Sperm Wars

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,{{cite web|url=http://www.robin-baker.com/books/sperm-wars/status/|title=Sperm Wars: controversies and science - Robin Baker|website=www.robin-baker.com|date=6 March 2011 }} 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.Gallup, G.G. & Burch, R.L. (2004) [http://journals.sagepub.com/doi/pdf/10.1177/147470490400200105 Semen Displacement as a Sperm Competition Strategy in Humans]. Evolutionary Psychology, 2, 12–23. 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.Baker, R.R. & Bellis, M.A. (1989). Number of sperm in human ejaculates varies in accordance with sperm competition theory. Anim. Behav., 37, 867–869. 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.Bellis, M.A. & Baker, R.R. (1990). [http://www.springerlink.com/index/wt374233132n7382.pdf Do females promote sperm competition?: Data for humans]{{dead link|date=February 2020|bot=medic}}{{cbignore|bot=medic}}. Anim. Behav., 40, 997–999.

A major focus of the book is sperm heteromorphism, in which not only are a variety of morphological typesBelsey, M.A. et al. (1987) WHO Laboratory Manual for Examination of Human Semen and Semen-Cervical Mucus Interaction. 2nd edn, Cambridge University Press, Cambridge 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.Quinn, P. & Marrs, R.P. (1991) Effect of concentration and total number of spermatozoa on fertilizationof human oocytes in vitro. Hum. Reproduc. 6, Suppl. 1, Abst 238 A similarly low proportion of fertile sperm is found in the ejaculates of mice.Siddiquey, A.K.S. & Cohen, J. (1982) [http://www.reproduction-online.org/content/66/1/237.full.pdf In vitro fertilization in the mouse and the relevance of different sperm-egg concentrations and volumes]. J. Reprod. Fertil. 66, 237–42 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.Baker, R.R. & Bellis, M.A. (1988). [http://www.springerlink.com/index/w677360445578627.pdf ‘Kamikaze’ Sperm in Mammals?]{{dead link|date=February 2020|bot=medic}}{{cbignore|bot=medic}} Anim. Behav., 36, 936–939. He cites data suggesting that in Britain at least 4% of children (but perhaps as many as 6–12%) are conceived via sperm competition,Baker, R.R. & Bellis, M.A. (1995) [https://books.google.com/books?id=pSnSAwAAQBAJ Human Sperm Competition: copulation, masturbation and infidelity]. Chapman & Hall, London. pp.201–2 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.Macintyre, S. & Sooman, A. (1992) Non-paternity and prenatal genetic screening. Lancet, 338, 839. 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,{{cite journal|journal=Whiterose.ac.uk|url=http://eprints.whiterose.ac.uk/197/|title=No evidence for killer sperm or other selective interactions between human spermatozoa in ejaculates of different males in vitro|date=7 December 1999|volume=266|issue=1436|page=2343|last1=Moore|first1=H. D. M.}} but Baker criticizes the protocol in this latter experiment.

Based on studies with a fiber optic endoscope attached to a man's penis during sex, Baker describes how the cervix behaves during sex and orgasm, dipping down "like an elephant's trunk" into the pool of semen so that the sperm can more easily swim into it, the so-called "upsuck theory" of female orgasm. This has also come under some criticism; see Cervix#Function. He theorizes about how the timing of orgasm can affect fertility, due to changes in the cervical mucus said to accompany orgasm, which then affect its ability to "filter out" sperm and prevent them from traveling into the cervix. Other studies have found no connection to sperm transport or fertility.Bancroft, 1989Masters & Johnson, 1966

Although Baker draws a clear distinction between predatory rape and date rape, the book has drawn criticism for portraying date rape and "rough-and-tumble intercourse" as being on the same spectrum of behaviour: a test of the male's strength and ability. Despite Baker asserting that the two behaviours are at different (illegal and legal) ends of that spectrum, his statement that the drawing of the line between criminal and non-criminal behaviour is a job for the legal profession and not for the biologist has led some to interpret his words as meaning that he sees no real difference between the two:[http://findarticles.com/p/articles/mi_m2372/is_n1_v35/ai_20746731/ Sperm Wars: The Science of Sex. - book reviews] "Baker makes few distinctions between these two types of aggression and attributes both to female testing of male partners' strength and ability to overcome her resistance"

{{quote|Initially, she can simply watch him in competition with other males. ... But finally, the only real test a woman can set is whether a man can negotiate and overcome her own defences. To test this, she has to resist first verbally, then physically. The stronger and more realistic her resistance, the better the test.}}

In this context he makes comparison with the many examples of aggressively induced ovulation in mammals, including the extreme example of mink in which if the female does not experience physical trauma at the male's hands, she does not ovulateFord, C.S. and Beach, F.A. (1952) Patterns of Sexual Behavior. Eyre and Spottiswoode, LondonVan Tienhoven, A. (1968) [https://www.cabdirect.org/cabdirect/abstract/19690102113 Reproductive Physiology of Vertebrates]. W.B. Saunders, London.

Regarding sexual orientation, he states that true homosexuals are rare; that only 6% of the male population engages in any sort of homosexual behavior in their lifetime, and that 80% of those also have sex with women, so he focuses on bisexuality. "It seems most likely that exclusive homosexuality is a genetic by-product of the reproductively advantageous characteristic of bisexuality. If so, homosexual behaviour joins the ranks of a number of other human characteristics that are advantageous when a person has inherited a few of the relevant genes, but disadvantageous if they have inherited more."

Bisexuality in both men and women is explained as an adaptive trait because it provides earlier opportunities to gain sexual experience, and more opportunities to practice skills such as infidelity and interacting with people of different personalities. Experience gained with a member of the same sex of a particular character type can help the bisexual to get the most out of a relationship with a member of the opposite sex of a similar character type. Although studies show that by the age of 40 years bisexuals have fewer children than heterosexuals,Essock-Vitale, S.M. & McGuire, M.T. (1985) Women's lives viewed from an evolutionary perspective: I. Sexual Histories, reproductive success, and demographic characteristics of a random sample of American women. Ethol. Sociobiol., 6, 137–54Baker, R.R. & Bellis, M.A. (1995) Human Sperm Competition: copulation, masturbation and infidelity. Chapman & Hall, London. pp.117 he maintains that at low levels in the population bisexuality is still an evolutionary advantage since the children are typically conceived earlier in life; by age 20 years bisexuals have four times the reproductive success of heterosexuals, and by age 25 twice the success.Baker, R.R. & Bellis, M.A. (1995) Human Sperm Competition: copulation, masturbation and infidelity. Chapman & Hall, London. pp.117–8 Homophobia is then explained as a natural response to the threat of this reproductive advantage (despite the fact that true homosexuals are not in competition for females), as well as "the bisexual's role in the spread of disease".

{{reflist}}

• {{cite web|title=Sperm Wars: The Science of Sex. - book reviews |url =http://findarticles.com/p/articles/mi_m2372/is_n1_v35/ai_20746731/|website=Journal of Sex Research |date= February 1998 |archive-url= https://web.archive.org/web/20070222190726/http://findarticles.com/p/articles/mi_m2372/is_n1_v35/ai_20746731/ |archive-date=22 February 2007}}
• [http://www.flatrock.org.nz/topics/men/sex_best_when_you_lose_your_head.htm Sex Is Best When You Lose Your Head] - on killer sperm and testicle size

Category:Books about evolution

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