Scientists in Ethiopia are baffled by a bull and a male donkey that keep choosing each other over females

University of Pisa researchers stumbled on the bond by accident during a lunch break in Ethiopia, when PhD student André da Cruz spotted a bull mounting a donkey in the distance and his colleagues refused to believe him. Over four days of observation, Achille the bull mounted Patroclo the male donkey 51 times, with 34 resulting in penetration — while Patroclo actively presented himself and even nibbled Achille back.

Other donkeys flatly rejected Achille with ear-flattening and threatening expressions; Patroclo never once kicked or avoided him.

The pair belong to different owners and only met when their herds shared the same pasture, making their selective bond even harder to explain.

'Nature does not always respect the boundaries we draw around it,' lead researcher Dr Elisabetta Palagi told scientists — the findings are published in Current Biology.