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In the primate world, plump breasts last only as long as breastfeeding doesexcept in humans. Women are busty all the time, even after menopause. According to some scientists, the trait is an evolutionary trick for snagging men and signals a woman's ability to feed her children.

But no one can confirm an answer as to why women are the only apes with sizable chests .

Even though they appear full, a woman's breasts are only filled with milk after a she gives birth. The rest of the time, they're mostly made up of fat.

"Human breasts could therefore be a kind of biological deception," according to David P. Barash and Judith Eve Lipton at the National Sexuality Resource Center.

However, Barash and Lipton report that full breasts could just as easily signal the truth about a woman's ability to store fat and her fertility. For example, flat-chested prepubescent girls are too young to bear children, and the sagging shrunken chests of older women may suggest they're past their prime.

Because breasts sometimes get in women's way, some scientists have developed an evolutionary theory they call a "handicap principle." According to this theory, heavy breasts honestly announce a woman's genetic health, but at a cost of her carrying them around.

Barash and Lipton explain that this same idea applies to creatures like the male peacock, which struts around with his awkward, ornamental tail in hopes of roping in mates.

One lesser-agreed upon theory, supported by Leonard Shlain, a surgeon and author of "Sex, Time and Power: How Women's Sexuality Shaped Human Evolution,"(Viking, 2003), suggests that women's breasts grew round after our early ancestors stood upright.

In this view, the breasts of the female ancestors of humans evolved over time, along with a gradual tilting of the pelvis, so that the vagina was more oriented to the front of the body. Together, these transformations encouraged face-to-face sex, and marked a departure from the position most commonly used by other apes, in which the male approaches the female from behind.

Ethologist Desmond Morris has also proposed this theory, and has suggested breasts are substitutes for the round, red buttocks of our female ape ancestors .

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