In my last post, I promised—threatened?—to explore a few human evolutionary mysteries, characteristics of our own species that are clearly the products of evolution by natural selection, but for which we don’t currently know the details. Let’s start by looking at breasts; after all, when given the chance, most men do … as do more than a few women.
[By the way, please don’t fall into the trap of interpreting the highly speculative, wildly uncertain hypotheses herein raised as evidence that evolutionary biology consists of “mere” Just-So stories rather than hard science. First of all, there is much to be said for Just-So stories, as the way problems are identified and a first step in solving them, and second, I’m specifically looking at things we don’t know rather than the immense amount that we already do.]
The mystery: Why do women have prominent breasts, even when not lactating? It isn’t simply because we are mammals, all of which, as the word implies, sport mammary glands that produce milk. There are, after all, about 4,000 species of mammals, and we are the only ones to do so prior to lactation—as a predictable component of sexual maturation—as well as after.
Some Hypotheses
1. A Dedalus Device? In James Joyce’s Portrait of the Artist as a Young Man, Stephen Dedalus was musing with a friend on the nature of female beauty, whereupon he concluded that “you admired the great flanks of Venus because you felt that she would bear you burly offspring and admired her great breasts because you felt that she would give good milk to her children and yours.” Plausible enough, but there’s a problem: Breasts (except when lactating) are composed almost entirely of fat, not glandular tissue. If anything, in fact, there is a negative correlation between prelactating breast size and capacity to make milk! This leads to the intriguing (may I call it “titillating”?) prospect that those Dedalus Devices can be downright deceptive, promising more than they can provide, but successful as a mate-attraction strategy nonetheless because men have noticed that breasts swell so dramatically during nursing, thereby making them—blockheaded men, that is, who are already prisoners of their rampaging hormones—easy to swindle.
Thus, prominent nonlactating breasts may well owe their evolutionary existence in our species not so much to the offspring they might help nourish but to men that they attract. I write this fully aware that indignant female readers might complain “How like a man, assuming that our breasts must be pointing at them.” But in fact, they probably are.
If so, then interestingly, men seem to have evolved a counter-strategy of sorts. There is a strong cross-cultural tendency for men to prefer women with an “hour-glass” figure, specifically a waist-to-hip ratio of .70, which provides a means of discriminating against potential mates whose breasts offer more fat than substance.
2. Buttocks Substitutes? Some downright silly ideas have been advanced, one of the most notorious by British ethologist Desmond Morris, in his best-selling book, The Naked Ape. According to Morris, conspicuous breasts evolved in part because natural selection favors emotional intimacy between men and women as a result of the need for devoted biparental care of offspring. Most mammals mate dorso-ventrally (“doggy style”), which—although feasible for human beings, too—is less personal and thus, less likely to generate emotional bonding than is face-to-face intercourse. To induce ventral-ventral, face-to-face love-making, then, evolution supposedly favored conspicuous bilaterally paired breasts, which essentially mimic the buttocks of “normal” quadrupeds, and assist in the transition from dorso-ventral to frontal copulation.
I wish I could embrace such a creative idea. Frontally. But the problems are too great. For one, there is no evidence of a correlation between women’s degree of breast development and their partner’s preferred sexual position. In addition, many nonhuman primates—including bonobos, gorillas and orangutans—copulate in a variety of positions, including ventral-ventral, yet in all these cases, nonlactating females are flat-chested. The supposed connection between sex-while-looking-into-each other’s-eyes and devoted monogamy also appears to be sheer fantasy. Monogamy is rare among mammals, but when it occurs, in some species of foxes, beavers, otters, the California mouse, pygmy marmoset and certain odd-balls including the fat-tailed dwarf lemur and Malagasy giant jumping rat, the pairs mate dorso-ventrally. (But of course, you already knew that …) So prominent breasts aren’t necessary for frontal mating, and frontal mating isn’t needed for pair-bonding.
It is also difficult to imagine that males of any species, including our own, have ever needed the visual image of female buttocks in order to feel horny. Behavior and pheromones are more than sufficient. Not only that, but quadrupedal mammalian buttocks aren’t naked, rounded globes but rather flattened and hair-covered … in short, not very breast-like.
Stay tuned: More and better hypotheses to follow.
