All the right curves: in an island hummingbird, the shape of the female's bill enables her to feast on local flora inaccessible to the male

Natural History, Nov, 2002 by Ethan J. Temeles

On what has become an annual expedition, each May and June I lead a group of my students to Saint Lucia, one of the eastern Caribbean islands that make up the Lesser Antilles. Our destination is the Quilesse Reserve, the largest of the four areas of rainforest on the island. Although Saint Lucia is small--twenty-seven miles long and fourteen miles wide--its topography is dramatic, and traveling inland to Quilesse takes no trivial effort. Only about 19 percent of the island's original rainforest still remains, all in reserves on the mountain slopes of the interior. The potholed six-mile feeder road leading from the east-coast highway to Quilesse takes an hour to drive. Once we reach the reserve, we get out and start hiking. As is typical on Saint Lucia, the climb is almost straight up. And though relatively few visitors to the island scale the remote slopes, even casual tourists can appreciate the steepness of the terrain: on the southwestern coast the national landmarks, two extinct volcanoes called the Pitons, rise 2,000 feet up from the sea.

As we first work our way along the muddy slopes of the Quilesse trail, we hear the squawk of Saint Lucia parrots. Twenty years ago the population of these parrots had dropped to 150, and the species had become the world's thirteenth rarest bird. Now, protected from hunting and from habitat destruction, the species has rebounded to about 500 individuals and has been named Saint Lucia's national bird. But we have come to study another, much smaller, island resident: the purple-throated carib.

Native to the mountain rainforests of the Lesser Antilles--from Saint Kitts and Nevis in the north all the way south to Saint Lucia and Saint Vincent--the purple-throated carib is one of the most beautiful of hummingbirds. Both males and females are predominantly black with a glittering, rosy throat patch, or gorget, and emerald wings. Many hummingbirds have iridescent gorgets, but the purple-throated carib is one of only two out of some 320 species of hummingbirds to have iridescent wings. Flashing green and black through the rainforest, these hummingbirds resemble imperial fighters from Star Wars. The analogy is apt, not only because of the birds' speed and maneuverability but also because of their pugnacious behavior.

Although alike in color, male and female purple-throats display one of the most extreme differences in bill size and shape of any bird. Males are 25 percent larger than females, weighing in at ten grams (roughly three times the weight of a penny). But the bills of females are 30 percent longer than those of males and curve downward at a 30 degree angle, compared with the 15 degree slant of the males'. Males and females of some other hummingbird species also differ in bill size, but the discrepancy among purple-throated caribs is so great that an observer can readily tell the sexes apart by bill characteristics alone.

Sexual differences in size and color are widespread in the animal kingdom. For example, male elephant seals, which battle each other for access to mates, are two to three times larger than females; the extravagant plumage of peacocks contrasts with the drabness of peahens. Among arthropods, such differences, or sexual dimorphisms, can be just as striking, though they tend to go the other way: the females, which carry eggs, tend to be the larger sex. Darwin was the first to suggest how such differences may have evolved. One possibility is sexual selection: a brightly colored male fish or a male deer sporting large antlers may have a better chance of attracting or fighting for mates than males that lack such traits. The second possibility is fecundity selection: if larger female arthropods produce more eggs than smaller ones, then genes for larger female size may predominate in succeeding generations.

But Darwin also proposed a third selective mechanism. Some sexual dimorphisms, he suggested, may have evolved through the ecology of feeding: the sexual differences simply enable males and females to consume different foods. The clearest examples are certain mosquito species in which the mouthparts of males are adapted for drinking nectar and those of females for imbibing blood. But other good examples have been hard to pin down, because in many species the size of an individual determines what the animal can eat, making it unclear whether sexual differences in diet are the cause, or the consequence, of sexual dimorphism.

Darwin was well aware of the difficulty of relating the evolution of sexual dimorphism to differences in diet. Hence he maintained that any scientific claim for such a relation should be grounded in dimorphisms associated primarily with the feeding apparatus. An example Darwin cited was the New Zealand huia. (Unfortunately, this bird was so highly prized by the indigenous Maori and, later, by Europeans for its large black-and-white tail feathers that it became extinct by 1930.) The sexes were similar in size and plumage, and both fed on insects. But their means of feeding and their bills were different. Males, which chiseled into wood for their prey, had short, thick, straight bills. Females, which probed crevices for insects, had long, slender, curved bills. Ornithologists and naturalists, including the renowned nineteenth-century illustrator John Gould, originally classified male and female huias as two different species.