14 DAY TRIAL // Recently, a team of Norwegian scientists from University of Troms? discovered that seals exposed to cold air shiver, but not when diving in chilly water. This produces a cooling of the body (which becomes hypothermic), allowing it to better cope with a lack of oxygen (hypoxia) and minimize brain damage that could result from long dives.
Scientists hope to understand more of the phenomena of hypothermia and hypoxia in humans by studying the seals. "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," explained Lars P. Folkow from University of Troms?.
Mammals and birds are programmed to shiver when body temperature drops below a threshold. The involuntary contractions of the muscles produce warmth, rising the body temperature. The team found that while on the surface, breathing, seals shiver like other mammals, but when they dive in icy water shivering is stopped. "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.
The researchers used an experimental tank filled with water of 2-3? C, and diving seals were monitored for shivering, heart rate, brain temperature and rectal temperature while on the surface or diving. When the seals stopped shivering while diving, their heart rates and temperatures dropped, but - once 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.
Hypoxia slows down the metabolism and reduces oxygen demand, not to mention that shivering is an oxygen-consuming process. Moreover, the team found that the seal's brain cooled 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.
Scientists think that seals try to avoid using their capacities at maximum. These marine mammals can dive to more than 1,000 meters and for more than an hour. But, usually, their dives are much shorter, and only occasionally do they 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.
Long diving can consume nearly all their oxygen, but they can recover with these special adaptations. Humans are intolerant to such low oxygen levels. "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.