Sex at Dawn, part 4

This covers part 4 of the book Sex at Dawn: The Prehistoric Origins of Modern Sexuality. (You can see my reaction to part 1 here, part 2 here, and part 3 here.) There are 5 chapters in this section.

Chapter 15: Little Big Man

This section starts out with a discussion of body size dimorphism, which is essentially that males and females in many species are different sizes than each other. When this is true in mammals, it is nearly always that the male is larger than the female. The degree of disparity between the two sexes in size correlates strongly with the mating system -- when a single male is capable of monopolizing many females, males tend to be much larger than females, while when a species is monogamous, the two sexes tend to be roughly the same size. In humans, chimps, and bonobos, males tend to be 10-20% larger than females. From this, the authors argue that humans are naturally promiscuous, as males are not as much larger in humans as they are in gorillas, while also claiming that polygyny requires sufficient political power and wealth to support multiple wives, and that this did not exist prior to agriculture.

This is not a logical argument. Within the US, adult males about 17% heavier than and about 8% taller than adult females according tot he CDC, so that's clearly in the right ballpark. In lions, males are about 12% longer, 20% taller, and 35% heavier than females, according to this conservation site. So male lions are a little larger in relation to female lions than male humans are to female humans, but not as severe as the discrepancy between male and female gorillas. Lions are notably lacking in agricultural wealth, yet prides are typically composed of multiple females and their young, and only one or two adult males. It is not a requirement that males either be as much larger than females as is true in gorillas, or else for males to have extensive material wealth, for polygyny to be possible in a species. My interpretation of the data is that human males (as well as chimps and bonobos) have been less polygynous over evolutionary time than have male gorillas, but that does not necessitate widespread promiscuity (nor does it refute it).

From here, they segue into a brief overview of sperm competition, which they get into more deeply in the next chapter

Chapter 16: The Truest Measure of a Man

Here the authors go into more depth on the idea of sperm competition. Their take is predominantly that in species with promiscuous mating habits, male competition is not for access to mates, but instead takes place at the level of the sperm. This accounts for larger testes in such species relative to body size. There are some pieces of evidence in favor of sperm competition in humans -- relatively large testes compared to most primates (though notably smaller relative to body size than in chimps or bonobos), and greater sperm production in men whose regular sexual partners have been away for several days, regardless of the man's other sexual activities. It is, however, not a settled point, which they oddly concede in an end note to a sentence that declares that there is no question that there is sperm competition in human reproduction. They conclude with an argument from pornography, namely that scenes involving multiple men and a single woman are popular, and that women rarely demand sex-inverted scenes in their pornographic preferences. To the authors' viewpoint, this clearly demonstrates an enjoyment on the part of men for visual depictions of sperm competition. Possible. The extent to which this is a popular preference is not discussed, which means it's difficult to interpret -- it could still be a relatively small fraction of the consumed pornography. But there are many other interpretations. Fantasies that this is how individuals they idolize (sports stars, successful musicians) behave. Access to an unobtainable woman in a way that is clearly demonstrated in front of friends. Subjugation of the woman as purely a sex object, rather than a person in her own right. It's very difficult to determine motives from this sort of (lack of) data.

Chapter 17: Sometimes a Penis is Just a Penis

In this chapter, the authors talk extensively about the size of the human penis in relation to body size, noting that it's longer and thicker than in other apes, and that it has more differentiated structures on it. From here, they conclude that these are features molded by natural selection to deal with sperm competition. I am not saying that that is not a possible interpretation, but I will say it's far from the only one. This entire chapter suffers, in my opinion, from pan-adaptationist thinking. The authors assume that every feature they can measure is the result of natural selection. It doesn't have to be that way. Some things are the way they are due to chance. Or history. I can provide plenty of links to papers showing that particular features of studied genetic systems are sometimes due to chance or history, rather than being selectively advantageous in their own right. And even when things are adaptive, it is not necessarily the case that adaptation occurs because such features physically improve mating success. Mating success has a social component as well, and thus is often shaped by sexual selection: individuals of one sex (often female) choosing which individuals to mate with. Sexual selection explains a lot of features in a lot of species, such as the brightly colored feather on many male birds compared to the drabber but much more camouflaged colors of females from the same species. It could be that proto-human women had preferences for particular shapes of male genitalia, and that this resulted in the human male package as it exists today.

In specific claims, they discuss the idea that the particular shape of the human penis coupled with typical human sexual positioning creates suction the removes previous sperm from a woman's reproductive tract. I've never been convinced by this argument. Both male and female reproductive morphology is highly variable in our species; it seems very unlikely to me that there would be a particular shape and size combination that would produce such a physical effect in a high enough percentage of partners for it to have a strong enough effect to be visible to natural selection. They also then claim that differences in relative penis and testicle size across ethnic groups is due to differences in intensity of sperm competition. Yet those differences, subtle as they are, could also easily be explained by different preferences by females in different regions, which resulted in different sexual selection pressures.

Chapter 18: The Prehistory of O

This chapter is nearly entirely about the history of Western culture's dismissal of female libido and/or its punishment of sexually assertive women. As this has nothing to do with science, I don't have much of a scientific reaction to it. It's unfortunate that so many societies had issues with female sexuality, but that's about all I have to say on the matter.

Chapter 19: When Girls Go Wild

This chapter is another one rife with pan-adaptationist thinking. The argument is put forth that since human females tend to be more vocal during orgasm than males are, and that they are capable of multiple orgasms over a short period of time, this must have adaptive value since it's too costly otherwise. I don't buy that. Humans haven't routinely been prey to large organisms (though have to diseases and parasites) in a fairly long time period. It is unlikely that an ancient hunter-gatherer would become dinner because his or her screams of delight brought a jaguar down on him or her. Even if female orgasm vocalizations are adaptive (which I've seen no evidence that they are -- this could just be a general neural firing), that doesn't require the sperm competition explanation that the authors favor. It could display status. Multiple matings could promote lack of paternity certainty, and thus decrease the risk of males committing infanticide. It could be something else. Without experiments to test these ideas, they're simply just-so stories.

This chapter also gets weird. It advances the idea that human female breasts are a replacement signal for swellings at the genitals or buttocks, as the changing demands of locomotion would have made swellings in these regions problematic for a physically active woman, while the breast tissue could easily be enlarged. This is somehow tied to the notion that female ovulation isn't actually hidden because human women make more effort to appear attractive in portions of their cycle when they are likely fertile. Even if we accept that women do make such an effort, that doesn't actually refute the notion that ovulation is hidden. All it states is that sexual behaviors can be influence at an unconscious level by hormonal changes, which I think is a pretty widely recognized fact.

I am once again disappointed in the arguments made in this section. Little evidence is provided, and what evidence is provided often has little to say on what mating systems would be "natural" for humans. Arguments are made from assertion more often than from experiments, and the experiments presented are not of conclusive value.