Feeding, not mating, made the shape, claims scientist
The roundness and up-tilt of a woman's breast evolved to prevent babies from smothering; they didn't take shape because they were a sexual lure to men, an English anthropologist suggests.
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Monkeys, with their jutting jaws, can suckle their flat-chested mothers without suffocating, but humans, whose faces are flatter, caused evolutionary pressure on breast shape, making them rounder and fuller to allow human infants to breathe while feeding, says the scientist.
"The idea came to me from breast feeding my own child when she was an infant," says Gillian Bentley, a Royal Society research fellow at University College in London, England. "If the human breast were flat during lactation, which is true for our closest primate relatives, human infants, with their flat faces, would suffocate."
The reason for the shape of the human breast
Evolutionary biologists have long reflected on the reason for the shape of the human breast. Because breasts don't develop until puberty, biologists have suggested that they helped early human females attract a mate and keep him interested in her welfare and the welfare of her children. Those with larger breasts were more successful, the theory goes, and therefore produced more offspring.
"My theory turns this around," Bentley says. "Humans have flatter faces because of the evolution of a larger brain, which necessitated a change in skull shape, as well as some changes in mouth structure, the tongue, the muscles that support the tongue and the length of the throat as language developed. The whole idea is that there was a co-evolution of the human flat face with breast shape, and thus there was evolutionary pressure on the shape of the breast to accommodate the infant."
Eroticising the breast
On the other hand, if sexual selection is the reason for the larger human breast, as many scientists have thought, why don't all cultures eroticise the breast?
"It is true that in many cultures around the world, the breast is not sexual," says Marilyn Yalom, author of A History of the Breast and A History of the Wife. "The breast is over-eroticized in the Western world, but if you look at cultures in Africa, the breast is for the baby, not for the male. It is the buttocks that have become the erotic focal [there]."
The same is true in other cultures; for example, the Chinese make a fetish of the foot, whereas the Japanese concentrated on the nape of the neck, says Yalom, who is a senior scholar with the Institute for Research on Women and Gender at Stanford University in Palo Alto, Calif.
Blame the male academics for the sexual selection theory, Bentley says. "The majority of people who used the sexual selection argument have been male and were part of Western culture with its eroticized view of the breast."
Her findings appear in the April 11 issue of New Scientist.
Why does the breast respond sexually?
But if sexual selection is not the reason for the shape of the human breast, why does the breast respond sexually?
Bentley says, "It could be a combo here. The pressure for the shape of the breast was initially from this need to feed the baby and then, secondarily, men may have evolved to find the breast of this shape and size more sexually interesting. The convergence of the two pressures could have evolved into more successfully reproducing females."
Yalom says the smothering hypothesis "doesn't ring true for me, with disclaimers of not being an anthropologist or an evolutionary biologist. The complexity of evolution doesn't, in my mind, point to a single factor, but is multi-factorial. While the shape of an infant's face could be a part of it, so is sexual selection."
Yalom says, "I'm also not convinced that it is necessary for the breast to be large in order to prevent smothering. Au contraire. Fat in the breast is also related to the production of milk in humans, which is not a part of Bentley's hypothesis."
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