Many male animals have evolved horns, so that they can fight over females. Until now, though, no one has wondered if male plants have done likewise. But that is what Andrea Cocucci of the Multidisciplinary Plant Biology Institute in Cordoba, Argentina, suspects has happened in milkweeds. These plants’ pollen, Dr Cocucci observes in a paper in the New Phytologist, is packaged into structures called pollinaria, which attach themselves intact to visiting insects and are thus carried to other flowers. Some pollinaria sport a pair of horns (see photo). Their job, he has found, is to fend off pollinaria from other flowers, stopping them being picked up by the same insect, and thus reducing competition for subsequent mating opportunities.