Human Sexual Behavior II. 100m Robert Sapolsky video.
Part 2 of 3 in a broad biological survey on the nature of human sexual behavior from neuroendocrinology to evolutionary history. This discussion starts with a general review of Part 1 then picks up where he left off on pheromones.
16. Human Sexual Behavior II
Releasing Stimuli for sexual behavior
Women have greater preference for the smell of males around the time of ovulation (their noses become more sensitive at this time; estrogen has receptors on olfactory neurons). Men rate the smells of females as less unpleasant when the woman is ovulating (depends on testosterone in their systems). Physiological effects: The Wellesley effect (synchronizing female menstrual cycles) is pheromone based. In rodents, the pheromones of an adult female will delay the onset of puberty of younger females and decrease the onset of induced ovulation (unless they are sisters: kin selection!). In rodents and some primates, when males smell the pheromones of dominant males it drives down their testosterone levels. In some species, the same dominant pheromones stimulates sperm production: "a coevolutionary arms race". In rodents, male pheromones stimulates onset of puberty in females and the onset of induced ovulation (such as in pigs where Boar-Mate, a variant of pig pheromone derived from truffles, is an ovulation inducer) (unless the two are siblings: kin selection). In lots of species female pheromones increases testosterone and sperm production in males.
Sapolsky describes the experiments in the paper "Effects of sexual activity on beard growth in man" by Anonymous in Nature 226, pp. 869–70, 1970 where the researcher measured beard growth after sexual activity vs. his extended periods of time alone. The effect is possibly due to pheromones, but there are many possible confounds and the sample size is n = 1.
Homosexual men prefer pheromones from men with higher testosterone levels suggesting some interesting biological underpinnings of homosexuality.
Other releasing stimuli: giraffes lick private parts to get gustatory information, auditory priming in moose. In human females their voices get a little bit higher during ovulation which is subliminally detected by men (no conscious awareness).
The most erogenous organ in humans is the brain. Thought as a releaser: it often bypasses the world of smell and sound. Coolidge effect: variety can stimulate sexual arousal even after a male is sexually sated.
Fear suppresses sexual behavior. Chronic stress is extremely suppressive of sexual behavior. Short term stress may stimulate or suppress arousal. For example, there are plenty of historical examples of violent sexual behaviors associated with warfaring.
In most species, females only engage in proceptive behavior around the time they are ovulating. In non-human primates the relationship isn't so strict. In humans one study found heightened arousal, proceptivity, and sexual activity (including likelihood of orgasm) near day 14 (around the time of high estrogen levels and ovulation) and day 28 (more relaxed about fear of pregnancy???). The study has not been easy to replicate except the part about day 14. Women tend to wear more provocative clothing around the time of ovulation. Geoffrey Miller reported that around the time of ovulation, lap dancers get larger tips.
Estrogen increases expression of receptors for progesterone. Estrogen increases the synthesis of oxytocin. In voles (and maybe humans), social affiliation of females is enhanced at the time of ovulation. Estrogen increases sensitivity to touch in some parts of the body. Estrogen both affects the brain and the periphery to increase proceptivity around the time of ovulation. In females, androgens (male sex hormones) play a role in proceptivity, arousal and motivation.
The time of year when males mate correlates to the time of year in which testosterone levels are elevated. In humans testosterone levels increase at puberty and start decreasing after age 30 with a correlative profile of sexual behavior. More sexually active men tend to have higher testosterone levels. There is a decrease in testosterone levels (and increased vasopressin) around the time they become fathers. Testosterone in fact does cause increased male sexual behavior: after castration, there is a big drop off in male sexual behavior; then when testosterone is added back in, sexual behavior levels return. But the subtraction experiment does not eliminate male sexual behavior (in all species!). Moreover, when adding testosterone back if the level is only 10% or even 200%, the sexual behavior returns to pre-castration levels: so those brain regions that drive male sexual behavior require sufficient testosterone but are not affected by quantity. The more sexual experience before castration, the more sexual behavior is retained afterward. So there is part of male sexual behavior driven by social experience and not the hormones. If testosterone levels are increased to supraphysiological levels (1000% of normal), then you do start seeing increases in male proceptivity (an issue for anabolic steroid abusers despite the partially compensatory down regulatory response). The same testosterone levels similarly affect male aggression. In sum, testosterone is playing a modulatory role.
Melatonin is a hormone that tells the body what time of year it is: it is sensitive to light. It drives seasonal mating patterns in some species. There is some evidence for a seasonal pattern to human sexual behavior, but it is small compared to many other species.
Perinatal factors: so far, the evidence is that there is very little about early life experience that affects the quality (the way) of later sexual behavior. Early experience is not about how to be sexual (those fixed action patterns are effectively built-in), but rather about learning the appropriate social context for proceptive behavior. The same pattern holds for aggression.
