Why Is Sex Fun?

To humans, the sex lives of many animals seem weird. In fact, by comparison with other animals, we are the ones with the weird sex lives. How did that come to be? We are the only social species to insist on carrying out sex privately. Stranger yet, we have sex at any time, even when the female can't be fertilized (for example, because she is already pregnant, post-menopausal, or between fertile cycles). A human female doesn't know her precise time of fertility and certainly doesn't advertise it to human males by the striking color changes, smells, and sounds used by other female mammals. Why do we differ so radically in these and other important aspects of our sexuality from our closest ancestor, the apes? Why does the human female, virtually alone among mammals, go through menopause? Why does the human male stand out as one of the few mammals to stay (often or usually) with the female he impregnates, to help raise the children that he sired? Why is the human penis so unnecessarily large? There is no one better qualified than Jared Diamond--renowned expert in the fields of physiology and evolutionary biology--to explain the evolutionary forces that operated on our ancestors to make