In recognition of the "Boobquake" that was recently celebrated around the world as liberated women shook their breasts (figuratively) in the face of the Iranian cleric who recently blamed earthquakes on female sexuality, a few words on the mysterious allure of the human female breast.

Considering its almost total lack of muscle tissue, the female breast wields amazing power. Curvaceous women have leveraged this power to manipulate even the most accomplished, disciplined men for as long as anyone's been around to notice. Empires have fallen, wills have been revised, millions of magazines and calendars sold, Super Bowl audiences scandalized ... all in response to the mysterious force emanating from what are, after all, small bags of fat.

One of the oldest human images known, the so-called Venus of Willendorf, created about 25,000 years ago, features a bosom of Dolly Parton-esque dimensions. Two hundred fifty centuries later, the power of the exaggerated breast shows little sign of getting old. According to the American Society of Plastic Surgery, 347,254 breast augmentation procedures were performed in the United States in 2007, making it the nation's most commonly performed surgical procedure. What gives the female breast such transcendent influence over heterosexual male consciousness?

First, let's dispense with any purely utilitarian interpretations. While the mammary glands contained in women's breasts exist for the feeding of infants, the fatty tissue that confers the magical curve of the human breast has nothing to do with milk production. Given the clear physiological costs of having pendulous breasts (back strain, loss of balance, difficulty running), if they aren't meant to advertise milk for babies, why did human females evolve and retain these cumbersome appendages?

Theories range from the belief that breasts serve as signaling devices announcing fertility and fat deposits sufficient to withstand the rigors of and breastfeeding, to "genital echo theory": females developed pendulous breasts around the time hominids began walking upright in order to provoke the excitation males formerly felt when gazing at the fatty deposits on the buttocks.

Theorists supporting genital echo theory have noted that swellings like those of chimpanzees and bonobos would interfere with locomotion in a bi-pedal primate, so when our distant ancestors began walking upright, they reason, some of the female's fertility signaling moved from the rear office, as it were, to the front showroom. In a bit of historical ping-pong, the dictates of fashion have moved the swelling back and forth over the centuries, with high heels, Victorian bustles, and other derrière enhancements.

The visual similarity between these two bits of female anatomy has been facilitated by the recent popularity of low-cut jeans that teasingly reveal the nether cleavage. "The butt crack is the new cleavage," writes journalist Janelle Brown, "reclaimed to peek seductively from the pants of supermodels and commoners alike. ... It's naughty and slightly tawdry," she continues, "but with the soft round charm of a perfect pair of breasts."

If your moon is waning, you can always don a "butt bra" from Bubbles Bodywear, which promises to create the effect that's been turning male heads since before men existed. Like the Victorian bustle, the butt bra mimics the full curves of the ovulating chimp or bonobo. Speaking of waning moons, it's worth noting that unless her breasts are artificially enhanced, as a woman's fertility fades with age, so do her breasts—further supporting the claim that they evolved to signal fertility (or at least, sexual availability).

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Human females aren't the only primates with fertility signals on their chests. The Gelada baboon is another vertically oriented primate with sexual swellings on the females' chests. As we'd expect, the Gelada's swellings come and go with the females' sexual receptivity. As the human female is potentially always sexually receptive, her breasts are more or less always swollen, from sexual maturity on.

But not all female primates have genital swellings that visually announce their ovulatory status. Primatologist Meredith Small reports that only 54 out of 78 species surveyed "experience easily seen morphological changes during cycles," and that half of these showed "only slight pinkness."

Once again, our two closest primate cousins stand out from the pack in terms of their decidedly indiscreet , being the only primates with such extravagant, brightly colored sexual swellings. The female chimp's red-light district comes and goes, reflecting the waxing and waning of her fertility, but as Small confirms, the bonobo's "swellings never change much, so that bonobo females always give a signal of fertility—much as humans do."

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Some of this material appears in Sex at Dawn: The Prehistoric Origins of Modern Sexuality.