Flamingos use less energy standing on one leg instead of on two, say scientists.

It may be their signature pose, but how and why the birds perch on one limb has been a longstanding puzzle.

Now, the BBC reports, a team from the United States has shown that flamingos employ no active muscular effort when they're unipedal, meaning they are also expending less energy.

A passive mechanism is engaged in the one-legged position, allowing flamingos to stand proud while having a doze.

Previously, researchers had wondered whether the one-legged position might help reduce muscle fatigue, as the birds alternated from standing on one leg to the other.

Other teams have proposed that this behaviour helps regulate body temperature.

Now, Professor Young Hui Chang, from the Georgia Institute of Technology in Atlanta, and Professor Lena H Ting, of Atlanta's Emory University, have uncovered the mechanical secrets behind this impressive trick.

The researchers conducted several experiments with both live and dead birds. Amazingly, they found that flamingo corpses could be made to stand one-legged without any external support.

They describe this phenomenon, as a "passive gravitational stay mechanism".

"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," Prof Chang told BBC News.

Dr Matthew Anderson, an experimental psychologist who specialises in animal behaviour at St Joseph's University in Philadelphia, said: "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."