How a flamingo balances on one leg

A question flamingo researchers get asked all the time — why the birds stand on one leg — may need rethinking. The bigger puzzle may be why flamingos bother standing on two.

Balance aids built into the birds’ basic anatomy allow for a one-legged stance that demands little muscular effort, tests find. This stance is so exquisitely stable that a bird sways less to keep itself upright when it appears to be dozing than when it’s alert with eyes open, two Atlanta neuromechanists report May 24 in Biology Letters.

“Most of us aren’t aware that we’re moving around all the time,” says Lena Ting of Emory University, who measures what’s called postural sway in standing people as well as in animals. Just keeping the human body vertical demands constant sensing and muscular correction for wavering.  Even standing robots “are expending quite a bit of energy,” she says. That could have been the case for flamingos, she points out, since effort isn’t always visible. 

Ting and Young-Hui Chang of the Georgia Institute of Technology tested balance in fluffy young Chilean flamingos coaxed onto a platform attached to an instrument that measures how much they sway. Keepers at Zoo Atlanta hand-rearing the test subjects let researchers visit after feeding time in hopes of catching youngsters inclined toward a nap — on one leg on a machine. “Patience,” Ting says, was the key to any success in this experiment.

As a flamingo standing on one foot  shifted to preen a feather or joust with a neighbor, the instrument tracked wobbles in the foot’s center of pressure, the spot where the bird’s weight focused. When a bird tucked its head onto its pillowy back and shut its eyes, the center of pressure made smaller adjustments (within a radius of 3.2 millimeters on average, compared with 5.1 millimeters when active).

Museum bones revealed features of the skeleton that might enhance stability, but bones alone didn’t tell the researchers enough. Deceased Caribbean flamingos a zoo donated to science gave a better view. “The ‘ah-ha!’ moment was when I said, ‘Wait, let’s look at it in a vertical position,’” Ting remembers. All of a sudden, the bird specimen settled naturally into one-legged lollipop alignment.

In flamingo anatomy, the hip and the knee lie well up inside the body. What bends in the middle of the long flamingo leg is not a knee but an ankle (which explains why to human eyes a walking flamingo’s leg joint bends the wrong way). The bones themselves don’t seem to have a strict on-off locking mechanism, though Ting has observed bony crests, double sockets and other features that could facilitate stable standing.

The bird’s distribution of weight, however, looked important for one-footed balance. The flamingo’s center of gravity was close to the inner knee where bones started to form the long column to the ground, giving the precarious-looking position remarkable stability. The specimen’s body wasn’t as stable on two legs, the researchers found.

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Reinhold Necker of Ruhr University in Bochum, Germany, is cautious about calling one-legged stances an energy saver. “The authors do not consider the retracted leg,” says Necker, who has studied flamingos. Keeping that leg retracted could take some energy, even if easy balancing saves some, he proposes.

The new study takes an important step toward understanding how flamingos stand on one leg, but doesn’t explain why, comments Matthew Anderson, a comparative psychologist at St. Joseph’s University in Philadelphia. He’s found that more flamingos rest one-legged when temperatures drop, so he proposes that keeping warm might have something to do with it. The persistent flamingo question still stands.