Female warblers go gaga for potential mates who serenade them with extensive song repertoires.
Red jungle hens favor a rooster not for his strutting, heft or fineness of feathers, but for the length of his comb and the brilliance of his wattle.
The female scorpion fly is a stickler for symmetry, losing her head over a fella with wings identical in length, width and shape.
While the criteria vary, the warbler, jungle hen and scorpion fly share the same biological motivation: selecting the healthiest suitor available, the better to produce disease-resistant offspring.
Scientists have established that warblers with big playlists, long-combed roosters with bright wattles and symmetrically perfect scorpion flies are healthier, stand a better chance at copulatory success and produce offspring that are more disease resistant than do their less body-beautiful competitors.
Human preferences are also built upon certain aesthetics that ensure a mate isn't passing along damaged goods. Like the scorpion fly, we size up the physical form of a potential mate as a semaphore of good health.
But Devendra Singh, a University of Texas psychologist, has ferreted out one mate preference detector that is singularly Homo sapiens: that universal measure of sexual allure, waist appeal.
Both sexes consider the va-va-va-voom from hip to waist in mate selection, but physique is more important to men, studies indicate.
Boys and men of various backgrounds, ages and nationalities were shown line drawings of female figures with varying waist-hip ratios and asked to rate them on attractiveness, sexiness, fertility and health. Women also ranked the drawings. The favorite was a figure of average weight with a waist 70% the size of her hips, or a .7 ratio. (Translation: roughly a 25-inch waist with 37-inch hips. Popular as they are, breasts were not factored in, but the bottom half of the hourglass figure fits the ratio.)
The only sex difference evident in the survey was women rated an underweight figure drawing as equally attractive as the normal weight figure while men did not.
While a smaller waist set upon wider hips is an undeniable characteristic of what is cross-culturally identified as "sexy," it's also a reproductive calling card: "I am a goddess of health and fertility." (It's no accident that we are celebrating the 50th anniversary of the bikini.)
"Human beauty ideals have biological meaning," Singh says. "Yes, people always said they liked the hourglass figure, but they did not know that the waist-hip ratio was a health certificate. If women don't have the right hormonal conditions, they won't have the right fat distribution. Notice I did not say a woman who is healthy, fertile and skinnier. A healthy, fertile woman can be 140 or 110 pounds and still have a waist-hip ratio .7."
Before puberty and after menopause, the sexes have roughly the same waistline. Post-puberty, the average girl amasses nearly 35 pounds of estrogen reproductive fat padding her hips and thighs, a savings account of fuel capable of sustaining pregnancy and a year of lactation (at a cost of 180,000 calories or hundreds of pints of Haagan Dazs) during a brief famine. Boys lose weight at the hips and thighs with testosterone fueling muscle mass in the shoulders, arms and chest.
Most women's waist-hip ratio ranges from .6 to .8 during the reproductive years, Singh says.
Growing evidence suggests that the waist-hip ratio is a reliable indicator of fertility and absence of conditions such as diabetes, hypertension, cardiovascular disease, gall bladder disease and cancer of the breast, cervix and ovaries.
A 1993 Dutch study of 500 women seeking artificial insemination, most because their partners were infertile, sterile or had genetic abnormalities, found that a woman's chance of conceiving decreased 30% for every 10% increase in her waist-hip ratio, regardless of weight. (The greater the difference between waist and hip size, the lower the ratio.)
Estrogen seems to lower the ratio while testosterone increases it. Women with polycystic ovarian syndrome have elevated testosterone and higher waist-hip ratios. Menopausal women taking estrogen-enhancing medication, who are not obese, develop lower waist-hip ratios than comparable women not taking enhancers, a study found. Still other problems--such as anorexia, obesity and production of excess androgen, a male sex hormone--cause ovulatory dysfunction while affecting waist-hip ratios.
But can it be that the attraction of below-the-navel curvaceousness can be isolated to nothing more than a biological courting trigger?
"Certainly there is cultural enhancement of it," says Donald Symons, an evolutionary psychologist at UC Santa Barbara and author of "The Evolution of Human Sexuality" (Oxford University Press, 1979). "But if it was cultural, it would be arbitrary [like face tattoos and lip elongation] and wouldn't cross cultures. Why would nuns conceal their bodies and why would women have ribs removed to make their waists appear smaller? Degenerative diseases like diabetes did not exist in ancient times, which suggests [the ratio] was used primarily as a cue to a woman's reproductive health."
Evolutionary psychologists hypothesize that since the "potential reproductive value" of women is concealed, unlike many primates and other species, the larger hips with a smaller waist has grown attractive to men because of its role as an unshakable reproductive gauge. During "initial selection," men (who are probably not thinking about babies) can separate the fertility goddesses from the reproductively out-of-order by looking at the back, side or front and from a considerable distance.
It also signals pregnancy. "Our forefathers' great question has been 'Is this my child?' " Singh says. "Pregnancy shows up in the waist within a month."
Women are leaner today, but the waist-hip ratio has been stable for decades, even centuries. Ancient statues of the femme ideal share few characteristics with Cindy Crawford, but they have the same waist-hip ratio. Playboy centerfolds and Miss America winners from 1923 to 1990 grew significantly thinner but the waist-hip ratios ranged from .68 to .72, Singh's research indicates.
The 19th century female beauty paragon ranged from the physically frail to the robust, but the corseted waistline was universal (and cinched to 18 inches, if possible). Anthropologist Desmond Morris reports that the first cosmetic surgery in England was removal of two lower ribs to enhance tininess of waist. Just last year, American women shelled out more than half a billion dollars for the flesh-persecuting, nerve-pinching, Lycra-squeezing phenomena called body reshapers.
Even Barbie, that paragon of fat-free womanliness, can't escape. Singh wrapped electrical tape around the icon's waist, increasing her ratio of .6 up to .8 and .9, then dressed her in glittery couture and showed her to test subjects. "People said 'I don't like this Barbie,' " he says.
To prove the dynamic is reversible, Singh filed Barbie's waist down to a .4 waist-hip ratio, which both sexes also judged "unattractive."
"The body is the absolute messenger of sexuality . . . the face is not," Singh says.
Women subject men to a similar acid test, but with much less fervor. Using computer images of Michelangelo's sculpture of David, the epitome of stony manliness, Singh altered the waist-hip ratio from the he-man ideal of .9 (narrow hips, broad shoulders, fanning chest) to the female ideal of .7. "His shoulders, back and feet were still masculine," he says. "And . . . they hated him."
In another survey of women, Singh tested the popular notion that the higher the man's income (solid resources, good mate choice), the more forgiving she is of his, well, potbelly (not a healthy specimen). Women aged 18 to 69 rated drawings of men, with waist-hip ratios of .7, .8., .9 and .10, and who had incomes ranging from $15,000 to $80,000. "Even if he is rich, they don't want him."
Of course, this last outcome applies only in the hypothetical perfect world of figure-drawn men. "You see all these fat, bald bankers with beautiful women," says Helen Fisher, a Rutgers University anthropologists and author of "Anatomy of Love" (Fawcett Columbine, 1992). "Women aren't too forgiving about lack of money. Men feel preyed upon for their wallet and lots of women feel pressure to stay young and beautiful. In some ways, it's easier and less dangerous to look beautiful than to be rich."