Why Women Have Breasts


Many people may suppose that the question of the title is a stupid one, given that the answer is so obvious: women have breasts for feeding babies. In fact, the question is a good one, because it is a mystery why the vast majority of women has breasts. Most women are, at this moment, not lactating, and yet they have breasts. If breasts were merely for feeding babies, then most women would not have them. They would develop them only during pregnancy, and would lose them again when they stopped breast-feeding. Humans are unique in the animal world, in that they develop breasts at puberty and retain them into old age, whether or not they ever get pregnant. This requires an explanation.
A heifer develops an udder when it is time to produce milk for feeding her calf. Once the calf has stopped feeding, the heifer (now a "cow") dries up and the udder disappears. Growing an udder, like any body tissue, is a cost. It takes nutrients to grow and maintain the udder, and the udder itself is a hindrance to quick movement. Indeed, prominent udders are not necessarily needed at all. Seals manage to feed milk to their young without any such protuberances which would spoil their hydrodynamic form. A woman pays a cost in nutrition and agility by growing breasts when she doesn’t need them. If there is a cost, then there must be a benefit to explain why this human trait has evolved. Given that the trait is unique to humans, it is very likely indeed that the explanation will be found in some other characteristic that is unique to humans.
There is surprisingly little literature on this topic. There are a few short papers, many written long ago, and most of which have been soundly criticised by more recent writers. There is little point in repeating the demolition of several early theories here. See the bibliography for further reading (Cant 1981 especially).
Herbivorous women, carnivorous men
Our ancestors were hunters, who survived principally from eating other animals. With other hunter species, like dogs, cats, and stoats, the adult females hunt, unless perhaps they are heavily pregnant or tending young offspring. Hunting is difficult enough without udders, and so the hampering effect of udders is minimised - they only grow them when lactating. Herbivorous herd beasts like cows, sheep, and deer must be able to escape predators by running faster than their neighbours in the herd, and so one with useless udders is just asking to be eaten. Most mammals rely on speed to escape being eaten, so even solitary herbivores would be badly advised to grow udders until the proper time.
Humans, however, are importantly different, in that they do not rely mainly on speed to escape predation. Few animals will attack a group of humans, and humans will use group tactics, fire, weapons, missiles, and the like to defend themselves. Any animal powerful enough to bring down an animal as big as a grown woman, can out-run her so easily that speed of foot is not the issue. Human females gather food in groups, and in parts of the world where this is dangerous, tribes post armed teenage boys, and the occasional older man, to guard them. Much of their time they spend in camps, in the presence of fire, and very few animals will venture there. Human females, therefore, paid a much smaller price for developing udders than other animals, during their evolution. Very few women in the Pleistocene of child-bearing age would have been lost to predators. They were free to be hampered by non-lactating breasts.
In other words, division of labour made nulliparous breasts possible. Our ancestral fathers behaved like carnivores, and needed to be able to run and jump freely to hunt, and our ancestral mothers behaved more like herbivores, harvesting in groups. A nullipara is a female who has not yet borne children. It is an interesting quirk of our culture that we all know the word virgin, but few know nullipara.
The above explains how it could have been possible for non-lactating breasts to evolve, but it still falls a long way short of saying why they actually did evolve.
Beautiful plumage, beautiful breasts?
Once it has been established that it is possible for breasts to evolve, then an evolutionary scientist can argue that breasts can evolve as sexual characteristics. They could be like peacocks’ tails – arbitrary signals to the opposite sex. Peacock tails are big, fan-shaped and green, with lots of “eye-spots”. Other birds have long tapering tails or bright red plumage. It seems that it doesn’t really matter what sort of plumage evolves, as long as it belongs recognisably to a certain species, and as long as the opposite sex develops a preference for it. Once peahens start finding fan-shaped green tails with spots sexy, then it is in their interests to mate with the male with the best tail of that description. Human breasts could have evolved in the same arbitrary way. This can lead to “Fisherian run-away selection” which very rapidly causes a species to evolve such things as bright plumage, wattles and combs, and, perhaps, breasts. Fisher’s logic would say that once men start to find breasts attractive, women have selective pressure on them to grow them, and thus do such women become sexier, and thus is there a greater pressure on men to find them sexy.
The classic Fisherian example is the peacock's tail, and I apologise to evolutionists who have read about this too many times already. In peacocks, however, the males are the pretty ones. In humans, somehow this is reversed: women are the pretty ones, and men are all astonishingly ugly. In most species, the females are the replicators, and must be sensible about camouflage and the like, and so can't afford to be showy, but even if most of the males die because of their hampering plumage, this doesn't matter, since those left can fertilise all the females, and will probably have the best genes anyway, since they managed to survive. This occurs in its most extreme form in species where the males play little or no part in the rearing of offspring. Humans have fairly high male parental investment, however, and so the males are more valuable, and as said before, mortality due to predation in adult females, even those hampered by udders, would be so low, that this rule could in humans be sex-reversed.
That breasts were selected for by sexual selection is convincingly argued by Geoffrey Miller in his book The Mating Mind (2000). Like other such traits, they exaggerate the different between the sexes; they are invisible on the skeleton; they vary greatly between individuals; they enlarge after puberty; they engorge with blood during sexual arousal; all cultures value them as sexual symbols and many mutilate them for sexual offences; all around the world they are emphasised to look sexy or played down to avoid sexual attention. That breasts vary so much in size is a good clue to their not being utilitarian like hands or eyes. Pre-lactating breasts consist mainly of fat. Lactating breasts produce the most milk about eight months after the start of lactation. During the first eight months of lactation, however, the breasts are at their largest. It seems that the fat is actually hampering to milk production, and that it takes eight months to change the design of the breast to maximise efficiency, and get rid of the decorative fat.
A peacock’s tail seems to have some innate qualities of beauty, in that humans find them beautiful, and they were not designed for us to gaze upon. A human breast involves a circle within a circle within a circle, all presented to the viewer. It is easy to imagine that there might be some innate beauty in this design. The nipple is not placed for efficient feeding. When tapping a barrel, one does not put the tap half way up the barrel. A much more efficient<