Squirrel sleeps at a fluid subzero.

Squirrel sleeps at a fluid subzero

Researchers have discovered for the first time a warm-blooded animal that can survive, without freezing, at a body temperature below the freezing point of water. The hibernating arctic ground squirrel, Spermophilus parryii, can drop as low as -2.9[deg.]C, reports biologist Brian M. Barnes of the University of Alaska at Fairbanks. Although scientists have found a few cold-blooded vertebrates that can live at subzero body temperatures, they previously had found no mammal that survives below 0.5[deg.]C, Barnes writes in the June 30 SCIENCE.

Barnes and his co-workers captured 12 arctic ground squirrels from their native habitat on the North Slope of Alaska. They implanted miniature temperature-sensitive radio transmitters in the squirrels' abdomens and released them in partially buried outdoor wire cages in Fairbanks. The squirrels dug burrows in the cages and hibernated for eight months starting in September, Barnes writes.

The scientists recorded the lowest body temperatures in February and March. The squirrels maintained these temperatures, which averaged -1.9[deg.]C, for over three weeks. Then several days before the animals' brief monthly arousal, the squirrels gradually warmed to about 0.5[deg.]C before rapidly climbing to normal body temperature, says Barnes' collaborator Alison D. York.

In a second experiment, the scientists held arctic ground squirrels in a -4.3[deg.]C laboratory chamber, where they could measure temperatures at different locations and examine their blood for clues of freezing survival mechanisms. They found subzero temperatures only at the rear of the animal, not in the brain or heart, Barnes says. And from measurements of molecules in blood plasma drawn from six squirrels with subzero body temperatures, Barnes concluded that the measured concentrations could not lower the blood's freezing point enough to account for the squirrels' survival at the temperatures observed.

Barnes' team also found that the freezing and melting points of plasma from these animals were identical, ruling out the presence of antifreeze molecules, which lower freezing points below melting points. And since these animals' fluids do not freeze, the only possibility, Barnes reasons, is that the animals use supercooling, in which a fluid is somehow prevented from freezing below its freezing point.

Despite decades of research, no previous scientist has found a mammal able to remain supercooled for longer than an hour. Prolonged supercooling is probably unique to arctic species, many of which must live for many months at extremely low temperatures, says H. Craig Heller of Stanford University. Barnes suggests that supercooling to -3[deg.]C could save 10 times the energy needed to keep up a body temperature above zero, giving the squirrels a selective advantage.