For those who wonder at the sight of a flamingo standing on one leg, the right question to ask may not be how, but why. New research suggests that a flamingo is more stable and requires less muscle effort when standing on one leg than when standing on two — whether awake, asleep, or even dead. In a study published in Biology Letters, biomedical engineer Lena Ting and neuromachinist Young-Hui Chang of the Georgia Institute of Technology, Atlanta, describe experiments they conducted on juvenile Chilean flamingos as well as cadavers.
They set up a cadaver in both one-legged and two-legged stances, and found that it was stable only when one foot was directly beneath the body, as in one-legged standing. “We showed that that the joints folded down into a compact and stable configuration if the leg was held at an angle similar to one-legged standing, but that the joints were not stable when the leg was held in a two-legged pose,” Ting told The Indian Express.
“One way to think about it,” Chang added, “is that there is a mechanism for standing on one leg passively, which would require very little muscle effort. The birds appear to only be able to use this mechanism when they are standing on one leg.” Passive mechanisms for balance suggested themselves again during the experiments with live birds, which compared how much a flamingo’s body sways when awake and when asleep. The researchers put active as well as quiescent flamingos (eyes closed) on a force plate and tracked their body movements. They found the sway several times more pronounced in birds that were active than in those that were quiescent.
“… As we did not observe large postural sway when standing on one leg, there may also be passive mechanisms for balance, which may be particularly important during sleep,” Ting and Chang write in their paper.
What had drawn them to the research was largely the anatomy of flamingos. In many birds, including flamingos, the knee is well inside the body while the joint that is visible — and bends backwards — is the ankle.
“The upper leg, or thigh, is oriented horizontally and adjacent to the main body of the flamingo. This is common in birds… and one of the main reasons we were curious about how flamingos could stand for so long with ‘bent knees’,” Chang told The Indian Express by email. “If a human were to adopt this posture, it would require great muscular effort from our thigh muscles. Flamingos are apparently able to do it with relatively little effort.”
The paper mentions two hypotheses about why flamingos stand on one leg. One is that it is to reduce muscle fatigue (which would necessitate alternating from one foot to the other). The experiment with the dead birds, however, showed that they could stand on one leg with no muscular effort and that the body support mechanism is passive.
The other hypothesis is that standing on leg reduces heat loss. “Previously it may have been thought that any additional energy expended to stand on one leg was worth the heat energy that was conserved,” Ting said. “Our work shows that heat loss may not be the only reason for the animals to stand on one leg.”