Seals protect brain, conserve oxygen by turning off shivering response on icy dives

The researchers presented the study at The American Physiological Society's conference “Comparative Physiology 2006: Integrating Diversity,” in Virginia Beach, Va., October 8-11. The researchers, Arnoldus Schytte Blix, Petter H. Kvadsheim and Lars P. Folkow hail from the University of Tromsø, located above the Arctic Circle in Tromsø, Norway.

The research provides insight into how seals allow their bodies to cool (become hypothermic) during a dive, presumably to better cope with a lack of oxygen (hypoxia). Research into hypothermia and hypoxia is important because they are problems that affect people under a variety of circumstances. Doctors often are called upon to treat people who have suffered accidental hypothermia, for example, as a result of falling into the ocean or becoming lost during the winter. In addition, several hundred thousand people die or are irreversibly injured each year following cardiac arrest, stroke or respiratory disorders which cause inadequate oxygen supply to the brain, Folkow explained.

Folkow will present a second study on hypoxia, involving diving birds, at the conference. The study “Neuronal hypoxic tolerance in diving birds and mammals,” examines how diving birds and seals preserve brain cell function in the face of oxygen deficits. The study is by Folkow, Stian Ludvigsen and Blix, of the University of Tromsø and Jan-Marino Ramirez of the University of Chicago.

Shivers produce warmth

Shivering is an involuntary response that consists of muscle contractions which produce warmth. Mammals and birds are physiologically programmed to shiver when body temperature drops below a certain “set-point.”

While breathing air, seals shiver just like other animals. But when they dive below the surface in frigid water, shivering is switched off, the study found.

By shutting down the shivering response, a seal allows its body temperature to drop and achieves the benefits of hypothermia: a slower metabolism and lowered oxygen requirements which extends the dive time, Folkow said.

Taking the plunge

The seal experiment took place in a tank in which the seals took a series of experimental dives into cold water of 2-3° C. The researchers recorded shivering, heart rate, brain temperature and rectal temperature while the seals were on the surface and while they were diving.

The seals shivered on the surface but stopped or nearly stopped shivering when they dove, even though their bodies continued to cool. Their heart rates and temperatures dropped while they dove, but when they returned to the surface they restarted their shivering nearly immediately.

Seals have a remarkable capacity to store oxygen in their blood and muscles – four times as much as humans – to which they add this oxygen-conserving step of not shivering, Folkow said. By allowing body temperatures to drop, they slow metabolism and reduce oxygen demand. In addition, since shivering itself requires oxygen, there is an oxygen-conserving advantage to not shivering when diving.

In addition to slowing metabolism and generally reducing the need for oxygen, the researchers found that the seal's brain may cool about 3° C during the dives. The cooler brain requires less energy and oxygen and reduces the chance of damage caused by hypoxia, Folkow explained.

Achieved while remaining active

Seals have this physiological adaptation available just in case. This study found the seals can dive to more than 1,000 meters and for more than an hour. However, they usually take dives much shorter than their maximum capacity, and only occasionally perform very long dives. By limiting dive duration, seals maintain aerobic metabolism, avoid lactate buildup that occurs in the face of insufficient oxygen and require little time to recover, Folkow explained. Seals often spend 80-90% of their time at sea underwater, he said.

Seals in the wild occasionally dive for so long that they use nearly all their oxygen, but they can recover with these special adaptations. Humans cannot tolerate oxygen levels nearly so low as a seal can.

“Somehow they tolerate hypoxia better, we don't know why,” Folkow said. The study of how seals handle this lack of oxygen may someday give us knowledge that is useful in treating people who have suffered severe hypoxia, although those advances are likely years in the future, he added.