Irradiated by LabRat
It seems that if I’m going to go with the will of the people, I have no choice but to write about the all-time most popular search string we ever get. So be it.
Although they have been a truly enduring fixation of humanity, the presence and purpose of the female breasts remain something of a puzzle and a source of speculation to evolutionary biologists and anthropologists. While all mammals have mammary glands and most of them have nipples that swell into teats when there are nursing young, the female human is the only one that has permanently swollen teats from puberty on. There are plenty of reasons for women NOT to have breasts; they’re unwieldly, metabolically expensive (as completely unnecessary fatty tissue), and when they’re particularly large, they cost their owners a good deal of pain due to the back strain of having what amounts to a pair of weights up front day in and out. They don’t even need to be larger than average to make running more problematic for a woman than they are for the average man. What’s even worse, they’re prone to cancer- and there isn’t even a nursing advantage for offspring in large breasts; it makes things only more awkward for the infant and mother alike. The question of why we’re saddled with them is therefore more compelling than it would be for a feature with fewer costs. It’s trivially obvious that breasts carry a large sexual selection advantage for the female, but why?
One of the earliest ideas I ever heard regarding the purpose of breasts was Desmond Morris’s, put forth in The Naked Ape: breasts, as well as fuller and reddened lips, are meant to be a sort of recapitulation of the gigantic swollen round red ass presented by female apes and monkeys in estrus. When hominids became bipedal (possibly our earliest major evolutionary divergence from the other apes), that anogenital pad would have become harder to see; much better to put the major features of sexual signaling up front, where our innovative ancestors would be most commonly gettin’ down to it. This idea becomes even more compelling when one considers the Gelada baboon, which has a bare chest patch that indeed looks very much like the ready-and-raring rear view.
Unfortunately, the neatly symmetrical just-so-ness of this is rather ruined first by the observation that the male Geladas have a chest patch as well, and second by the observation that nearly every feature OTHER than the breasts on the female that fits into the “front imitates rear for greater sexiness” paradigm have other, better explanations…. and that there are much biologically cheaper ways to do that anyway. Like making the chest bright red instead of expensively expanding its size by several orders of magnitude. Or, at least that’s what the other biological anthropologists said; not having to get along within a group of them, I feel free to point out that when you spend months at a time in the field where the single most interesting and action-inspiring point of interest for miles around is a baboon’s swollen red ass, it can start to take on slightly greater importance in your mind than is perhaps warranted.
Making the “your chest reminds me of your butt” hypothesis look rather staid and conservative is the explanation Elaine Morgan and her followers presented as part of the Aquatic Ape Hypothesis. The overall AAH asserts that hominids went through a lengthy semi-aquatic stage in our evolution, which explains many of our oddities: our hairlessness, our unusual affinity for water (for apes), our relatively high proportion of body fat, our fondness for full-frontal action when mating, and, of course, the breasts, which supposedly attained their large and hemispherical shape to make nursing in the water easier for the infant. (In the event of a milk emergency, this teat is designed to serve as a flotation device…) Unfortunately, that does precisely nothing to explain why they are there from puberty on, nor does it address the problems of a large and swollen breast in pressing against the mouth and nose of the infant on land or water, who would seem to have more than enough problems already. The idea that the relatively flat-faced human infant drove selection for rounder, fuller breasts because it would suffocate against a flat chest is also lacking- the flat-faced infant would benefit from the swollen but droopy teat sported by many primate mothers during lactation, not a bigger round one with a lot of extra tissue to raise the suffocation problem all over again.
Getting away from the nuttier theories, a more standard and run-of-the-mill sexual selection theory has been proposed: that females have bigger breasts for the same reason male peacocks are supposed to have bigger tails*: sexual selection. A chick who can afford to pack fat by the pound into a completely useless structure must be pretty robust and well set-up to have lots of healthy babies, if she can afford that kind of metabolic extravagance on pure display. Naturally, there are several immediately obvious problems with this as well.
1) Why the breasts? Why is fat in general not particularly attractive? The reason that breasts are so prone to cancer is that they’re glandular tissue, not just fat- and what’s worse, all that fat it’s packed in produces estrogen on its own, and being constantly bathed in hormones is a cancer risk in and of itself for a particular tissue- which is why ovarian cancer is the other big common cancer for women.
2) Big, exaggerated display-only features driven by sexual selection are almost exclusively the province of males in the rest of nature. Females need to be equipped to do all the biological heavy lifting in the business of reproduction; males have much more to prove in the realm of “my genes are so superior you can afford to put in this huge chunk of time and energy betting on them!”.
