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By definition, all female mammals have mammary glands that produce milk for feeding offspring. Any discussion about the evolution of breasts has to take this mammalian heritage as the starting point. Milk-substitute manufacturers have worked very hard for almost a century to convince women that they are not mammals and have no business breast-feeding. Even many science journalists support this view, as when some recent research was reported as showing that "breast-feeding raises IQ by five points," rather than "bottle-feeding reduces IQ by five points"—as if bottle-feeding was the biological norm. The popularity of bottle-feeding and breast implants should not mislead us into viewing breasts as nothing more than sexual ornaments.

During human evolution, female breasts would have been producing milk about half of the time between puberty and menopause. Babies probably nursed for at least a year or two, as they do in hunter-gatherer societies today. Without contraception, after a mother stopped nursing one baby she would typically have conceived the next baby within a few months. Assuming that the average female hominid produced at least 20 fluid ounces of milk per day when breast-feeding, and she spent a total of ten years breast-feeding in her life, the average hominid breast would have delivered over 35,000 fluid ounces (nearly 300 U.S. gallons) before menopause.

This high level of milk production does not itself explain why female humans breasts are so much larger than those of other apes. Most primate females are quite flat-chested, even when producing milk. Milk output depends on the amount of active glandular tissue in the breast, not the volume of fat. Human breasts have an unusually high ratio of fat to glandular tissue. They do not seem to be optimized for milk production. Most experts on breast-feeding claim there is no correlation between breast size before pregnancy and milk production ability after birth (though I know of no good data on this point). Milk output seems limited more by a woman's overall nutritional state than by her pre-pregnancy breast size. So, we have to distinguish between mammary glands, which evolved for milk production, and enlarged human breasts, which must have evolved for something else. It seems likely that sexual selection played a role. But how?

Perhaps breasts evolved as cues of sexual maturity. Human breasts enlarge at puberty long before they are required for breast-feeding the first baby. Just as bipedal walking may have allowed female choice to focus more on the penis, bipedalism may have allowed male choice to focus on female breasts as a maturity cue. However, maturity cues do not have to be so dramatic. Males have evolutionary incentives to distinguish mature women from infertile girls, women have evolutionary incentives to advertise their fertility, and girls have evolutionary incentives to advertise their infertility. Given these shared interests, signals of sexual maturity could be very inconspicuous. Males of most other species have no trouble distinguishing mature from immature females using relatively subtle cues.

It seems likely that male choice shaped breasts not to distinguish girls from women, but to distinguish young women from older women. Here, the informative thing about breasts is the way they droop with the effects of age and gravity. There is a relatively narrow age window in which large breasts can appear pert before repeated cycles of pregnancy and breast-feeding cause them to sag. There were no bras or breast-lift operations in the Pleistocene. As we saw in the previous chapter, hominid males probably favored younger women for their higher fertility. Any indicator of youth, such as large, pert breasts, would tend to be favored by males. A male preference for size and pertness would spread at the expense of male preferences for droopiness and flatness, because the latter preferences would generally lead men to choose older, less fertile partners.

This argument sounds fine from the male point of view, but it takes a bit of thought to see why females should evolve youth indicators. The most informative cues of youth are also the most informative cues of age. Youth indicators might make women more attractive when they are truly young, but might make them less attractive when they are older. A mutation that caused an enlargement in breast size might benefit its carriers when they are in their teens and twenties, but impose high costs when they are in their thirties and forties. The question is whether the early benefits would outweigh the later costs. The answer is probably yes, because it is almost always better to have babies earlier than later in life. Females tend to be more fertile in youth, produce fewer birth defects, are in better shape to care for offspring, and are more likely to have living sisters and mothers to help with child-care. Also, fast breeders produce more generations per century, so can increase their population numbers faster than slow breeders. For these reasons an attractiveness benefit in youth can often outweigh an unattractiveness cost in older age. This is why it can be in the interest of females to evolve youth indicators such as large breasts that tend to droop, fine skin that tends to wrinkle, and buttocks that tend to develop stretch marks. This is one of the most counter-intuitive applications of Zahavi's handicap principle.

