Researchers for the first time have found a species of mammal that can survive for weeks at body temperatures below freezing, according to a study published today in the journal Science.
Studying the eight-month hibernation pattern of the Arctic ground squirrel, researchers at the University of Alaska, Fairbanks, found that the animal routinely enters states of torpor during which its body temperature falls to as low as -2.9 degrees Celsius, nearly three degrees lower than the temperature at which water freezes.
The scientists noted that their study is aimed at a better understanding of how animals hibernate and does not hold implications for efforts to freeze humans and then revive them later.
The squirrel will remain at that temperature for about two weeks, the researchers found, at which point it spontaneously arouses itself and, over a period of several hours, raises its body temperature back to normal. The animal then urinates and returns to its state of near-frozen suspension for another two-week cycle, the Alaskan team said.
Many mammals can withstand subzero temperature in the air around them but must maintain a higher body temperature.
Although scientists have previously cooled mammals such as laboratory mice and rats to very low temperatures and successfully revived them, this is the first time such a process has been found to occur naturally, according to Alison York, a UC Berkeley graduate student working at the University of Alaska's Institute of Arctic Biology. In addition, York said, mammals artificially cooled to near-freezing temperature can remain chilled for less than one hour before they die, making the Arctic squirrel's two-week periods marathon in comparison.
"You can get one of those (laboratory) animals to go down to right around zero, and if you then artificially warm them, they'll survive," York said. "But no one has ever shown that a mammal can do this kind of thing reversibly."
It has long been known that some fish, reptiles and insects are able to withstand subzero body temperatures by either freezing solid, as is the case with some types of frog, or through chemical substances in the blood that serve as a cellular antifreeze.
However, the squirrels do not allow themselves to freeze, and in tests on blood plasma taken from the hibernating squirrels, no such antifreeze was found. In this respect, "there's no difference between their blood and ours," York said.
Instead, York and others suspect that to survive, the squirrel takes advantage of a phenomenon known as supercooling, in which liquids at freezing temperatures sometimes do not solidify.
Ice crystals require a particle called a nucleator around which to form. Humans and all other mammals have such particles in their blood.
In the absence of such a particle, a liquid can remain fluid despite ultra-low temperatures. In the case of the Arctic squirrels, "they seem somehow to be ridding their body of nucleators, but we don't really understand what the mechanism is yet," York said.
In addition to purging themselves of nucleators, the Arctic squirrels maintain varying temperature levels throughout their bodies, the Alaskan researchers found, with some areas significantly warmer than others.
"It appears that they're dropping the body temperature of the back half of their bodies, their legs and their lower abdomens," York explained. "They're keeping their brains and their hearts warmer. It may be because they need to maintain more active metabolism there, but we're not sure about that either."
During this process, squirrels' heart rates drop to between two and six beats a minute and they periodically stop breathing for several minutes at a time.
In the six-month study, York and her husband, Brian Barnes, an assistant professor at the University of Alaska's Institute of Arctic Biology, used temperature-sensitive radio transmitters surgically implanted in the abdomens of 12 captured squirrels.
The animals were monitored in burrows surrounded by wire mesh so they could not escape. The outside temperature never fell under 7 degrees below zero Celsius, York said. Because Arctic squirrels normally hibernate at temperatures between -12 and -18 degrees Celsius, the animals might be able to survive body temperatures even lower than those recorded by researchers.
However, both Barnes and York believe that significantly lower body temperatures might be too much for even the hearty squirrels, and hope to find out in coming months by exposing the animals to still lower temperatures.
"I would be very surprised if they could go much lower," Barnes said.