Why do women have breasts?

At first glance the reason behind breasts seems obvious: they’re there to feed our children. All mammals have lactating mammary glands – in fact the word ‘mammal’ itself originates from the Latin mammalis, literally meaning ‘of the breast’.

Yet strikingly, no other mammal has developed such large and instantly recognisable mammary glands as ours, which are mostly made up of fat rather than milk-producing glandular tissue. The fact that other mammals only develop swollen breasts during lactation, which then shrink away to nothing as soon as the offspring is weaned and the milk dries up, has for many years proved puzzling to the evolutionary biologist.

Over the years, various theories have sprung up behind the reasoning for breasts, some with hypotheses more firmly rooted in fact than others. In 1840 the eminent English anatomist and surgeon, Astley Paston Cooper, claimed in his book On The Anatomy of the Breast that fatty breasts both keep the temperature of the milk warm and ‘enable women of the lower classes to bear the very severe blows which they often receive in their drunken pugilistic contests’…

Moving on to the more universally accepted Darwinian theory of natural selection, in 1967 English zoologist Desmond Morris published his bestselling book, The Naked Ape, a bold examination of the human species. To Morris, breasts provided a visual reminder of the buttocks purely for mating purposes: ‘The protuberant, hemispherical breasts of the female must surely be copies of the fleshy buttocks, and the sharply defined red lips around the mouth must be copies of the red labia.’ Morris went on to explain that most mammals mate dorsoventrally (from behind), with the males attracted by the females’ fleshy buttocks. To have the buttocks ‘recreated’ in the form of breasts allows men to mate with women while facing them. This is more personal – we can look into each other’s eyes – encouraging us to pair bond and eventually become monogamous parents who work together to raise our children.

All the better for the species, or so it seemed. Although immensely popular, The Naked Ape and its tendency to present evolution from a very masculine perspective was controversial, especially as it emerged simultaneously with the women’s lib movement. Scientists criticised Morris’ ‘buttock theory’ as it didn’t follow through logically – gorillas and orang-utans also mate facing each other, yet they have flat chests. And the couple theory didn’t quite work either because it was argued that frontal mating isn’t necessary for pair bonding. Of the various species that also choose to pair bond and work together as parents – including pygmy marmosets, otters and certain types of foxes – they all mate dorsoventrally.

After reading The Naked Ape, Welsh writer Elaine Morgan, who despite not being a scientist authored several books about evolutionary anthropology, introduced her Aquatic Ape hypothesis in 1975. She claimed that humans had for a time gone through a ‘water phase’ and breasts evolved mainly to allow us to survive in the water, literally as flotation devices for the woman, and for something to hold on to for the children. Presumably the men would be holding on for dear life as well – heaven forbid you had a large family…

Aside from the ability to nourish our babies, breasts are undoubtedly seen as both social and sexual signals to those around us. Socially, a flat chest suggests a pre-pubescent girl, while a young woman with a developed bosom, broadly speaking, might be taken more seriously as she is clearly older. From a reproductive perspective, breasts are a clear indication of sexual maturity and fertility, as basic (and offensive) an equation as:

Undeveloped = not ready yet
Full = ready to go
Empty and/or sagging = too old

and therefore males can ‘select’ the ‘ripest’ female.

Erotic, social signalling of this kind is well known in the animal world. Think of the big red bottoms paraded by baboons and macaques; these are clear seasonal and sexual signs that the females are ready to breed. Evolutionary theorists frequently cite the peacock and his ridiculously long and brightly coloured tail. No doubt the showiest peacock will score with the relatively demure peahen of his dreams, but evolution’s not really tending to his needs. The same thing that attracts his mate’s attention also makes him stand out to predators, and surely it must quite simply get in the way. There are parallels to be drawn here with larger breasts and the need for supportive underwear, likewise breasts’ susceptibility towards cancer.

Certainly men will be drawn to an ample, pert bosom, but on an evolutionary scale this still begs the question posed by Florence Williams in her book Breasts: A natural and unnatural history: ‘If big, firm breasts tell a man that a woman is fertile and ready for sex, then why would her breasts be biggest and firmest when she’s already pregnant or lactating?’

Breasts are also known quite basically as a ‘fat store’, as on average, reproductive-age women store twice the fat that men do, and they store it in this area as well as around their hips and bottoms. Some scientists speculate that in the past this may have come in handy during times when food wasn’t so plentiful – and it’s also useful during pregnancy and breastfeeding when more calories are burned than normal. While breasts may well be a clear visual signal of physical health and genetic quality, perhaps the idea of ‘woman as larder’ would be equally appealing at various stages throughout history!

In Breasts: A Natural and Unnatural History, Florence Williams makes an interesting case for a slightly different evolutionary reason. She describes how when our ancestors lost their fur, our young were no longer able to cling on from an early age; something that other primates take advantage of as their ‘hands-free’ mother is able to swing from tree to tree and forage for food while her baby suckles. Without fur and until our young are strong enough to both support their own heads and cling on, we are forced to hold our babies in our arms.

Williams explains that the shape and pendulous nature of the breast and its flexible nipple allows babies to suckle easily. The skull shape of baby is the driving factor here – our large brains mean our heads are approximately five times that of an equivalent primate with a similarly sized body. Also newborns have distinctively ‘flat’ faces – a by-product of having to squeeze through our narrow bipedal hips during childbirth. As flat faces and flat chests don’t work well together, today’s squishy, fatty and malleable breast shape and moveable nipple helps babies latch on and breathe during feeding.

In Breasts: The women’s perspective on an American obsession, author Carolyn Latteier raises the concept of deception theory. Returning momentarily to the pair bonding aspect of evolution, this theory suggests that, as enlarged breasts denote a lactating or pregnant female, a prehistoric woman’s mate might assume she is temporarily infertile and thus loosen his hold over her, allowing her to wander off and mate with whomever she chooses. In this case it is assumed females were pre-programmed to search for superior genes for their offspring whenever the chance arose – although doesn’t that sound a bit like a male characteristic? Evolution’s a funny old thing.

Quoting anthropologist Robert L. Smith, Latteier also discusses the possibility that breasts exist primarily to ‘secure maximum support from reluctant males.’ Indeed, this is where provisioning theory comes in. As enlarged breasts suggest pregnancy and/or nursing, two periods in which a female would be less physically able to go out and forage for food, the theory is that evolution supplied female hominids with permanent prominent mammaries in order to gain food, shelter and protection from the group as a whole, because males are pre-programmed to protect their young, even in utero. This would supposedly work for males other than the father of her supposed child. Interestingly, if this theory were correct, then lesser-endowed females would have received far less food and protection than their more voluptuous friends. Sometimes life just isn’t fair.

‘Evolutionary theories are journeys in search of the truth, but they also serve as metaphors and justifications for our present-day culture as myths of origins we spin in response to the questions that trouble us,’ Latteier notes with good reason. It is likely that we’ll never know exactly how or why breasts evolved into the uniquely human shape we all know and love, although as Latteier continues, ‘It would be nice to have a clean and simple theory, one that somehow rises above women’s history of surviving for millennia by pleasing, by manipulating, by influencing, and by nurturing.’

If only we could live long enough, one thing that would be interesting since the advent of plastic surgery and breast enhancement, would be to see how these natural symbols of sexuality, fertility, comfort, age and nourishment continue to evolve in the future.

by Bryony Sutherland