Breasts also make good fitness indicators because they come in symmetric pairs. I mentioned earlier that many bodily ornaments in many species advertise an aspect of fitness called developmental stability When body traits grow in pairs, perfectly symmetric development of the pair indicates high fitness. The paired traits tend to grow large to make their symmetry more obvious during mate choice. Evolutionary psychologists John Manning and Randy Thornhill have shown that women with more symmetric breasts tend to be more fertile. It is possible that bipedalism made breasts a useful potential cue of developmental stability for male mate choice. Once men started paying attention to the symmetry of breast development, high-fitness women could better display the symmetry by evolving large breasts. The larger the breasts, the easier it is to notice asymmetries. Perhaps single mastectomies are so distressing to women because breast symmetry has been such an important fitness cue during human evolution. Large human breasts may have evolved to advertise fitness through their symmetry, not just youth through their pertness.

Finally, breasts are pretty good indicators of fat reserves. In the Pleistocene, starving was more of a problem than overeating. It was harder to have good fat reserves than to be extremely thin, because women had to use their own energy and intelligence to gather food from their environment. It would be possible to spread one's fat evenly over the whole body surface, like a porpoise, but that would make it hard for men to compare females, and it would give females too much insulation under the scorching African sun. Females who concentrated their fat-displays in breast and buttocks could attract male interest without overheating. Also, by not depositing too much fat on the abdomen (as males tend to), females could avoid appearing pregnant already—a sure sign of not being fertile at the moment, which might inhibit male sexual attention. Breasts appear to have evolved as highly condition-dependent indicators of a woman's nutritional state. Most women who have tried dieting know that breast size is the first thing to shrink when food intake is restricted.

The role of breasts as fitness indicators may help to explain why there is so much variation in breast size among women. If large breasts were critical for breast-feeding, which is one of the single most important stages in mammalian reproduction, all women would have large breasts. But as we have seen, fitness indicators do not tend to converge on a single size in a population. They maintain their variation indefinitely, due to the effects of genetic mutation and variation in condition. It has sometimes been argued that men's preferences for larger-than-average breasts must be an artifact of modern culture, because, if it were ancient, all women would have already have evolved large breasts. This argument is wrong if breasts evolved as fitness indicators. Bra manufacturers offer a range from A-cups to D-cups because evolution amplifies the variation in each fitness indicator rather than using it up.

However, even more important in explaining such variation is the fact that each sex assesses the other using a wide range of fitness indicators. This leads to surprising and subtle effects. Imagine that each indicator advertises a different aspect of physical or mental fitness. Because each indicator is costly (so it works according to the handicap principle), there are trade-offs between indicators. This allows scope for individuals to differ in their allocation of resources to different indicators. One individual may grow very tall and muscular; another may grow very symmetric breasts; yet another may grow very intelligent. Each may advertise the same general level of fitness, but may advertise it in a very different way. If height, breast symmetry, and intelligence are all fitness indicators, then—by definition—they must all correlate with fitness, so they must also be positively correlated with one another to some extent. However, such correlations might be quite modest. This implies that even if individuals select mates for their overall fitness, sexual selection may not have the power to drive every fitness indicator to its maximum value. Instead, sexual selection may produce a great diversity of strategies for allocating scarce bodily resources among different indicators. Variation in overall fitness level, combined with variation in these allocation strategies, may account for the rich human variation that we observe. It also explains why not all women have very large breasts—many women may be genetically programmed to prioritize other indicators of physical and mental fitness.

Like penises, breasts have given us some practical information about mate choice in the Pleistocene. The amplification of female human breast size beyond what was useful for milk production reveals the importance of male mate choice in human evolution. If males had not been picky about their sexual partners, female humans would be as flat-chested as chimpanzees. The clitoris does not yield evidence of male mate choice, but breasts do. This opens the door to the possibility of male mate choice influencing the evolution of female brains as well as bodies. Breasts seem to act simultaneously as indicators of youth, indicators of developmental stability, and indicators of foraging ability. We shall see that many of the human mind's most distinctive abilities seem to serve the same range of functions.

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