12 November 2009

10 Things You Didn't Know About Sexual Biology (Sex Edition)

For those of you who have been following this series, I hope you've enjoyed learning about many of the interesting nuances of the biology involved in sexuality. This will be the final entry, and I'm sure you'll find it to be the most interesting. I had a lot of pleasure reading up and compiling these lists and I am amused at their popularity. Who knew sex could be so... viral?

1. Male homosexuality may be caused by the same gene that makes women more fertile1. We know that homosexuality exists in nature, but it usually manifests itself as a distinctly different kind of behavior than what we're used to. For many animals such as monkeys, giraffes, dolphins and manatees, male-on-male sex is more of a social bonding and tension diffusing device2. When there aren't enough females to go around, they make love with each other, not war. Alternatively, the human homosexuality that we all know is exclusive attraction to members of the same sex. Because of this difference, scientists are looking beyond behavior and into genetics for its root. An Italian study has found that female relatives of gay men produce more offspring than those of straight men. Simply put: the mothers, aunts and grandmothers of gay men tend to be more fertile. How does this end up affecting their children? It is likely that the same gene in the X chromosome that is responsible for a female's fecundity is being activated in their male offspring, thus making them attracted to males in the same way their mothers are. Alas, this is only a study that observes a correlation, but not causation. It certainly is worth delving into with future studies.

2. Marital problems? There's a gene for that3. A hormone in our brain called vasopressin can determine if we are venturous lovers or loyal companions, depending on where it is detected. Our genes are responsible for that very modulation: the location of the vasopressin detectors. A Swedish study looked for one of those genes responsible for this in hundreds of men and came to a telling conclusion: "Men with two copies of the allele (named RS3 334) had twice the risk of experiencing marital dysfunction... compared to men carrying one or no copies." The study went on to say that women married to these men also felt less satisfied with their relationships. This shouldn't bum you out though, if there is one thing that a skeptical reader will say, it is that humans are very capable of defying their own nature. Nobody is a slave to their genes.

3. That 4 hour erection that Viagra ads warn you about is necrosis4. You may remember my description of blue balls in the Male Edition. This is pretty much the same thing, but on a more extreme level. If you missed it the first time, it goes like this: blood necessary to produce an erection gets trapped in the pelvic region by constricted vessels. This reduces circulation to the heart and lungs, thus depriving blood in the penis of oxygen. If this lasts long enough, the blood can clot, leading to gangrene. At that point, you can kiss your willy goodbye. As a bonus fact, there is erectile tissue in your nose. It typically regulates the flow of air within your nostrils. That is why Viagra users often report nasal congestion as a side effect.

4. Females have a prostate. Big deal, right? Well, this little fact is at the center of many other interesting notes about a female's enjoyment in sex. For instance, the female prostate is positioned around the urethra, but comes in all shapes and sizes5. The famous G-spot is a location on the anterior wall of the vagina, right about where the prostate is on some women. Incidentally, G-spot stimulation is the most common way of achieving female ejaculation. And what is that fluid that women squirt out when they do ejaculate? It shares many common elements with male semen and Cowper's gland secretions, products of the glands around the prostate6. Women who squirt also experience fewer urinary tract infections as a function of the anti-microbial properties of their ejaculate. It's all coming together.

5. Frequent oral sex with the same partner may increase the chance of a woman getting pregnant by them through intercourse7. Stop glaring at me, I didn't make this up. There is a very reasonable way to explain this. To a woman's body, sperm are foreign invaders. While elements of semen help protect the sperm from the female's immune system, repeated exposure to a single male's sperm can help the immune system learn not to attack the friendly little swimmers. In this way, oral ingestion of sperm (as well as other likely methods of love injections) paves the road for successful conception. Beyond helping conception, oral exposure to semen has been shown to help reduce the risk of a woman's blood pressure rising too high during pregnancy8. You've never encountered a better excuse to ask for a blowjob.

6. Three words: homosexual duck necrophilia9. For every human sexual behavior, there is a counterpart to be found in the animal kingdom. The lesson in this point is that no matter how perverted you think humans are, there is always an animal that shows us how well-behaved we can be. Ducks are no exception as they have been observed buggering the corpses of their flock. We're not talking about a quick, demoralizing gesture of dominance; an hour and fifteen minutes says this probably felt quite... I don't know... natural. Of course you'd like to know why this happens, but nobody knows the answer yet.

7. Herpes can cause hypersexuality. I know what you're thinking, but this isn't a good thing. Herpes is more than just warts on your package, it can attack your brain and nervous system, too. This is called herpes simplex encephalitis. HSE can cause damage to your amygdala, which helps you process and memorize emotions. What can happen, in rare cases, is a disorder called Kl├╝ver-Bucy syndrome10, which leads to hypersexuality. Hypersexual people often exhibit inappropriately sexual behavior in public, much to the detriment of their own health and safety. Luckily, this condition is not a likely one and it fades over time.

