If you stood in the water all day, what would happen to your skin? You'd look like a prune, most likely. Well, wading birds have the same problem. This is where one of the theories comes from: Maybe flamingos stand on one leg to dry their other foot off. Since flamingos alternate which foot they've got in the water, this theory does seem possible, but it's not terribly popular.
Some experts think the one-leg balancing act might have to do with a flamingo's brain. Many animals, including dolphins and ducks, only turn off one side of their brain at a time when they sleep. Ducks have also been known to stand on one leg for long periods of time (along with herons, storks and geese). If flamingos, too, keep half their brain awake while they sleep, that could explain why they sleep on one leg. The leg controlled by the side of the brain that's awake stays on the ground to maintain balance while the other leg and foot get to rest up for a while.
The most common theories, though, relate to hunting and energy conservation.
Since a flamingo's legs are so long and make up the majority of the bird's height, it takes a lot of energy to pump blood through both legs. That's a big strain on the heart. It's possible that pulling up one leg to rest, and tucking it up into the body, allows the heart to more easily pump blood through the body, since only one leg is fully extended. At the same time, it could conserve body heat. It's like wrapping your arms around your torso to stay warm in cold weather; the more compact you make your body, the easier it is to warm up. Since flamingos do sometimes live in cold weather, it's likely that standing on one leg has something to do with saving body heat. It's an imperfect theory, though, since flamingos stand on one leg in hot weather, too.
Another common theory relates equally to both warm and cool climates. There are many scientists who think flamingos keep just one leg in the water in order to better camouflage themselves. A lagoon has lots of long, thin objects in it, including reeds and small trees. A flamingo on one leg could resemble a tree with a thin trunk leading to a much bigger top -- especially when seen from in the water, which is where a flamingo's prey lives. This theory would make more sense if the flamingo's prey were a bit more sight-driven, though. Shrimp, mollusks and algae don't seem to spend much time looking out for hungry birds.
So nobody is sure exactly why flamingos do it. But there's an interesting anatomical mechanism that may help explain how they do it. A flamingo's ankle is where you'd expect the knee to be -- near the middle of the leg. The knee is so far up on the leg, it's usually hidden by the body. That ankle has something a human ankle doesn't -- the joint actually snaps shut to literally lock the foot-to-leg junction in place. Some scientists think this locking mechanism might be the key to a flamingo's incredible balancing act [source: Necker].
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