Rhesus monkeys and others reared (unethically perhaps) without the social context of any parents or peers grow up with the same sexual fixed action patterns, but without any understanding of the social context appropriate to those behaviors (e.g., the normal male fixed action pattern may occur with a towel or bowl of food or ignores the social dominance hierarchy). Kibbutz literature suggests that if you spend lots of time with someone before you both turn six, chances are that when you grow up they will not be considered a mate but more like a sibling (pseudokin).
Early 20th century literature espoused the absence of a father model for making boys gay (the idea is the father may provide training for the appropriate context for appropriate proceptive behavior). The other leading model for explaining homosexuality was the neurotic mother explanation. No evidence for either model: they are complete nonsense.
Perinatal hormone levels affect sexual behavior in many species. In humans, the jury is still out and more research is needed. The "organizational effect" of a hormone is its impact on development (on the organs of the body but also its impact on the brain). The "activational effect" of a hormone is how it activates aspects of behavior (such as the sexual behavior apparatus that develops during puberty). In rodents, sex hormones have huge organizational effects that dramatically influence sexual behavior. Some 300 species have been documented to have homo- and hetero-sexual behavior in natural settings. In primates, If and only if there is prenatal masculinization (exposure of fetus to testosterone), then testosterone will have an acute activational effect (leading to male fixed action patterns). In human females there are two known ways to become masculinized: congenital adrenal hyperplasia (a genetic disorder) where the adrenal glands pump out lots of testosterone and through the drug DES (diethylstilbestrol) which is converted to androgens. There is a higher likelihood that prenatal masculinization leads to lesbianism, but there is a strange confound: the masculinization also created weird intersexual genitals which resulted in plastic surgeries and all sorts of other social consequences as well as the hormonal effects. Sapolsky concludes that there is weak evidence that prenatal masculinization is associated with homosexual behavior as an adult (Wikipedia sites two later studies with ambiguous results).
sexual identity: what sex you feel you are
intersexual people have gender ambiguous genitalia
secondary sexual characteristics: those features that develop during puberty
In some parts of the body, testosterone (a steroid hormone with its hormone binding domain and DNA binding domain) is biochemically changed to dihydrotestosterone which behaves somewhat differently (this effect occurs in the genitalia and the locations for secondary sexual characteristics). In other parts of the body, testosterone is biochemically changed to estrogen to cause male behavior! This estrogen effect only happens in target cells (not in blood circulation) generally in the brain. What? Why don't the masculinizing effects of estrogen affect female brains causing everyone to have a male brain? Alpha-fetoprotein (AFP), the most abundant plasma protein found in the human fetus, during pregnancy it binds to estrogen breaking it down so that circulating estrogen never has an effect on the fetus (at least in rodents according to Wikipedia). So the only fetuses with estrogen are males who made it from their testosterone. Where do prenatal feminizing effects come from? Perhaps, the female brain is the default brain and it takes testosterone to masculinize it. Sapolsky argues that there are other hormones that produce the feminizing effects. The effects of gender on our brains are very complex!
Testicular-feminizing males are phenotypically female, but genetically male. It is a genetic condition caused by ineffective testosterone receptors. Although testicular-feminized males experience no masculinizing effects directly from testosterone and dihydrotestosterone, they still have testosterone in their brains biochemically changed to estrogen and therefore they have masculinized brains. Inference: testicular-feminized males develop a female sex identity due to socialization and not the masculinization of their brains. So do the prenatal hormonal effects matter?
Genetic effects: what do genes have to do with sexual behavior?
Genes determine which gonads you make as a fetus which determines the kind of sex hormones released into the blood stream which determines what genitals you develop and the type of secondary sexual characteristics. Genes play a role in sex determination.
In monozygotic twins there is about 50% concordance with sexual orientation; in dizygotic twins it is 22%; and 9% with other siblings. So there is some suggestion that genes and/or pre-natal environment play some role in sexual orientation. Openly gay NIH scientist Dean Hamer reported finding genetic markers that were more likely to be shared between gay siblings. It was reported as "the gay gene": it is not a gene, it is a genetic marker: a statistical pattern in a genetic analysis. The marker showed no consistency: different pairs of twins shared different markers. Moreover, the finding has never been replicated despite substantial effort. Simon LeVay's INAH3 brain region size differentiator for homosexuality is scientifically compelling; Dean's genetic markers much less so.