8. Humans are the only mammals who long outlive their fertility11. This fact isn't a stretch to realize; even though apes usually decline in fertility around their 40's, their life expectancy doesn't forecast them to live beyond that either. Conversely, humans live more than twice as long. We can point to technology and medicine as contributing factors to this phenomenon, but this also gives humans an adaptive advantage. What evolutionary benefits do we gain by living far past our prime? You could argue that the presence of grandparents, specifically grandmothers, provides insurance that protects the survival of young. A study in Gambia indicated that children who lose their mothers before the age of two are twice as likely to survive if they have a grandmother to take care of them12.

9. Some animals take birth control13. Oral contraceptives work by using hormones to trick a female's body into believing it is pregnant so that ovulation doesn't occur. There just so happens to be a variety of plum that is rich in phytoprogestogens, which mimic female hormones. Who eats those plums? Monkeys. The result is that the plums slow the reproduction of the animals that eat them, preventing overpopulation and thus over-grazing. Red clover also has the same effect on the animals that graze on it. It was discovered during a sheep-breeding crisis in Australia. Who knew that it was just the local plant life protecting itself from being overwhelmed?

10. If you were on The Pill when you met him, going off The Pill can lead to relationship problems14. Introducing all of the manipulative hormones into your body through The Pill is bound to throw a few things off. These changes are not limited to a woman's olfactory preferences. As explained in the Couples Edition, a female's sense of smell can lead her to a compatible mate, as well as away from an incompatible one. The Pill changes the situation a bit, altering a woman's tastes. This can cause problems when a woman decides to go off birth control with her partner. The supply of artificial hormones stops and her body returns to its natural functioning state. All of a sudden, preferences have changed and Prince Charming doesn't seem as delightful anymore. Of course, there is more to love than just the scents that subtly attract us, but if there isn't much depth to a relationship while The Pill is in effect, there probably won't be any once it is not.

Previous Editions:
Female Edition
Male Edition
Couples Edition

1 A. Camperio-Ciani, F. Iemmola and S. R. Blecher, "Genetic Factors Increase Fecundity in Female Material Relatives of Bisexual Men as in Homosexuals," J Sex Med 6, 2(2008): 449-455
2 Joan Roughgarden, quoted in J. Lehrer, "The Gay Animal Kingdom," Seed Magazine, June 2006
3 Walum, H., Westberg, L., Henningsson, S., Neiderhiser, J. M., Reiss, D., Igl, W., et al. "Genetic variation in the vasopressin receptor 1a gene (AVPR1A) associates with pair-bonding behavior in humans." Prot Natl Acad U S A, 105(37) (2008), 14153-14156.
4 http://en.wikipedia.org/wiki/Priapism
5 D. Sundahl, Female Ejaculation and the G-Spot (2003)
6 F. Wimpissinger, K. Stifter, W. Grin, and W. Stackl, "The Female Prostate Revisited: Perineal Ultrasound and Biochemical Studies of Female Ejaculate," Journal of Sex Medicine 4, no. 5 (2007): 1388-1393.
7 S. A. Robertson, W.V. Ingman, S. O'Leary, D. J. Sharkey, and K. P. Tremellen. "Transforming Growth Factor Beta - A Mediator of Immune Deviation in Seminal Plasma," J Reprod Immunol 57, no. 1-2 (2002): 109-128.
8 R. B. Ness, D. A. Grainger, "Male Reproductive Proteins and Outcomes," Am J Obstet Gynecol, 198, 6 (2008):620el-4; C. A. Koelman, A. B. Coumans, H. W. Nijman, II Doxiadis, G. A. Dekker, and F. H. Claas, "Correlation Between Oral Sex and a Low Incidence of Preeclampsia: A Role for Soluble HLA in Seminal Fluid?" J Reprod Immunol 46, no. 2 (2000): 155-166.
9 C. W. Moeliker, "The First Case of Homosexual Necrophelia in the Mallard Anas Platyrhynchos (Aves: Anatidae)," DEINSEA 8 (2001): 243-247
10 http://en.wikipedia.org/wiki/Kl%C3%BCver-Bucy_Syndrome
11 M. Emery-Thompson quoted in R. Hooper, "Menopause Sets Humans Apart from Chimps," New Scientist, 2007.
12 D. P. Shanley, R. Sear, R. Mace, and T. B. Kirkwood, "Testing Evolutionary Theories of Menopause," Proc Biol Sci 274, 1628 (2007): 2943-2949.
13 J. P. Higham, C. Ross, Y. Warren, M. Heistermann, and A. M. Maclarnon, "Reduced Productive Function in Wild Baboons Related to Natural Consumption of the African Black Plum," Hormones and Behavior 52, no. 3 (2007): 384-390.
14 S. C. Roberts, L. M. Gosling, V. Carter, and M. Petrie, "MHC - Correlated Odour Preferences in Humans and the Use of Oral Contraceptives," Proc Biol Sci, 275, no. 165 (2008): 2715-2722.