Abstract
We hypothesize that the general differences in breeding systems between birds and mammals results from a different outcome of the between-sex arms race. The arms race has been won by male mammals that can impose their mating decision over females, but has been lost by male birds because of the extreme specialization of forearms that do not allow males to trap and hold females during copulation. Male birds compensate for their inability to force copulation by developing “Don Juan” behaviour to persuade females to accept copulation.