Evolutionary History of Human Sexuality
"Organisms have sex for the good of the species." Sexual behavior is about reproduction: passing on copies of your genes. But humans and bonobo ("pigmy") chimps have non-reproductive sex. Bonobos are much different from Jane Goodall's tournament species (they are highly aggressive, fashion weapons, etc.). On the other side of the Congo, bonobos have virtually no sexual dimorphism, female dominance, and they have astonishing amounts of sex. What? A pair-bonding species (little sexual dimophism) with low aggression that is the most sexually promiscuous species on Earth? They have lots of non-reproductive sex: female not ovulating, homosexual, non-fertilizing sex (does he mean oral or anal sex?). Conclusion: sex must not be just for passing on copies of genes! Joan Roughgarden formerly at Stanford argues that there is far more non-reproductive sex in lots of species than the classical Darwinian notion of sexual selection can account for.
In bonobos sex appears to be about promoting group cohesion. Social grooming is important for social cohesion in many primate species (baboons will groom after a scare from a predator), decreasing individual tensions, reconciliation, etc. Solly Zuckerman's favored the theory that sexual behavior is purely for promoting group cohesion and decreasing violence. It became the dominant theory early in the 20th century. Sapolsky criticizes it severely: sexual behavior is the cause of huge amounts of aggression in many species.
Costs of sexual behavior: sperm do not cost much; eggs, pregnancy and post-natal care are very expensive. So sociobiology suggests that females are more selective about who they will mate with than males because of the differential costs of reproduction. Pair-bonded species like the marmosets provide an interesting exception: the male does as much child care as the female (they have twins). When the male caloric expenditure exceeds the female, you start to see cuckoldry (females abandoning their children to find a better mate) which is common in bird species. So the sociobiological argument that female selection is always more important has an important exception. In many species, the male attempts to control female reproductive behavior (is that common in humans?). In primates mate guarding is a male's attempt to keep his female away from any other guy. In humans, we find clitorectomy and chastity belts in cultures where males tend to go away for long periods of time such as nomadic pastoralists or during the Crusades.
The linear access model of reproduction suggests that male dominance rank is entirely predictive of male reproductive success: the top ranked N males get access to the N females that are in estrus (in Baboons for example). Copulatory plugs occur in many canine (dog) species where semen hardens into a plug to block access by other males. In some fly species the penis is barbed so that after sex it detaches (he can grow a new one) and lodges in the female blocking access to other males. In some fly species there is also sperm competition where sperm use toxic substances to kill each other. In other species the male biochemically decreases the sexual attractiveness of the female subsequent to mating (by decreasing pheromones or proceptivity). "Viciously clever ways for males to control female reproductive behavior after they have left."
Females have developed counter strategies to male strategies to control female sexual behavior so the female gets more choice in the matter. Hidden ovulation in humans is a tactic to decrease the certainty of paternity (Sapolsky discounts the effects of pheromones and voice raising during ovulation which are subliminal and so consciously undetectable). One argument suggests hidden ovulation may reduce competitive infanticide while another suggests it decreases the male interest in controlling female sexual behavior because it is less clear when one needs to. Non-reproductive sex serves to fool the male when ovulation is actually occurring. Pseudo-estrus helps females in some species prevent competitive infanticide.
The leading cause of aggression in social species is male-male competition for reproductive access to females including humans. In monogamous species males produce small amounts of sperm (small testes) whereas in polygamous species males produce lots of sperm and have larger testes. Chimps have gigantic testicles per body size but not Gorillas. Humans are intermediate in testes size between pair-bonded and polygamous/tournament species.
Evolutionary aspects of female choice: In classic tournament species, females have very little choice (linear access model). By 1980s it became clear that there is little evidence for strict linear access models in actual behavior (female primatologists entered the field and started looking for evidence). Female choice was discovered! In tournament species, there is sexual dimorphism (big males), so females must be more clever than the brute force techniques of males in these species. For example, she can exhaust the guy: every time he sits down or takes a nap, she moves away forcing him to move to maintain his control over her sexual access. In baboons, a female will walk right in front of the dominant guy's worst rival again and again and again until those two fight, then she runs off to the bushes and has a "stolen copulation" with the guy she is actually interested in. There is a suggestion in the data (unproven) that female baboons like to mate with male baboons who are nice to them. Barbara Smutz coined the term intersexual friendships to describe nonsexual male-female interactions. So being a nice guy is an alternative male strategy in a tournament species since there is female choice. Paternity studies have shown that the strategy works because although nice guys mate less often, then do not suffer the injuries and complications of fighting to maintain a high dominance position. So these "alternative strategies" can work.
In orangutans, an alternative strategy for the low-ranking males who never get to mate (they are physiologically smaller) was first discovered by pioneer researcher Birutė Galdikas when she observed low-ranking males raping females (rape is defined as the violent process of mating with a female against her will). Some fish species have an alternative mating strategy where some males pretend to be female (coloration, etc.).