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Beauty is only skin deep. How sweet that old chestnut is, equally comforting to the unbeautiful, who know they have so much beyond physical appearance to offer the world, and the beautiful, who, after years of being pursued for their prettiness, really do want to be loved for their inner selves.

The only problem with the cliche, say evolutionary biologists, is that it may not be true. In the view of a growing number of researchers who study why animals are attracted to each other, a beautiful face and figure may be alluring not for whimsical aesthetic reasons, but because outward beauty is a reasonably reliable indicator of underlying quality.These biologists have gathered evidence from studies of species as diverse as zebra finches, scorpion flies, elk and human beings that creatures appraise the overall worthiness of a potential mate by looking for at least one classic benchmark of beauty: symmetry.

By this theory, the choosier partner in a pair - usually, though not always, the female - seeks in a suitor the maximum possible balance between the left and right halves of the body.

She looks for signs of exquisite harmony, checking that the left wing is the same length and shape as the right, for example, or that the lips extend out in mirror-image curves from the center of the face. In searching for symmetry, she gains essential clues to the state of the male's health, the vigor of his immune system, the ability of his genes to have withstood the tribulations of the environment as he was growing up.

The new emphasis on the importance of symmetry to mate choice is one of those annoying developments in research that lends oblique validity to ingrained prejudices - in this case, to a fairy-tale view of the world, in which princes and princesses are righteous, strong and lovely, while the bad folk are misshapen and ugly.

Biologists emphasize that symmetry is just part of the story of how animals make their choices, and that much remains to be learned about what, in any given species, the possession of a perfectly proportioned body announces to one's peers.

Nevertheless, symmetry does seem to play a role in desirability. Reporting in a recent issue of the journal Nature, Dr. John P. Swaddle and Dr. Innes C. Cuthill of the University of Bristol in England found that when they put a variety of colored bands on the legs of male zebra finches, the females vastly preferred males with symmetrically banded legs over those given bands of different colors on each leg, a manipulation that apparently made the males look as goofy to potential mates as somebody wearing mismatched socks.

Writing in the January issue of the journal Trends in Ecology and Evolution, Dr. Paul J. Watson and Dr. Randy Thornhill of the University of New Mexico in Albuquerque sum up the data gathered thus far on the role of symmetry in mate selection. In their own work, they have shown that female scorpion flies can detect a male with symmetrical wings either visually or simply by sniffing the chemical signal - the pheromone - he emits. (For some reason, there is an association between the symmetry of a male's wings and his scent, but scientists don't know why.) Given the choice between the pheromone of a male with wings that differ very slightly in length and the cologne of a suitor with matched wings, she will move toward the scent of the even-keeled fly.

Researchers who study elk have determined that the males who possess the largest harems of females not only sport the largest racks of antlers, but also the most symmetrical ones.

It turns out that a male elk who loses a fight to another male - and who is thus likely to lose all or part of his harem to that victorious competitor - will grow an asymmetrical segment on his antler the following year, the sorry obverse of a scarlet letter.

By the new hypothesis, a symmetrical body demonstrates that the male's central operating systems were all in peak form during important phases of his growth.

A well-proportioned body may indicate that the male possesses an immune system capable of resisting infection by parasites, which are known to cause uneven growth of feathers, wings, fur or bone. Or it may signal a more global robustness, one capable of withstanding such threats to proper development as scarcity of food, extreme temperatures or ambient toxins.

In theory, females will select a symmetrical male either for the superior genes that he can donate to her offspring or because he is likely to be in good enough shape to help out with rearing and protecting their young.

"The individuals who have had a good developmental background come out more symmetrical," said Thornhill. "They're put together better and they'll do better in competition for resources and mates."

The new work is part of the larger study of sexual selection, an intellectually vigorous discipline that is yielding a host of novel proposals of why females opt for one male rather than another.

Researchers believe that many outstanding traits found in male animals, from the extravagant plumage of a peacock to the percussive calling of a cricket, have been shaped over generations by female taste, and biologists have sought to understand the sources of that taste. Some scientists have suggested lately that female choice on occasion can be explained by copycat behavior, with females choosing those males whom they have noticed other females fancy.

For every new proposal, there are outspoken detractors, and the study of symmetry is no exception. Critics of the theory complain that some of the differences in body proportion that scientists are now measuring are very tiny, noticeable only when a researcher puts a pair of calipers up to a creature's wingspan.

Is it likely, they ask, that these minute variations are obvious to potential mates? They also argue that the case remains to be proved that a symmetrical individual possesses especially hardy genes.

"People are embracing this idea of symmetry because it's something you can go out and measure," said Marlene Zuk, an evolutionary biologist at the University of California at Riverside. "But it's a black box as to what it means and what it's indicating to the female."

Whatever its precise relevance to animal sexuality, symmetry is an artistically appealing concept, one that painters, sculptors and architects have been exploring for at least 5,000 years, ever since the Egyptians began building their severely, even rigidly symmetrical temples. In his famed paintings of the School of Athens and the dispute over the sacrament, for example, the Renaissance master Raphael perfectly counterpoised the people on the left side of his canvas with figures of equal numbers and arrangements on the right.

"A sense of cosmic order and structure can be conveyed through a symmetrically planned composition," said Kathleen Weil-Garris of New York University, a scholar of Renaissance art. "In both medieval and Renaissance cosmologies, symmetry and proportion, the square within the circle, were part of God's plan."

So, too, does symmetry seem to be part of nature's plan. Most animals have bilaterally symmetrical bodies, with limbs and features mirrored on either side of a central axis, and many flowers are radially symmetrical, all their petals bursting forth in equal arrangements from a central point. Many viruses exhibit an almost mathematical degree of symmetry, as do important structures within the cell that control the cleaving of one cell into two.


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"People are embracing this idea of symmetry because it's something you can go out and measure. But it's a black box as to what it means and what it's indicating to the female."

-Marlene Zuk

evolutionary biologist