In all animals, mating is a deal. But few creatures behave as if sex is a simple transaction. Many treat it with reverence, suspicion, angst and violence. In the case of the Black Grouse, the bird at the centre of Matt Ridley's investigation, the males dance and sing for hours a day, for several exhausting months, in an exhausting and sometimes deadly ritual called a 'lek'. To prepare for the ordeal, they grow, preen and display fancy, twisted, bold-colored feathers. But why are males the eager sellers and females the discerning buyers? Why do increasingly baroque and bizarre males put themselves at risk of attack by circling hawks and rival birds? And why are these displays considered beautiful by humans at all?