Aug. 2, 2000 -- Men and women may battle over mundane matters like in-laws or housekeeping, but male and female hummingbirds must spar over something a little more critical: food.
Food is especially contentious for hummingbirds since the creatures move their wings invisibly fast (up to 80 flaps a second for some species) and operate at such high metabolic rates that they can starve within one hour. In fact, these hovering birds burn more calories relative to their size than any other animal except flying insects.
“They’re like motorcycles with small fuel tanks,” says Bill Calder, an ecologist and longtime hummingbird expert at the University of Arizona. “They’re light and fast, but they’re always running on nearly empty.”
Fortunately, nature has found a way to divvy up food resources between the sexes of some hummingbird species.
Designed for Access
On the island of St. Lucia in West Indies, biologists have noted physical differences among the local male and female purple-throated carib humming bird. And their observations show that two kinds of nectar-bearing flowers are the most likely reason for the birds’ different designs.
“Usually sexual dimorphism [a feature that differs between males and females] is attributed to mating purposes, like males having brighter feathers to attract females,” says Ethan Temeles, a biologist at Amherst College in Massachusetts who has spent the past two summers on St. Lucia. “Here, ecology appears to have a role, which is very rare.”
The evidence, Temeles explains, is in the beak.
The males sport a straight bill (as Temeles says, “imagine if you stuck a yardstick on your nose.”) The female’s bill, meanwhile, is a third longer and twice as curved. It turns out both shapes are uniquely fitted to probe the stalks of two separate species of the nectar-filled Heliconia flower.
Observing the iridescent birds in the cool, dark rain forests of St. Lucia, Temeles and his students noted that 15 of 15 males fed on patches of H. caribaea while 11 of 18 females chose H. bihai instead. The flowers the males select have short tubes to their nectar, while the females’ choice have long curved stalks leading to their sugary source.
“It’s quite ingenious architecture,” says Temeles. “The flower is driving sexual dimorphism of the shape of the bill.”
Bolstering that finding, Temeles and his team also observed that both male and female caribs fed more quickly on the flowers that best matched their bills.
Temeles believes that evolution may have worked both ways. The birds may have also caused the flowers’ tube shapes to change to adapt to the birds’ bills since the hummingbirds are the only Heliconia pollinators on this 20-mile stretch of island.
Male Gets Choice Flower
So which sex won access to the better flower? The males, it appears, are slightly better fed.
While both flower species provide sweet nectar, the red colored H. caribaea produces more flowers than the females’ green H. bihai. Temeles thinks that males won their way to the preferable flower by their heft. Although the sexes are similarly sized now, they descended from a family of hummingbirds in Central America in which the males outsize females significantly.
“The females also take care of the young so they don’t have time to fight over flower territory,” adds Temeles. “So the females are left with the dregs.”
Charles Darwin long ago suggested that food competition may cause differences between males and females of some species. In fact, Darwin once singled out the hummingbird as a possible example.
But evidence for the theory remains scarce.
Tracing Differences to Food
The problem, Temeles points out, is that most other species feed on a wide range of sources. So it’s difficult to detect if males and females are making different selections and if they’re better suited to feed on those particular selections.
“Tracking what a primate is eating and being able to count them up and link it to evolution is a hard thing to do,” says Temeles.
There is, however, at least one other animal that shows distinctive feeding patterns among its sexes. The male mosquito is primarily a nectar feeder while the female is equipped with a long, piercing-sucking proboscis that she uses to suck blood.
Incidentally, Temeles and his students are quite familiar with the female mosquito’s feeding habits. He says the setting where he and his students conduct their studies is “extremely beautiful” — except for the pesky female insects.
“You’re sitting there in the forest, watching the birds that glitter like diamonds in the dark forest. You hear the parrots fly overhead, the tree frogs croaking. It’s very pleasant,” he says. “But that’s when my students always stop me and say, ‘Yeah, but you’re forgetting about the mosquitoes.’”