Promiscuity: An Evolutionary History of Sperm Competition

When mammals have sex, many sperm race to fertilize one egg. Does chance alone decide which sperm succeeds? What happens when sperm from different males chase the same egg or eggs? How are things different for the male and female gametes of squid, poultry, starfish or sharks? And how might female organisms benefit from choosing more than one mate? Such questions are the province of biologists who study sperm competition, an intriguing, sometimes bizarre field that draws on evolutionary theory, biochemistry and old-fashioned animal watching. Birkhead (Great Auk Islands), professor of behavioral ecology at Britain's University of Sheffield, has written an engrossing, accessible explanation of sperm competition and related elements of animal biology. Birkhead succeeds on two levels at once. He sets out evolutionists' nuanced arguments about sperm competition and sexual selection, and shows how their hypotheses have been tested. He also offers a fantastic array of biological believe-it-or-nots. The fish called capelin in effect mate in threes; two males at once assist the female capelin in pushing spawn out of her body. Male giant squid shoot long needlelike spermatophores from a penis nearly three feet long; the spermatophores stick in the female's skin, and no one knows how the sperm reach the eggs from there. Nobody's sure why such systems evolve: studies of house mice, Australian fairy wrens and Panamanian pseudoscorpions, though, might help explain them. Birkhead's work is solid and intriguing, a clear picture of many out-there phenomena: no one who cares for biology should miss it. (Oct.)