A NOTE ON TERMS AND DEFINITIONS - Winter World: The Ingenuity of Animal Survival - Bernd Heinrich

Winter World: The Ingenuity of Animal Survival - Bernd Heinrich (2003)


Terms help and sometimes almost define what we think. However, as much as possible I have tried in this book to let empirical reality be the ultimate arbiter, with terms serving only as handy abstractions to encapsulate concepts. Unfortunately, concepts change and keep changing as new information becomes available, so that the terms should change as well. Throughout this book I have used terms that have had various meanings at different times and to different people. To minimize potential confusion and partly as a review, I here try to clarify some of these terms that refer to the winter adaptations of animals.

In Winter World, I primarily use the Celsius scale to measure temperature. For weights and lengths, I use the U.S. as well as the metric system. For those readers who need to brush up on converting between Celsius and Fahrenheit or between U.S. and metric measures, here are a few quick formulas: 1 ounce equals 28.35 grams, and 1 inch equals 2.54 centimeters.

Since so much of Winter World is about temperature, fuller Celsius to Fahrenheit conversion details are given in the scale that follows:

Converting Celsius to Fahrenheit.

Most of the other terms in the book relate to hibernation, and even this term has caused confusion because of assumptions associated with it. Traditionally, hibernation simply meant winter inactivity, and it thus applies equally to frogs that have buried themselves in the mud under the ice, some insects and other frogs that are frozen solid while above ground, bears lying in their dens while maintaining a high body temperature, or ground squirrels and bats spending most of the winter with a low body temperature but periodically warming themselves up to be active for a day or more.

Hibernating animals are most (but not necessarily all) of the time in torpor, a state of inactivity achieved primarily (but not exclusively) by a greatly lowered body temperature. Hibernation refers specifically to an evolved suite of adaptations to the winter season, whereas torpor can either be a pathological breakdown of temperature regulation, or an adaptive response for conserving energy. Its duration can be for hours, days, or months.

After it became known that winter torpor, by setting down the body’s thermostat, can be adaptive for warm-blooded animals, then low body temperature became almost the defining characteristic of hibernation. Precisely the same mechanism of adaptive torpor was then observed in some animals that survive not only winter conditions, but also inhospitable seasonal conditions in the desert. In this new context the “hibernation” physiology of torpor was then defined as aestivation.

Initially, the strict definition that joined the mechanism of hibernation (or aestivation) to body temperature implied that only mammals (and potentially birds) hibernated. However, since other animals that never regulate 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 state of development. All insects are in arrested development when they hibernate (in part because low temperature, if not also freezing, retards or stops biochemical processes unless special mechanisms are invoked to circumvent the cold), but they are not strictly in diapause unless they do not respond with resumed development as soon as they experience warming. Many (but by no means all) moths arrest their development in the pupal stage during late summer and fall when it is still warm, and then hibernate as diapausing pupae. Others, depending on the species, hibernate in the egg, larval, or adult stage. Special adaptations are required for arresting development and combine with other traits for withstanding the cold during overwintering. Diapause also occurs in the absence of hibernation. For example, some adult insects enter reproductive diapause in the summer when they migrate or search for host plants.

The muddle in terminology of what hibernation means can be avoided if hibernation is defined not in terms of body temperature or some other specific physiological or behavioral phenomenon characteristic of a given species, but in terms of its adaptive function. In most animals hibernation and/or aestivation are seasonal periods of adaptive torpor that allow the animal to survive regularly occurring famine. Cold, heat, and aridity are factors that exacerbate the seasonal famine that hibernation has addressed through the evolution of various adaptations.

Even more confusion of terminology could be avoided by realizing that making ever more precise or restrictive definitions does not generate greater precision in the understanding of any animal. Animals are dynamic. Each animal’s choices fit in somewhere in a long continuum of almost anything that can be measured or imagined. Different terms may apply in any one animal in varying degree, depending on circumstances, but ultimately the species, and often the individual, fashion their own solutions to fit the situation or the occasion. We gain understanding not so much by lumping and defining, but by differentiating the specifics from the generalized features. The latter have a tendency to become enshrined as rules or laws that are ultimately statistically derived descriptive artifacts. But animals don’t follow rules or easily allow us to pigeonhole them into convenient intellectual boxes. A “rule” is nothing more than a consistency of response that we have deduced animals exhibit because it serves their interests. Rules are the sum of decisions made by individuals. They are a result. The chaos, and the art, of nature remains.