Why Birds and Bees, Too, Like Good Looks
Published: February 8, 1994
(Page 2 of 2)
Dr. Nancy Burley, a professor of ecology and evolutionary biology at the University of California at Riverside, has revealed all sorts of inexplicable preferences among the female birds that she studies. She discovered, for example, that when she put little white feather caps on the heads of male zebra finches, who do not have feather crests of their own, the males became extremely popular with females. By contrast, males given red feather hats gained no advantage from the adornment.
Neither preference appears to have any functional significance, said Dr. Burley. The females could not be choosing white feather caps as evidence of strong zebra finch genes, because zebra finches are not supposed to have hats of any color. Instead, Dr. Burley and others cite results like these as support for the so-called sensory exploitation theory of female choice. By this notion, some female preferences may be the coincidental and still-mysterious reflection of how an animal's sensory system and brain work -- what the brain focuses on and what it ignores in the environment -- rather than the outcome of a female's careful appraisal of the male's genetic assets.
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 Dr. 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. Bigger, but Not Better
Scientists did not begin to appreciate the possible relevance of symmetry to mate assessment until 1990, however. Dr. Anders Moller, an evolutionary biologist at the University of Uppsala in Sweden, was studying barn swallows, a species in which the males have extremely long tail feathers in a wishbone pattern. He had determined that females like long tails on their males, the longer the better. But in attempting to manipulate the feathers experimentally, he made another discovery: females also like symmetrical tails, when one side of the wishbone configuration is the same length and coloration as the other.
Playing around with parameters by adding tail feathers, clipping tail feathers or painting them different patterns, Dr. Moller found that length and symmetry figured more or less equally in female choosiness. In other words, a long, slightly uneven tail and a short, symmetrical tail rated about the same, but when given the chance for a male with a lengthy, balanced tail, there was no contest. Such a well-endowed male was invariably enticing.
In other experiments, Dr. Thornhill and his co-workers upended expectations that the biggest male always prevails by showing that female scorpion flies preferred symmetrical males over bigger males with asymmetrical wings.
The work on symmetry meshed with scientific understanding of how disease and pollution affect animal development. For example, wildlife experts had long known that fish swimming in polluted waters spawn asymmetrically shaped fry. Evolutionary biologists began to suspect that a female animal could make similar assessments of her mate's overall robustness by checking him for symmetry.
Seeking evidence that symmetrical animals are indeed sturdier than their slightly misproportioned counterparts, Dr. Moller has lately determined that barn swallows with symmetrical tails are less likely to be infected with parasites than are males with asymmetrical tails. Through test-tube experiments, he has also discovered that the immune cells of symmetrically appurtenanced males are comparatively stronger, although these results are preliminary and have yet to be published.
In studies with starlings, Dr. Swaddle and his co-workers at Bristol have found that they could influence the symmetry of a bird's plumage by giving it more or less food while it was molting. The less food the bird got, the less proportioned the regrowth of its feathers later on. Hence, the balance of a bird's plumage is a handy and sensitive bellwether of a bird's overall health that season: the more symmetrical the plumage, the more nourished the male, and presumably the better a provider he is for his family, rather as somebody who starts the day with a good breakfast is likely to have more energy by the afternoon.
Symmetry may influence the performance of many traits that a male rallies to lure a female. A cricket creates its chirruping serenades by rubbing its legs together, and scientists are now testing the possibility that females prefer the songs of symmetrically limbed mates over the melodies of a male whose bow and fiddle do not quite match.
The apparent benefits of symmetry are not limited to males. Dr. Thornhill has discovered that the most symmetrical female scorpion flies also are the most adept at gathering and hoarding food, fighting off competitors, dominating their peers and otherwise behaving like members of a ruling caste.
Drawings: "Possible Causes Of Asymmetry" Influences that mar symmetry usually act early in development, affecting the individual in the egg or womb or in the larval stage. Scientists conjecture that the ability to develop normally despite such stresses could have a genetic basis. These are some likely stress factors: * Poor nutrition * Unusual temperature conditions * Pollution * Increased maternal age * Radioactivity * Parasites * Disease microbes * Attacks by predators * Fights with competitors of same species Elk antler Male elks with the largest and most symmetrical racks of antlers have the largest harems. Elks that lose fights may lose antler symmetry -- and females. Barn swallow tail Female barn swallows prefer a long-tailed male with a symmetrical wishbone pattern of feathers the same size and color on both sides of the tail. Scorpion fly A male scorpion fly (above and left) with symmetrical wings can be detected not only by sight but by scent. For some reason, there is an association between wing symmetry and hormone signals. Even minute differences count. (Sources: Tree (journal); "Insects of the World," Wooton (Facts on File)