A socio-sexual bond between a bull and a male donkey

Same-sex sexual behaviour is widespread in animals^1,2, yet to our knowledge observations of such behaviour between individuals of different taxonomic families remain rare. In a grazing area on an Ethiopian plateau (Debre Libanos, Oromia), Achille, a bull (Bos taurus/indicus), and Patroclo, a male donkey (Equus asinus), actively engaged in reciprocal socio-sexual interactions, thus forming an unusual association. Although interpreting this behaviour is challenging, the mounting attempts, taken together with the repeated and mutual affiliative interactions between the two animals, may signal the quality of their relationship. The interactions must have reproductive opportunity costs, in time and energy diverted away from potentially functional reproductive activities, and it would seem that these costs are outweighed by the strength of the animals' social bond³.