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By Root 1265 0

in squirrels equipped with radio transmitters indicate that body temperature of torpid squirrels declines in parallel with the sinking soil temperature. Remarkably, however, as burrow temperatures continue to decline to -15°C by December, the squirrel’s body temperature does not continue to decline, to -15°C. Instead, it stabilizes at between -2° and 2.9°C. That is, the torpid squirrel’s body temperature is then no longer allowed to be passive. It is regulated some eight to nine degrees lower than a hibernating chipmunk’s but twelve to thirteen degrees above soil temperature. No other animal had previously been shown to regulate its body temperature near 0°C, much less two or more degrees below the freezing point of water. Furthermore, the squirrels did not turn into blocks of solid ice as their core body temperatures declined to as much as -2.9°C. Barnes wondered if they might have antifreeze. To find out, he removed blood plasma from hibernating squirrels and tested its freezing point in the lab. The blood turned to ice at approximately -0.6°C. It therefore did not contain antifreeze. These results deepen the mystery of winter survival: Why should the blood freeze in the lab but not in the animal? The riddle is not yet solved, but the best tentative explanation so far is that the squirrels supercool.

Pure water has a freezing point of 0°C (32°F). Adding one mole (molecular weight) of a substance to a liter of water lowers its freezing point by -1.86°C. Although pure water and solutions of specific concentration have precisely predictable freezing (and melting) points, it is sometimes possible to lower temperatures below the predicted freezing point without having ice crystals form. Such solutions are said to be “supercooled.” Supercooling occurs due to the absence of “nucleation sites”—places where ice crystals can begin to grow. The best nucleation sites are other ice crystals. Thus, if one adds an ice crystal, say a snowflake, to a vial of pure liquid water that is supercooled to -10°C, then the whole vial full of water will turn instantly into a solid block of ice. But this ice won’t melt until it is heated to 0°C. This difference between freezing and melting points (called thermal hysteresis) defines supercooling. Supercooled liquids are unstable—they can turn to ice unpredictably and with little apparent provocation. Mere stirring can be enough. The greater the thermal hysteresis, the greater likelihood that freezing will occur, and the quicker the sample “flashes” into ice, giving off a measurable pulse of heat in the process as the energy of the motion of the liquid molecules is released when they stop their motion after release from the ice crystal lattice.

The absence of antifreeze in the blood of the squirrels, and hence the likelihood of supercooling to as much as 1° to 2°C ought to be risky. A single stray ice crystal in the blood could mean death. Why do squirrels risk it? Why don’t they regulate their body temperature 1° to 2°C higher, to avoid supercooling and thus be immune to turning into a block of ice and being killed? Barnes believes the advantage that outweighs the cost is related to energy economy; supercooling to -2°C would save the squirrels ten times the energy expended by maintaining a body temperature of 0°C (Barnes 1989). The squirrels also have a mechanism that reduces the risks normally associated with supercooling. Generally freezing would start at the coldest point, such as a toe, and Barnes has nucleated (started the freezing process) squirrels’ toes and found that the animals were then alerted—they rewarmed quickly before the ice could spread.

Barnes’s other remarkable discovery was that the squirrels can and do arouse spontaneously to warm themselves up from a body temperature of less than 0°C, to heat themselves all the way up to their body temperature when active, 37°C. Many other animals can survive cooling to or below 0°C, but none had ever been able to spontaneously arouse unless they were first artificially warmed by being taken to much higher air (and body) temperatures, where