At a singles cocktail party, the ending is often predictable. A female may choose a male from several because he is attractive. But why does she think he is good looking? Scientists, stumped by that question throughout the animal kingdom, hypothesized that something more than chemistry drives mate choice.

Looks are certainly important for the peacock, with his absurdly bright, burdensome train that he shows off to attract a female.

Peahens often choose males for the quality of their trains -- the quantity, size, and distribution of the colorful eyespots. Experiments show that offspring of males with more eyespots are bigger at birth and better at surviving in the wild than offspring of birds with fewer eyespots.

This way of choosing a mate is just one type of sexual selection: members of one sex mating in disproportionate numbers with members of the opposite sex that possess some "showy" feature. It might be ornate peacock plumage, large antlers on a deer, or a bird's particularly melodious mating call. Another type of sexual selection is combat among members of the same sex to choose an available mate.

But bigger is only better up to a point. If peacock trains become too big or too colorful over time, they may no longer confer a selective advantage. Exaggerated trains might attract a new kind of predator or become too heavy to carry around. Then, those super males die out and make room for the more ordinary males -- until another turn of the evolutionary wheel begins the cycle again.