Science

Flamingo Beaks Are Upside Down (Good Thing They Eat Upside Down, Too)

On the cover of evolutionary biologist Stephen Jay Gould's book "The Flamingo's Smile," there's a strange image that looks like the legless body of a bright pink swan. Flip the book over, though, and the image is clear: it's a flamingo. Gould chose this image to highlight what might be the strangest fact about that bird: not its peculiar color, or that it likes to stand on one foot, but the fact that most things about it are, well, upside down. It's a striking example of evolution in action.

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I Evolved This Way

If you've ever checked out the flamingo exhibit at the zoo, you're familiar with the bird's feeding habits: they stick their beaks in the water and swing their heads back and forth. That's because, like whales and oysters, flamingos are filter feeders. They swing their heads to help water flow into their bills and through a natural strainer that collects food. Sometimes that food is as large as crustaceans, mollusks, and insects, and sometimes it's as small as single-celled plants.

To accomplish this, they bend their long necks down toward the water and position their heads upside down. As a result, what looks like their lower bill when they're upright becomes their upper bill when they're feeding. In most birds, the upper beak is larger than the lower one. But since feeding upside-down has helped flamingos thrive—and therefore produce more offspring—evolution has selected that upside-down position as the "correct" one. And there you have it: small upper beak, massive lower beak.

That's not where the topsy-turvy adaptations end. Not only do most birds have an upper beak that's larger than their lower one, but the upper beak is also stationary, while the lower one moves (think about your own mouth, with its stationary upper teeth and moveable jaw). So does the flamingo have a stationary lower jaw and moveable upper jaw?

A Centuries-Old Mystery

That's a question that Gould says scientists have asked for 2,000 years. In the early 1800s, after naming the many researchers that had examined the strange beak, the naturalist Georges Buffon wrote, "But none of them have examined it with such attention as to decide a point which we should be glad to ascertain, viz. whether, as many naturalists allege, the upper mandible is moveable, while the lower is fixed."

Surprisingly, it wasn't until 1957 that the question was finally answered. That year, Penelope M. Jenkin published a study in the Philosophical Transactions of The Royal Society B that solved the mystery once and for all: both beaks move independently via ball-and-socket joints. When preening their feathers, flamingos move one or the other beak. But when feeding, they keep their lower beak stationary and move the upper beak against it, just as scientists had always suspected.

Stephen Jay Gould sums this up well: "Evolution as adaptation to particular modes of life—Darwin's vision—gains strength from an extreme test imposed by life upside down." In other words, score another one for evolution.

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