A new book by Robert Martin, Curator of Biological Anthropology at the Field Museum and affiliate of the University of Chicago, tackles the evolution of human reproductive biology (see his Psychology Today blog here too: http://www.psychologytoday.com/experts/robert-d-martin-phd
). The book focuses more on female issues such as pregnancy
, lactation, and maternal care, but also offers insights into male contributions too. This is not the book to learn about human sexual behavior or childcare, but it is a stimulating dose of comparative reproductive biology written in an engaging style.
Here, we focus on one issue Martin recognizes concerning novel evolutionary aspects of human reproduction. This is the use of male fertility control. “Deliberate interference in our own breeding is unique to humans, with a history stretching back thousands of years. Unlike [other] topics covered, there is no evolutionary precursor to birth control.” (p. 198) Although Martin doesn’t specifically note these examples, we can be assured that no primatologist has watched a tamarin male practice coitus interruptus (withdrawal before ejaculation) or a titi monkey use a condom. If males have their testicles removed, like occasionally can happen in a brutal male-male chimpanzee attack, that’s not to create a sterile eunuch, but part of taking out a member of the reproductive competition.
So how do human males sometimes constrain their reproductive output? There are a variety of ways, putting aside the larger reality that male competitors and the practice of female choice are the primary ways in which male reproductive output in curbed. Celibacy is one way—deliberately avoiding sex altogether. However, the reproductive imperative has made sexual desire an adaptation, making celibacy an uphill battle against human will.
Coitus interruptus and the rhythm method (timing intercourse to avoid presumed fertile phases) have been employed, but both have their problems too. For the rhythm method, the challenge has been accurately identifying when sex can give rise to conception, a topic that “How We Do It” addresses at length. Until early in the 20th century, the prevailing biomedical understanding was that conception was most likely to occur around the time of menstruation. Incidentally, cross-cultural conception beliefs have commonly held that a new life starts with the mixing of menstrual blood and male semen. The science of fertility gradually showed that instead sperm can survive typically around 2 days but sometimes even 5 or so days, while eggs tend to be viable around a day. If coitus and ejaculation are timed a day or two preceding ovulation, then a small fraction of the billions of sperm in the ejaculate may wend their way to safer spaces in the womb or oviducts, with the rare lottery winner fertilizing an egg. As a take home message of the book, however, the accurate assessment of timing fertility is more challenging than one imagines; the most fertile time is likely in the mid/late follicular phase, but ovulation can occur at lower probabilities across virtually the entire cycle. The result is that the rhythm method often fails.
Condoms were invented centuries ago. Falloppio noted that use of a linen sheath could reduce the spread of syphilis in the 1500s, with it also recognized that use of such a device could reduce pregnancy. “The first rubber condom was produced in 1855 and latex versions were introduced in 1920, although “lambskin” condoms made from intestines are also available.” (p. 200) The contemporary condom market has an even wider innovativeness: there are various colors, flavors, and such. As for other techniques for controlling male fertility, you may have heard that wearing tight pants, driving long-distance trucks, or even holding (as I am at the moment) a warm laptop on your lap can reduce sperm counts and maybe have impacts on fertility. But these don’t seem typical or reliable ways today for constraining male fertility, even if “one simple approach to male contraception, reputedly having a long tradition in the Middle East and particularly in Turkey, is to heat the testes…Various experiments, some conducted by John Rock, indicate that heating testes is safe, effective, and reversible.” (p. 220) Efforts have been put forth developing, and preliminary trials conducted, for male hormonal contraception. This is thus a potentially viable direction, and yet for whatever reasons (political will? possible mood and behavioral consequences?) it hasn’t pushed as far forward as it could. An extract from cottonseed oil—gossypol—has been subject to experiments in China, but with side effects sufficient to discontinue the effort.
Sterilization for men entails severing the vas deferens in a vasectomy. While sperm continue to be produced in the testes, they never make it out of the pipeline to be ejaculated or possibly fertilize an egg. This has a very low failure rate, and may be used if men feel that they have achieved their desired completed fertility, resulting in use at later ages than, say, condoms. Perhaps the most vivid of all male fertility control techniques is castration. By removing the testicles, a male loses the ability to produce sperm and the source of most of his androgen production. “Intentional castration to produce eunuchs is first recorded for the Sumerian city of Lagash around 4,000 years ago….At the end of the Ming Dynasty there were some 70,000 eunuchs in the service of the emperor, including some inside the Imperial Palace.” The Old Testament, incidentally, describes a world of eunuchs too, but as these anecdotes might suggest this was typically a male “operation” taken not by a male himself to constrain his fertility but by family or social superiors to constrain his abilities to impregnate women while fulfilling some other social function.
We likely don’t immediately envision that the mark of human uniqueness is our species’ use of male fertility controls. But as this discussion of the issue, inspired by “How We Do It” suggests, this can indeed be recognized as a derived feature of our reproductive behavior. This focus on male fertility control also flies against the logic and evidence that men often seek to enhance rather than constrain their fertility, including relative to female fertility preferences (http://www.psychologytoday.com/blog/the-evolving-father/201303/sex-differences-in-fertility-preferences). Yet sometimes there are good reasons—maybe it’s not the right time now, or you’ve already had enough kids—for wishing to prevent fathering a child. That’s just how we do it.
Martin, R. (2013). How we do it: The evolution and future of human reproduction. New York: Basic Books.