Sperm Wars
Front cover of original 1996 publication
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| Author | Robin Baker |
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| Country | United Kingdom |
| Subject | Evolution, Human sexuality |
| Publisher | Fourth Estate, London |
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Publication date
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1996 |
| Media type | Print, Paperback |
| Pages | 384 |
| ISBN | |
Sperm Wars is a primarily nonfiction book by evolutionary biologist Robin Baker. Originally published in English in 1996, it has since appeared in 25 languages and in 2006 a 10th anniversary edition was published in the United States. Through a series of short fictional stories and discussion following them, Baker proposes evolutionary functions for sexual habits, mostly on the principle of competition between sperm of different men for a prized egg. The "sperm wars" include both literal battles between sperm inside a woman's reproductive tract, as well as figurative battles between men competing for the chance to mate. The book is controversial, both because of its explanations of homosexuality, sexual assault, and prostitution, and because some critics have claimed that several of the hypotheses in the book are not supported by scientific research.
Oral sex is explained as an opportunity for partners to judge each other's reproductive health, and for mates to detect recent infidelity (also proposed by Kohl & Francoeur, 1995 "The Scent of Eros"). The shape of the penis and the thrusting during intercourse serve to remove other men's semen. Male masturbation is said to discard old, dying sperm, so that an ejaculate contains younger sperm that will stay active inside the cervix longer, with more of a chance of being present during the window of ovulation. Baker also proposes that men adjust the amount of sperm they ejaculate based on the time their mate has spent away from them. Likewise, women are found to be more likely to engage in extra-pair copulation and retain larger amounts of sperm during their most fertile phase of the month, and more likely to have sex with their regular partner during the infertile phase.
A major focus of the book is sperm heteromorphism, in which not only are a variety of morphological types of sperm apparent in every normal human ejaculate, but also at any one time fewer than 1% seem capable of responding to and fertilizing an egg. A similarly low proportion of fertile sperm is found in the ejaculates of mice. Baker calls these fertile sperm "egg-getters", and claims that the rest of the sperm in the ejaculate are infertile "kamikaze sperm" or "blockers", whose primary purpose is to prevent other men's sperm from getting to the egg. He cites data suggesting that in Britain at least 4% of children (but perhaps as many as 6–12%) are conceived via sperm competition, and claims that this lower figure is consistent with the earlier finding that 10% of children have a biological father who is other than their supposed father. Baker describes in detail how "killer sperm" actively seek out rival sperm and kill them with poison from acrosomes to prevent them from getting to the egg. This literal sperm warfare was not observed in a subsequent experiment, but Baker criticizes the protocol in this latter experiment.
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