For example, the belief that play helps some to practise for skills needed later in life is true - for some animals, and then just for young ones. Predatory animals play at predation, those that are preyed upon play at escape. Social animals beat each other up to establish rank and hierarchy. Of course, all animals play at sex. Animals play at mounting.
Humans play doctor.
But the difficulty is explaining why adults engage in play, an activity that has no goals at all. Which is why in most mammalian species, the adults leave playing to the young. Chick proposes that just as humans have selectively bred the wolf into the dog specifically for playfulness, so we have bred playfulness into our own selves by sexual selection. Males, he argues, can be dangerous. They rape and kill, especially when one deposes another in a social group.