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Slow ride. After deep dives, king penguins pause on their way up and ascend at an angle.

How Penguins Avoid the Bends

Penguins are Olympic-class divers. After a deep breath, they can plunge hundreds of meters for many minutes, bob up briefly, and dive again. This ought to cause the "bends," or decompression sickness, but penguins seem immune. Now researchers have discovered a diving habit that may help explain why: On their way up from the deep, Adélie and king penguins slow down and surface at an oblique angle--in effect mimicking the careful decompression of human divers.

Marine animals have a variety of strategies to prevent the bends. In human divers, increased underwater pressure forces nitrogen in the air within body cavities to pass into the blood. If divers surface before the nitrogen is cleared, they can suffer contorted joints, difficult breathing, and even paralysis. Many whales and seals have blood and muscles adapted to conserve oxygen; they can also collapse their lungs before diving to squeeze out air. Penguins don't have it that easy: Their lungs don't collapse, and the buoyant divers need a good dose of oxygen to swim hard.

To find out how penguins move about at depth, Katsufumi Sato of the National Institute of Polar Research in Tokyo and colleagues attached data loggers to Adélie and king penguins off the shores of Antarctica and Crozet Island, about 1000 kilometers away. The instruments measured the depths, speed, and acceleration and deceleration effects from wing strokes for more than 650 dives. From these data Sato's team estimated air volumes in penguin lungs during their descents and ascents.

The dive profiles revealed that the penguins flapped their flippers continuously on the way down. On return trips, after swimming halfway up, they stopped and let their natural buoyancy give them a free ascent. But surprisingly, instead of shooting straight up the penguins veered at an oblique angle, thus significantly slowing their ascent, the team reports in the May issue of the Journal of Experimental Biology. This increases the amount of time the penguins spend in shallow water with little prey, but it could provide time for nitrogen, under lower pressure, to return to the air inside body cavities.

Those findings intrigue marine biologist Dan Costa of the University of California, Santa Cruz: "They've made careful and insightful measurements of the fine-scale diving behavior of two penguins, supported with very sophisticated models of lung volume, and they may be correct." However, he cautions, there are alternate explanations for why penguins would slow their ascents, such as looking out for predators.

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