Young-Hui Chang has two sons he likes to take to the zoo, where they enjoy watching, among other animals, the flamingos. While at the flamingo exhibit, it's not unusual for Chang to overhear other kids ask their parents how the top-heavy pink birds manage to stand on one leg with such nonchalance. But most parents don't have a good answer, instead stumbling through an expanation. So Chang, a physiologist at Georgia Tech, decided to do some experiments to find out.
Take a moment or two to stand on one leg. That wobbly feeling—maybe located in your ankle, or elsewhere in your leg—is your leg muscles working to keep your weight centered over your foot. Each time you tip forward, muscles contract to pull you backward, and vice versa.
Physiologists have a nifty device called a force platform that can measure that off-balance sway. “I call it kind of a glorified bathroom scale,” Chang says. It measures the direction of force as well as the amount of force, and then generates a drawing that displays those changes in your center of gravity: The bigger the drawing, the more sway and the more muscles are working.
Chang and his colleague—with help from keepers at Zoo Atlanta—put eight juvenile Chilean Flamingos on a force platform. The birds fussed around for a while, craning their necks and chattering. Then they gradually settled in, picked a leg up, and quieted down, sometimes even falling asleep. The force platform drawings showed that active flamingos—which shift between standing on one leg and on two—exhibit seven times more sway than the quiet, sleeping birds. Those results suggested that the birds have some trick to keep their balance during one-legged sleep that doesn’t rely on their muscles.
To find the mechanism that made their easy stability possible, Chang asked a local zoo for the corpses of two Caribbean Flamingos to study. He dissected them and examined their legs, which for flamingos are unusual even among birds. Their knee joints are buried beneath their feathers where you’d imagine their hip joint might be; what we see as a long flamingo leg is actually just its calf, ankle, foot, and toes.
When he positioned the flamingo corpse on one leg, its hip and knee joints naturally aligned so that the foot was positioned beneath the center of its body. This posture pinned the whole leg in place; Chang could rest the flamingo’s toes on his hand and the pose held steady. “It was almost like a flamingo lollipop,” Chang says. “It wasn’t falling over, the joints weren’t coming undone.”
It’s a convenient trick that keeps the bird stable—and since a dead bird’s body can do it, it can’t possibly require energy. Previous scientists had guessed that the pose might reduce muscle fatigue, but Chang’s work proves the bird doesn’t really need to use muscles at all.
Flamingos are the most notorious of avian balancers, so understanding their secret may help scientists see less obvious balancing mechanisms in other species, from ducks and herons to sandpipers and pigeons. “Flamingos, they just represent the extreme of this behavior,” Chang says. “Now you can look for it in other species because you know what to look for.”
Humans could stand to benefit as well. Chang spends most of his time studying human amputees and exploring how people walk with prosthetics. “Studying how nature solves problems of legs would be helpful for engineers,” he says. One of the big problems with robotic prosthetics is that they use up battery power quickly. He says studying how other animals support themselves might inspire a new design that holds up a body with less effort.
And in the meantime, at least, all those parents at the zoo will finally have a good answer for their curious kids.
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