looking for narrow, preconceived explanations for unusual animal behavior may simply miss how often it occurs. The team plans further observation of the pair, and the paper's authors say similar interspecies bonds, if looked for directly, may turn out to be less rare than assumed.
