Winter World_ The Ingenuity of Animal Survival - Bernd Heinrich [2]
Only if I knew how and why a golden-crowned kinglet survives a Maine or an Alaskan winter would I understand the story of winter survival. Like other animals of the north, its life is played out on the anvil of ice and under the hammer of deprivation. For those that endure until spring, existence is reduced to its elegant essentials. The kinglet is thus iconic not only of winter, but also of adaptability under adverse conditions. This bird symbolizes astounding and ingenious strategies that animals have evolved for coping in the winter world. It is emblematic of the winter world that I will here explore, since its diminutive size and its presumed diet of insects, when insects are hidden and in hibernation, combined to produce an unsolved mystery. For me, it was the kinglet that led me further and further into the winter world of the north woods, and into this book, spurring me on to find the miraculous.
A NOTE ON TERMS AND DEFINITIONS
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