It began with an unsuccessful stakeout at Zoo Atlanta. "We really wanted to do an experiment where we just walked over and gave them a little prod," Chang told The Atlantic magazine. "But the zoo wouldn't let us." So they put out a call to area zoos. Alabama's Birmingham Zoo had just euthanised two flamingos, which they sent to the researchers. The duo began examining them, when something astounding happened. Chang held one of the cadavers up by its leg, which immediately snapped into place. He was able to place the dead bird on the table, where it stood as if it were merely sleeping. "It was a lightbulb moment," Chang said. "We weren't expecting it to be stable, but it totally was."

Even more surprising? The cadavers couldn't stand on two legs. Instead of using active muscle force to keep their balance on one leg - as a human might, which is why we feel tired after that yoga class - the flamingo's unique skeletal and muscular systems allow for gravity to do the trick, the scientists reported in a study published in The Royal Society's Biology Letters on Wednesday. The bird's skeleton appears to be the key. As with humans, flamingos have two main joints in their legs. The one you can see, that bends backwards, is not the knee. That's actually the bird's ankle. Its knee, meanwhile, is hidden in the bird's feathers at the fatter part of its body. When the flamingo is ready to nod off, it lifts one leg and instinctively moves its body so its single foot isn't under its hip. Instead, it's centred directly under the carriage of bird. Meanwhile, pulling the other leg up forces the knee to bend, which the flamingo rests on. All the joints essentially snap into place.

"If you look at the bird from the front, while they're standing on one leg, the foot is directly beneath the body which means that their leg is angled inward. That's the pose you have to strike in order to engage the stay mechanism," Chang told the BBC. As the flamingo remains nearly perfectly still while sleeping, gravity does the rest, keeping the bird in place. While the how doesn't necessarily explain why flamingos sleep on one leg, it does suggest that the bird isn't merely reducing muscle fatigue. After all, it could stand without using its muscles at all, which suggests it might be conserving energy. Matthew Anderson of St Joseph's University, who studies flamingos, called the study a "significant step forward" but said questions remain. "They begin to answer the question of how flamingos are able to rest on one leg," he told the BBC. "Importantly, these authors do not examine when and where flamingos actually utilise the behaviour in question, and thus this paper does not really address the issue of why flamingos rest while on one leg.

"Providing evidence of the mechanism that supports/allows for the behaviour to occur does not necessarily provide insight into why it happens in the first place." He also pointed out to The Atlantic that, if the position's main purpose was to conserve energy, then "one would expect flamingos to employ the one-legged resting stance constantly". Chang and Ting told Discover magazine more research was forthcoming. "We still don't know what anatomical mechanisms are engaged in the one-legged stance that allows for this, but as far as we can tell, it is related to the skeletal anatomy," Chang said. "We would need to directly image the skeletal anatomy during this behaviour - for example with X-rays - to really get at it, which is a direction for future research."

So far, though, the duo have carried out the research in their own time, without funding. "It was a labour of love - doing science simply for the sake of learning how nature works," Chang said. The Washington Post