The animal kingdom is filled with wild extravagances, and a lot of them have something to do with sex. Hermit Fiddler crabs wave their claws, swordtail fish flash their swordtails, manakins leap and buzz their wings. Darwin considered these displays so important and so puzzling (“the sight of a feather in a peacock’s tail, whenever I gaze at it, makes me feel sick!” he wrote to a colleague), that he dedicated half of a book to the subject.

Darwin argued that many extravagant displays in male animals were the result of a special kind of evolution he called sexual selection. Females preferred males with certain traits over other males, and so those males had more offspring, which inherited their traits.