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Swims like a duck. These schematics show the water flow patterns and forces set up by a swimming bird's foot.

The Power of Webbed Feet

Competitive rowers might want to look to swimming birds for coaching tips. A simple lift of the foot after a strong paddle speeds up the aquatic avians, new research suggests. The push as the birds paddle plus the lift as they pull their feet forward add up to a powerful and efficient means of locomotion, one that might also propel frogs and other animals with triangular feet. Although in many instances the birds use paddling alone, this maneuver may quicken their pace when they need extra speed.

Some aquatic animals, such as sea lions and penguins, move by wiggling their flippers or fins back and forth like a scuba diver does. This creates lift, which moves them forward. For years it was thought that most swimming birds propel themselves more like a paddling canoer, by pushing their feet back against the water to thrust forward. Now it looks as though some birds use a combination of the two techniques: They drag their feet back to a point and then deftly lift them up and away from their bodies.

Diving into the question of how birds swim, Christoffer Johansson, a zoomorphologist at Harvard University, and colleague Åke Norberg of Göteberg University in Sweden picked the great cormorant as their test subject. The team filmed the birds swimming and diving and used a computer program to estimate how fast the animals' bodies and feet moved. After working out the math, the researchers fashioned a mock bird with two triangular aluminum feet attached to a wheeled platform. When this pseudo-bird was pushed forward under water, the aluminum feet dragged and lifted in the same way the live birds' feet did on film. Using the model, the researchers investigated how the various movements benefited the birds' swimming. They concluded in the 3 July issue of Nature that triangular feet are ideal for the paddle-lift phenomenon and that other swimming animals with triangular feet or fins, such as arrow worms and frogs, probably do the same thing.

The added lift may raise efficiency only marginally, says Frank Fish, a biologist at the University of West Chester in Pennsylvania. But swim coaches should find these results interesting, he adds, as they are always looking for techniques their swimmers can use to increase speed.

Related sites
The Lauder lab, where Johansson works
Frank Fish's home page