A52 Hypothermia and frostbite

Topic

An insect facing sub-zero temperatures does not respond to the cold in a single way; instead, it follows one of two completely different strategies regarding hypothermia and freezing: the freeze-tolerant insect allows a portion of its extracellular water to freeze, whereas the freeze-intolerant insect does the exact opposite, preventing ice formation within its body at all costs and keeping its water liquid even below the freezing point—a state known as supercooling.

Fundamentally, both insect strategies rely on the same physical phenomenon: ice nucleation—that is, the moment the first ice crystal forms within the insect's body water. This nucleation follows highly non-linear kinetics: the rate at which new crystals appear depends exponentially on a critical energy barrier that must be overcome for a crystal to form. This barrier, in turn, depends heavily on the water's interfacial tension—the higher the tension, the higher the barrier—and decreases significantly as the water cools below its freezing point, dropping more sharply the further the temperature falls below that point.

However, this energy barrier preventing nucleation can be drastically lowered if the insect's body water contains small nucleating particles, such as bacteria or dust, which provide a surface upon which ice can form more easily. This process—known as heterogeneous nucleation—poses the greatest risk to the freeze-intolerant insect, which relies on maintaining a high barrier to remain in a supercooled state. Once ice nucleation has occurred, the fraction of the insect's water that freezes does not increase at a constant rate over time; instead, it follows a sigmoidal curve—as described by the Avrami equation for phase transformation—advancing slowly at first, then accelerating, and finally slowing down again as nearly all available water has frozen.

Both freeze-tolerant and freeze-intolerant insects possess their own molecular defenses: antifreeze proteins and cryoprotectants (such as glycerol and trehalose) lower the freezing point of the insect's body water, thereby extending the temperature range in which the insect can remain supercooled without ice nucleation occurring. At the same time, these molecules modify the growth rate of ice crystals once freezing has begun, giving the freeze-tolerant insect greater control over how ice forms within its body.

Ultimately, the extent of the damage caused by freezing—whether the insect adopted it as a strategy or its supercooling failed—depends primarily on the rate at which ice crystals form within the body: if freezing proceeds slowly, large crystals form outside the insect's cells, dehydrating them via osmosis but allowing the insect to survive; conversely, if freezing proceeds rapidly, crystals form inside the cells themselves, and these intracellular crystals prove lethal.

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