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By Root 1217 0

a high body temperature also engage in adaptive winter inactivity, a new term had to be invented, or else the old one needed to be discarded. The solution was to invent still a fourth term, brumation. Coined in the 1970s, this term refers to winter sluggishness or torpor of presumably cold-blooded amphibians and reptiles. Later still it became widely known that some mammals and some birds routinely enter torpor to conserve energy, not just seasonally but also on a daily basis in summer. The behavior/physiology of torpor could then no longer be retained as the defining characteristic of hibernation even in the warm-blooded animals. Finally, as ever more numerous and varied ways of surviving winter were discovered, an all-inclusive definition of hibernation has become out of reach.

Body temperature turned out to be an especially inappropriate criterion for defining hibernation because many insects, presumed to be “cold-blooded,” were found to regulate at times the same or even higher body temperature than the majority of birds and mammals. Like birds and mammals that at times allow body temperature to decline, they shiver (simultaneously contract opposing muscles otherwise used for locomotion to produce heat but little movement) so that they can become capable of rapid movement, in this case flight. Other insects stay active without ever heating up, either by shivering or by basking (increasing body temperature by orienting to capture solar heat rather than by shivering), and a few are even active with a body temperature at or slightly below the freezing point of water.

Activity and body temperature, as relating to the winter world, cannot be understood without rudimentary knowledge of the physical properties of water, and concepts and terms such as freezing point depression, antifreeze, ice-nucleation sites, thermal hysteresis and supercooling that will crop up later in the text and that I here foreshadow. In general, the freezing and melting points of water are the same temperature. For pure water it is common knowledge that solid-liquid transition occurs at a point defined as 0° on the Celsius and 32° on the Fahrenheit scale. (I shall refer primarily to the international, Celsius scale.) Solutes in water lower the freezing point predictably; adding one mole (molecular weight, which is a specific number of molecules) of any substance to a liter of pure water, for example, lowers the freezing/melting point by 1.86°C. Many animal adaptations to the low temperature in the winter world relate to physical “tricks” of altering the predicted freezing point, by exploiting other physical phenomena associated with the freezing/melting point of water. First, the freezing/melting point depression is not always strictly a function of the molar concentrations of the dissolved substance in the water. Some substances—those special ones we call “antifreeze”—interact with the water molecules and cause a freezing point depression (lowering of the freezing point) that is greater than that predicted by concentration alone. An even more important phenomenon that some animals (especially insects) exploit is the separation of the freezing from the melting points. This anomaly is called thermal hysteresis. When a solution of water (regardless of whether it is pure or has solutes that may or may not be antifreezes) is in the liquid state when at a temperature below its predicted freezing point (i.e., in thermal hysteresis), then it is defined as being supercooled. Normally ice crystals form on and around some molecule or other ice crystal, and supercooling of a liquid is possible only in the absence of so-called nucleation sites around which ice crystals grow. Adding a nucleation site—such as a single ice crystal or a dust particle—to a supercooled liquid results in it all “instantly” turning into ice; and since supercooled liquids are not in a physically stable state, they can potentially freeze at any moment.

Another (nonexclusive) term sometimes used for overwintering insects is diapause, which is, however, more strictly defined as an arrested