If you had to rate the most comfortable positions to sleep in, standing on one leg is going to be right down the bottom of the list, just above standing on your head. But for flamingoes, snoozing while supporting their whole body weight on one stick-thin leg seems to be a natural first choice, and scientists don't really know why. Now, researchers at Georgia Tech believe they've cracked the case, and their finding could lead to better-balancing robots or prosthetic limbs.

It may seem like a simple question to ask a zookeeper, but why flamingoes stand on one leg for hours at a time has long stumped scientists. There are a whole flock of theories out there: maybe it's to regulate their body temperature in chilly water, or to keep their feet from pruning up like your fingers in the bath. Another suggests it makes them look like a reed from underwater so they don't spook their prey, while others assume flamingoes just find it more comfortable.

As it turns out, that last point is on the right track. The Georgia Tech team investigated the biomechanics of the birds, and found that this position requires no muscle activation, so the leg itself won't get fatigued after long periods of time. Millions of years of evolution has tuned the flamingo's anatomy to make the most of the pose.

"The biomechanics are such that when they stand on one leg, they become very stable and are able to maintain that posture without activating muscle," says Lena Ting, co-author of the study. "If they deviate from that posture to two legs, that no longer holds. It's very posture-specific, a one-legged posture that can support their body weight."

If you've ever wondered why the knees of birds seem to bend backwards, that's because the joint we see in the middle of the visible leg is actually the animal's ankle – the knee is hidden up under the wing. Previous studies have concluded that the ankle bones of flamingoes "lock" into place to keep the leg straight, but the Georgia Tech team found that the mechanism is more like a sling, and it helps create a "passively engaged gravitational stay apparatus."

The team studied juvenile flamingoes at Zoo Atlanta, but it was actually dead specimens and skeletons that provided the eureka moment for the researchers. A cadaver was stood up on one leg, and it stayed put by itself.

"Here we have a non-living animal able to stand on one leg," says Young-Hui Chang, co-author of the study. "Obviously, if it's not alive, then the muscles are not activated."

Along with unlocking this long-standing mystery, the researchers believe the lessons learned could be applied to robots and prosthetic limbs that can balance themselves more naturally and efficiently.

"If you design the biomechanics of a robot in the right way, not so much sensing but a sort of feedback control, then they would have this passive ability, and they would be more robust in uncertain environments," says Ting.

The paper was published in the journal Biology Letters.

Source: Georgia Tech