BioEd Online

Diving headfirst into a tank of chilly water would cause even the most stoic of us to shiver, but not the hooded seal (Cystophora cristata). Although the plucky marine mammals shiver on cold, dry land, they stop as they plunge into nippy waters a strategy that probably helps them to conserve oxygen and minimize the brain damage that could result from long dives.

Researchers have spent decades trying to fathom the seemingly impossible diving physiology of seals. The animals, which can spend up to 2 hours underwater in one dive, don't seem to be able to hold enough oxygen to allow them to survive this feat.

"Stopping shivering is just one of the tricks that lets these animals dive for long periods of time," says Lars Folkow and colleagues from the University of Tromsø, Norway, who reported his findings at The American Physiological Society conference in Virginia Beach, Virginia yesterday.

Folkow's team took muscle, heart rate and body temperature recordings from a dozen captive-reared hooded seals trained to lie still on a specially designed board. When the board was above water and the room temperature was lowered to an icy -35C, the animals shivered vigorously in an attempt to keep warm. But when the board was lowered into a chilly saltwater pool, they stopped shivering instantly.

Stopping shivering helps the seals' body temperature to drop, explains Folkow. This slows their metabolism and means the animals need less oxygen.

The strategy isn't perfect for a warm-blooded creature that needs to maintain a certain temperature to survive. But the chance to stay underwater long enough to catch a meal seems to be worth the sacrifice of being chilly. The animals can warm up again once out of the water.

Keeping a cool head

As the animals were submerged, their brain temperature began to drop, falling a full 3C over 15 minutes. A cooler brain requires less oxygen, and so is less likely to be damaged during a long dive, says Folkow.

"Seals can tolerate hypoxia much better than humans," says Folkow. Understanding how this happens could help researchers to better understand the situation in the human brain when it is deprived of oxygen, such as occurs during a stroke or heart attack.

The seals' heart rates also dropped from around 90 to 10 beats per minute. This well-known dive response also helps the animals to conserve their limited oxygen supply, diverting blood to the organs that need it the most.

Taken together, the adaptations help seals, which spend more than 80% of their time at sea underwater, forage for food. "It's the icing on the cake," says Mike Fedak from the University of St Andrews in Fife, UK, who studies diving and foraging behaviour in seals, "a nice, direct observation of the physiological control mechanisms that allow body temperature to fall when an animal dives."

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