Flowers, not flirting, make sexes differ.

Let's not get so obsessed with attracting the opposite sex, cautions a hummingbird research team.

Sex appeal may seem like all that makes the world go round, especially to anyone reading recent scientific studies about why males look different from females, remarks Ethan J. Temeles of Amherst (Mass.) College.

Evolutionary pressure to charm and fight for a mate, or sexual selection, is usually easier to test for than natural selection, or the bottom-line pressure for survival, says Temeles. Yet he and his colleagues have found a Caribbean island where they say they can distinguish between the two.

Among purple-throated caribs, Eulampis jugularis, the largest hummingbird on St. Lucia, it's food and not flirtation that's driven males and females to develop bills in his-and-hers models, the researchers argue.

"This is the first really unambiguous example of ecology playing a role in the morphological differences between the sexes," Temeles says.

To be fair, he points out, biologists never claimed sexual selection explained all gender differences. Darwin himself proposed that specialized diets led to bill differences in a New Zealand bird, the huia. Males' stubby bills allowed them to drill into trees for insects, while the females' long, curved bills pried insects out of crevices. Verifying Darwin's theory has been difficult since the bird went extinct more than a century ago.

Evidence has been thin to verify any claim of ecological causes for sex differences. Some studies have suggested that food preferences may foster gender-related size differences in snakes, weasels, and predatory birds. Based on anatomical studies, scientists have argued that in certain mosquitoes, male mouth parts look ideal for sipping nectar while the females' counterparts look better for sucking blood.

On St. Lucia, "you couldn't design a better system" for studying food contributions to sex differences, Temeles crows. During breeding season, male hummingbirds sip nectar almost exclusively from Heliconia caribaea, whereas females eschew that flower for Heliconia bihai, the researchers report in the July 21 SCIENCE.

Measuring each gender's bills revealed that their curvature and length best fit the preferred flower. Indeed, females fed in just three-quarters the time when sipping from their preferred bloom instead of the males', Temeles reports.

Hummingbird expert Larry L. Wolf of Syracuse (N.Y.) University considers Temeles' explanation for the sex differences "quite reasonable." Elsewhere, he notes, hummers have developed species-specific bills to fit their bloomin' diet. Caribs are just extending that bill specialization to gender, he observes.

Their preferred flowers are also exciting, says Heliconia specialist W. John Kress of the Smithsonian Institution's National Museum of Natural History in Washington, D.C. He marvels that carib females prefer nectar from green flowers when many hummingbird-pollinated flowers blaze red or orange. In fact, the Heliconia genus--related to the bird-of-paradise--can get downright gaudy. Kress can think of only a few other green-flowered Heliconias, all pollinated by bats.

The two species on St. Lucia appear on other islands bearing more colorful blooms, he notes. Why such differences evolved is far from clear, but, Kress enthuses, "it's a great system."