by Whit Gibbons

May 26, 2013

Q: I recently visited the North Carolina Zoo in Asheboro. Among its many outstanding features is a nice flock of flamingos. While watching them, I asked a zoo worker why some of them were standing on one foot. She said flamingos often do stand on one foot (which was already obvious to me) but she did not know why. I know that flamingos are pink because they eat some kind of shrimp. But does anyone know why flamingos stand on one foot?

A: Flamingos are not the only birds that characteristically stand on one leg. Ducks, geese, and swans often do as well. But flamingos are the poster birds for standing around nonchalantly on one leg for long periods of time, and numerous explanations have been proposed for their doing so. One hypothesis is that letting one leg relax reduces overall muscle fatigue. Another is that having only one leg down at a time minimizes exposure to harmful parasites or fungi that could be in the water. Contact between the bird and noxious organisms would be cut in half if only one leg at a time was in the water.

Among what seem to me to be less plausible proposals are that one-legged behavior relates to sleep patterns in which an animal keeps one side of the brain awake while the other is asleep. The idea is that the leg controlled by the sleeping side of the brain rests while the other stays awake and practices a balancing act. Another suggestion has been that one leg provides a type of camouflage from predators or prey in an aquatic habitat by making the flamingo look less like a bird and more like a tree, albeit a pink one. Yet another view is that when both legs are down the energetics cost to a bird is higher because the circulatory system has to work harder to keep blood flowing from the feet to the heart. But all of these are simply guesses without proper studies to confirm or refute the explanation for the behavior.

Using a more scientific approach, Matthew J. Anderson and Sarah A. Williams of Saint Joseph's University in Philadelphia conducted observational studies to address the age-old question of why flamingos so often choose one-leggedness. Their study, published in the scientific journal "Zoo Biology," concluded that when flamingos are standing in water, the one-legged stance conserves body heat and that the principal reason for the phenomenon is thermoregulation. Among their statistical findings was the observation that the proportion of the pink birds that stand on one leg increases as water temperatures get cooler. Likewise, they noted that a flamingo on land is less likely to stand on one leg than one standing in water. The explanation is that having only one leg at a time in cold water reduces the rate of body heat loss. The converse observation, that most flamingos keep both legs in the water during hot weather, further supports the notion of thermoregulation.

Nonetheless, the investigators state that the "correlation is far from a perfect relationship" and that the results of their study "do not eliminate the possibility that there may be other benefits associated with this behavior." In other words, scientists cannot with certainty give a sole reason why certain birds often stand on one leg. The answer is not a simple, clear cut one. In fact, thermoregulation does not explain why all birds that stand on one leg do so. Most observations of ducks standing on one leg occur when the ducks are on land.

Conserving body heat may indeed be the prime reason for a one-legged stance for flamingos, but the complete answer probably involves multiple factors in response to a wide array of environmental variables. Scientific findings are often not absolute and some questions about animals do not have a simple answer. Even with proper scientific study we can still be left with uncertainty about something as basic as why a flamingo would choose to stand on one leg.

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