3) Even more inconveniently, for even the apes big, swollen breasts are a sign of a female who is busy lactating and therefore highly unlikely to be currently fertile. By straightforward logic, they should be a turn-OFF.
Human sexual biology is very rarely straightforward, for one major reason: concealed ovulation. Aside from breasts that only inflate when necessary, our primate relatives all signal clearly when they are fertile; the human female, in contrast, is sexually receptive all through the year and gives only the barest clues to her fertility status. This is what makes pair-bonded monogamy a viable reproductive status for humans at all: without any indication of whether or not the females he mates with are fertile, it dramatically swings the best fitness interests of the male away from promiscuity and toward heavy investment in the female and her relatively helpless offspring. In this context of perpetual confusion, permanently swollen breast tissue might just be one more signal regarding fertility or infertility that has been “switched off”.
This series of observations has led at least one scientist to the most novel hypothesis I’ve seen yet in the largely-lacking field of ways to come up with an explanation for big breasts and men’s attraction to them. No one who understands how sexual selection works has an issue with how breasts came to become big and permanent once males became attracted to them; the problematic part is how that could have ever come to pass in the first place.
One of the reasons that the most straightforward ideas of “the alpha male gets all the best mates, therefore all males are evolutionarily driven to be as badass alpha as they can be” have been crumbling in the studies of mate selection is the steadily increasing piles of examples of males in species whose sexual systems appear to be just that simple who have different, but successful strategies. One of the most common methods, especially among birds whose mating systems rely on female mate choice among a series of displays, is simply to exploit highly successful “big birds” who are courting more females than they can keep track of: stay on the periphery of their territory and pick off the females that are just that bit more interested in having fertilized eggs sometime this year than they are in being twentieth in line for Supermale. With brighter animals- primates- things get more interesting; the female has so much to worry about from violent males (one of the biggest causes of infant mortality in social primate groups), that it’s in her best interest to mate with a gentle one- even if she has to duck behind the alpha’s back to do it. The “be her friend” strategy for nice-guy monkeys who invest so much in grooming and interacting with the females that are *not* in estrus pays off for them when the female becomes willing to mate with him while the top two or three males are beating the shit out of each other. As it turns out, it’s a strategy with a pretty high payoff- fighting for the top few slots is a dangerous business with a high chance of gaining little or losing everything. You might not get to spend that spring with your pick of the hottest chicks, but you do get a fairly slow but steady supply of offspring.
This hypothesis of breast evolution is based on the ascent of the male with an eye to the future: the majority of males became attracted to the big, swollen breasts not in spite of their association with lactation, but because of it. A nursing female hominid is a female hominid with very high nutritional needs at that moment. She is likely to appreciate help with that greatly, and while she may not be fertile now, she will be later, and she has a long memory. Better yet, if she gets more and better grub while she’s nursing, she’ll be back to being fertile quite a bit faster. (This is why lactation can postpone fertility for a year or longer in lean hunter-gatherer societies, but a well-fed woman in Western society can easily have children nine months and ten minutes apart.)
Provisioning is the term for males bringing females food as part of a mating ritual, and it is found scattered widely in nature- either as a purely symbolic gesture, or as part of a hard slog of keeping a mate too occupied to feed herself well enough surviving to keep the kids alive. It’s found, unsurprisingly, among primates as well, although like most mammals they’re quite a bit more reluctant at it than the mainly-monogamous birds. The essential idea behind this theory of the breasts is that males who were willing to hang around a lactating female and give her food in hopes of retaining her goodwill for future mating purposes gained a reproductive advantage- much more so when the infertile female they were provisioning was lactating because she was carrying or feeding his own offspring. Males who immediately went looking for more fertile fields would suffer a disadvantage compared to males who stayed to maintain investment, and males who were uninterested in the goodwill of females who weren’t immediately fertile might likewise suffer in lost future opportunities. While it sounds like- and is- a highly unlikely scenario for promiscuous or harem-having apes, it’s a scenario that fits rather neatly with rapidly increasing pressure to form pair bonds from infants that were more and more helpless for longer and longer as brain size rocketed upwards for hominids.
As of yet, no hypothesis has emerged as truly compelling, or at least none has become the dominant one in anthropological circles. However, if the devoted-male theory ever does catch fire, perhaps we can even see breasts newly welcomed as a symbol of family values. We can only hope.
*Unfortunately, peacock tails have recently lost their status as the icon of ridiculous results of sexual selection. Guess it’s back to the Birds of Paradise.