Even in the rainforest, couples squabble over dinner. But purple-throated carib hummingbirds on St. Lucia, an island in the West Indies, have worked out a compromise. Female birds sport bills that are longer and more curved than males', and now researchers have linked these bill shapes to feeding habits. It's a rare example of food supply spurring the evolution of a sex difference, researchers say.
Charles Darwin long ago suggested that competition for food could cause, or maintain, different male and female hummingbird bills, but the scientific evidence has remained scant. Most evolutionary biologists explain male-female differences by sexual selection. Male peacocks, for instance, grow flashy plumes to attract mates. By comparison, few studies have shown that the sexes might, when faced with a new environment, evolve in order to divvy up food.
To see whether male and female caribs dined differently, a team of researchers led by evolutionary biologist Ethan Temeles of Amherst College in Massachusetts spent 4 weeks watching the birds at distinct patches of Heliconia plants. A pattern soon emerged: All 15 males that they observed fed on patches of Heliconia caribaea, whereas 11 of 18 females chose H. bihai instead, the researchers report  in the 21 July issue of Science.
The team then measured the carib bills and their favored flowers to check the fit. The male birds sport short bills that curve down at a 15° angle. Their preferred flower curves out at about 21°. By contrast, the bills of female caribs curve down twice as much, at a 30° angle. Accordingly, their favored flower, H. bihai, averages a 31° curve. What's more, Temeles says, both male and female caribs feed more quickly--and presumably efficiently--at the flower that best matches their bill.
How, exactly, did it happen? Temeles speculates that thousands of years ago, when hummingbirds first arrived on St. Lucia, the larger, dominant males probably favored H. caribaea, a plant that bears more flowers. That left females with the less effusive H. bihai. Over time, Temeles says, the bills of both male and female caribs have adapted to fit their flower of choice, enabling the birds to make the most of their food source. "Food is really running the show," he suggests.