You’re all familiar with this image, right? It’s on the “hello out there!” plaque included with the Pioneer unmanned spacecrafts 10 and 11 in the early 1970s. I was myself about eight years old at the time and a rather intense Star Trek fan, so I was taken with the excitement of formally introducing ourselves to the universe. An artier version, which reminds me of some of the communes I recall from childhood, may be found here.
Criticism of and reflection upon the image itself as representing humanity is rife, but I at least give it credit for summarizing human sexual dimorphism not too badly. morph means “shape,” so technically the term leaves out such things as hormonal profiles and some related developmental phenomena, but let’s interpret it broadly as saying “anything different and roughly dichotomous.”
Some of it is explained by an easy version of evolutionary theory, plain old basic selection. For example, although this may surprise you, the baseline size difference in sex for animals is a slightly larger female, among the many species for which she incurs heavier reproductive costs. Other examples include elaborating on the platform of differing hormonal physiology, i.e., working off of different environmental cues. I discussed some of the same/different details for mammals in Hermes and Aphrodite.
Technically and narrowly, Darwinian sexual selection concerns (i) secondary sexual features (i.e. not gametic or genital) affected by mate choice, such that the features of one sex are subject to selection via the preferences of the other. Mate choice is a really broad topic, even if you subdivide it into courtship vs. competition. Given a number of further subcategories and interesting case studies, this is what’s given rise to quite a bit of our most showy and photogenic features across enormous numbers of living things, ranging in the human gaze from some of our highest standards for beauty to our most fascinating-horrific moments of grotesquerie.
Its basic mechanism is easy, called Batemann’s Principle: given a difference in energy expenditure per offspring, the sex who spends more exerts stronger selection on the features of the other sex. Particularly those features which distinguish it from other individuals in terms of health, among other things. Size is often involved, and if the male is the selected-upon sex, then the size difference I mentioned above is overtaken by this other selective effect. This happens so often that we can mistakenly think of larger male size as intrinsic to the sex difference in the first place.
When this kind of selection outweighs that of, for example, predator avoidance (note: health is not the same as longevity!), then you get one plain-looking sex and one gaudy sex who lives fast, probably doesn’t get to get laid (because only a minority do), and dies both young and horribly. Don’t romanticize the “proud peacock” or fancy that the lion “rules his harem.”
The primary refinement to this line of thought, especially in the 1980s, concerns male-female conflicts of interest. There’s a bewildering and varied body of of work on this, some of which I was more-or-less close to during grad school via the students of H. Jane Brockmann.
Another detail concerns developmental tag-alongs like male nipples, which get tricky because on the one hand, it means that biologists’ propensity for celebrating selection gets applied inappropriately, but on the other, nothing invites later selection like something that’s “just sitting there.” I showcased a particularly aggravating non-debate about this in Graven images.
Glancing briefly back at my long series of posts about mammalian genitals, it may be surprising that the “obvious” example of genital dimorphism is probably not actually a matter of classical sexual selection, or that it might be so in some creatures and not in others. That’d bring us past the basic aspects of shape and function that I discussed to the jaw-dropping elaborations one finds among, say, octopi and dung flies, and whole libraries of similar examples. Uncharacteristically, I’ll leave the fig leaf in place and stay with the more straightforward body shape-and-composition features.
Whew! Let’s be egocentric and take a look at ourselves the bipedal primate, and even more so our very own species. Again, I’ll be focusing on anatomy and leave the daunting swamp of behavioral distinctions for some other, scarier time. The first layer is easy: there’s the basic mammalian mammary difference, the definite but actually-overlapping pelvis difference associated with our big infant cranium, the important and typical importance of timing for female fertility, and perhaps some interesting details which favor the female like pain threshold.
All that gets some extensive add-ons by what looks like medium-level, quite standard sexual selection: greater male size, shoulder proportions (which incidentally don’t provide core strength, for which women are notable), distinctive heavier construction in face, remarkable and consistent difference in voice. Possibly the uncharacteristically large penis relative to other living apes too. We really don’t know when and in whom the latter and other soft-tissue structures appeared among the at least six or seven species of Homo.
I should stress one of those primary aspects of such selection that tends to bypass the initial observer, addled as we are by the “more and bigger” aspects – that such features almost always go hand-in-hand with inferior immunity (which is hindered by testosterone levels) and less longevity. In this, we are pretty typically sexually-selected too.
This topics have received plenty of attention for a long time, both within-species and among primates, and it ties to the Harcourt studies I’ve mentioned before concerning social mating systems and primate testis size. It also intersects strangely with psychology as a discipline, and doubly-so with gender studies. But something has caught my eye, ever since those halcyon days of beer-fueled grad student discussions, that I don’t think I’ve seen a whole lot of researched aimed at.
You see, humans are a little weird in something – that women are not the “plain Jane blah-colored, blah-shaped female” that one sees all the time, in literally thousands of species which display classic Batemann-style sexual selection slanted toward the other sex. So you don’t get the wrong idea, that same selection can slant the other way, in which case it’s “plain Joe.” But women have … showy things of their own. I’m looking with narrowed eyes at breasts, which is to say, a lot more than merely teats. The only thing that would clinch them as classic sexually-selected features in the female direction (i.e. as with overall size in the male) would be a consistent male preference or observed attention toward them … oh wait.
So, we don’t display only showy/plain classic sexual selection. Nor – and this is important – do humans display the classic array that accompanies the most monogamous species, which is to say, dramatically minimal dimorphism.
You see what I’m getting at? We look like we’ve undergone classic Darwinian sexual selection, all right – but in both sexes, for different things.
That also makes me look at hair distribution differently, either replacing or in addition to the notion that men’s facial hair is a peacock tail, because I’m thinking whole-body now. OK – first, stay with me, hair cover reduction is the trait I’m thinking about. (Also, ape hair lacks underfur already, so enough with the warm/cool issue.) In that context, body hair reduction in humans is consistent with with the common phenomenon of an intensely selected degree in one sex, and a less extreme and more variable version in the other, with the female as the selected sex. This feature also needs some context in the effects of self-domestication, though, which set up a lot of the qualities of human hair like “the shock” on the head, and so I acknowledge I’m only gesturing at it rather than explaining it.
Addressing this as a research question brings up the constant aggravation of being a relict species. We simply don’t know which of these exact things are unique to Homo sapiens and which we’ve inherited from, say, H. ergaster or whoever. It’s intriguing to imagine that our genus included classic sexual selection toward the male (e.g. size, ‘robustness’ of feature and voice), then in a narrow subset of that includes us, or as part of our own species’ origin, or perhaps quite recently in our history, other selection occurred which targeted the female. But regardless of precisely when and in which species, going by Batemann’s Principle, that’d imply a new energetic demand upon the male, regarding offspring, resulting in sexual selection upon the female appearance. That’d be in addition to the older, ongoing, and obviously profound demand on the female that had resulted in sexual selection upon the male.
Thoughts on what that energetic demand for the human male might be, and when and how it might have appeared, strike me as a real-live valid question. If I’m simply ignorant of a school of work on this topic, and you know about it, please hit the comments with some links.
Links: the Wikipedia page for sexual dimorphism, which as of this writing is actually pretty good
Next: Just look at the label