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December and January are months of wild weather swings. We have a family video of a December 30 from many years ago in which Deborah and I were out playing with in the yard with our children. We are all wearing light shirts, and it was sixty degrees and sunny. We may even have had lunch out on the picnic table that afternoon! I also have clear memories (no home movies, though) of sitting at my writing desk on a December 30 of another year looking out on a frozen, windblown snowscape. The air temperature had plunged to nine degrees (Fahrenheit) below zero and there was a steady, swirling wind of 23 mph. The wind chills were in the -35 degree range.
There are no typical December days here in Western Pennsylvania!
The animals that really suffer from these wild fluctuations in temperature are the larger mammals like the white-tailed deer. They have spent several months building up a remarkably insulating winter coat and fat layer ideally suited for cold, blustery weather. When we get a robust southern flow of air, though, and the temperatures go up thirty or forty degrees from normal, the deer’s activities become very limited, and they hide out in the shade of the nearby woodlots and orchards. Smaller mammals, though, like gray squirrels and cottontail rabbits do not seem nearly as impacted by the winter heat wave as the deer.
There is an ecological rule that might help us understand these observations.
“Bergmann’s Rule” (articulated by Carl Bergmann in the middle of the Nineteenth Century) states that larger sized individuals (of a species or among closely related species) are found in colder climates, and that smaller sized individuals are found in warmer climates. Many studies on endothermic (“warm-blooded”) species and ectothermic (“cold-blooded”) species have demonstrated the validity of Bergmann’s Rule for half to maybe two-thirds of studied species. Those statistics, of course, clearly illustrate numerous exceptions to this rule, but its fundamental premises and implications are still quite important.
Bergmann’s explanation of his observations concerned a phenomenon of three-dimensional geometry that was first articulated by Galileo: as a three dimensional object gets larger, its volume increases at a much large rate than does its surface area. Consequently, large objects (or we could say “large animals”) have a smaller surface area relative to their body volumes than do small objects (or, “small animals”). This is significant in the energy dynamics of an animal since a great deal of metabolic heat is radiated out through the surfaces of their bodies. So, an animal that needs to dissipate body heat (an animal living in a warm environment) would be able to accomplish this more easily if it was small (i.e. if it had a high surface area to body volume ratio), and an animal that needs to retain its metabolically generated body heat (an animal living in a cold environment) would be able to accomplish this more easily if it was large (i.e. if it had a small surface area to body volume ratio).
Wolves, bears, foxes, wild boar, tigers, many birds (like the wild turkey (photo to the left) we saw regularly in our front yard last winter), and a number of species of deer all demonstrate body size increases with increasing latitude or increasing altitude of their habitats. Humans also have positive Bergmann Rule tendencies as can be seen in the blocky, heavy bodies of high latitude peoples (like the Inuit) compared to the long, lanky bodies of lower latitude people (like the Masai and Dinka of East Africa). The arms and legs of these warm climate people also tend to be longer than those people of the cold climates (which is a corollary ecological law to Bergmann’s Rule called “Allen’s Rule”).
But, when environmental temperatures fluctuate from cold to warm, and do so in highly predictable ways, would the larger bodied mammals of a cold region be selected against in favor of smaller bodied mammals that could better dissipate their excess body heat even through their layers of insulating hair and body fat?
One aspect of climate change may not so much involve warmer winter temperatures but a greater degree of temperature fluctuation throughout the season. These fluctuations might be observed through some Bergmann’s Rule (or even some Allen’s Rule) observations and measurements.
Humans living on the southern edges of the expanded ice sheets during the last Ice Age were subject to powerful stimuli for biological and cultural evolution. Many of our current responses to the cold undoubtedly have their roots in this do-or-die evolutionary moment. Biologically when we are subjected to sustained cold we tend to crave and consume more high calorie (high fat) foods and use these abundant calories to fuel a higher metabolic rate and a more robust generation of body heat. We lay down more subcutaneous fat for surface insulation and even, in some peoples, increase the amount of fat wrapping around the internal organs. We shiver and use these muscle contractions to generate heat. We decrease the blood flow to our dermal blood vessels via vasoconstriction (but also set up a long-term oscillation of vasoconstriction followed by vasodilation so that the skin does not freeze and die). This varied vascular response is called the “Lewis Hunting Phenomenon.”
Culturally and behaviorally humans living in the extreme cold learned to construct warm shelters and make insulating clothing. They used fire (and a wide variety of fuels to maintain that fire) to heat their habitations and frequently clustered and slept closely together inside of these shelters. When they went outside they tended to maintain vigorous and continuous physical activity in order to generate high levels of metabolic heat. Many of these cold-adapted people also synthesized and drank alcohol to generate at least the illusion of being warm (alcohol dilates blood vessels to the limbs and to the skin and, thus, makes a person feel warmer. In reality, though, this dilation accelerates heat loss from the body and can, unless other mechanisms of heat retention are in place, have disastrous, even fatal, consequences!).
We are the product of mixed messages from our evolutionary history and from our culture and intellect. Our ability to add and then as needed remove layers of insulating clothing lets us glide through winter days that have wide variations in temperature. We do start craving fatty comfort foods, though, as the days get shorter and the temperatures get colder, and maybe even the occasional glass of a beverage containing alcohol. Maybe tonight I will make lasagna and have a glass of Chianti and something chocolaty for dessert. My Paleolithic genes are demanding it!