Male guppy fish increase their reproductive success by mating with unfamiliar females. When President Coolidge famously attributed the sexual ardour of roosters on a government farm to a steady stream of new hens1, he identified a strategy whereby males increase their reproductive success. By preferentially copulating with different females, a polygamous male will facilitate the spread of his genes and sire more offspring2. But in order to benefit, a male must either have the cognitive skills to recognize familiar females, or behaviour patterns that increase his likelihood of encountering new ones. We show here that wild guppies, Poecilia reticulata, do both.