January 6, 2013: I am sitting in front of my usual window watching very unusual things outside. The air temperature has plunged to nine degrees (Fahrenheit) below zero and with a steady, swirling wind of 23 mph. The wind chills are in the -35 degree range.
This is not a typical January day here in Western Pennsylvania! (Photo by D. Sillman: hiking with Dr. Bridges on Laurel Hill (February, 2013))
I won’t talk about the Polar Vortex and the possible linkage of its latest southward dip to climate change, but I am thinking about how species adapt to this unforgiving force of cold. Many past ecologists have studied the responses of species to cold temperatures and have authored “rules” that describe some tendency or trend that they have observed. Probably the most well-known of these individuals is Carl Bergmann, a German biologist who published his definitive papers in the mid-nineteenth century.
“Bergmann’s Rule” states that larger sized individuals (of a species or among closely related species) are found in colder climates, and that smaller sized individuals are, conversely, 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 was based on a phenomenon of three-dimensional geometry: as a three dimensional objects 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, bear, foxes, wild boar, tigers, many birds, 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 (a corollary ecological law to Bergmann’s Rule called “Allen’s Rule”).
I have observed (and previously written about) Bergmann’s Rule right here in Western Pennsylvania. Chickadees living in the cooler Laurel Highlands are larger than the chickadees in the warmer, lower habitats near Pittsburgh, and the migrating robins that come from higher latitudes (the “Newfoundland Robins”) are as a group larger (and darker colored (another ecological rule!)) than their fellow robins that originate from warmer habitats.
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 eat more high calorie (high fat) food 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 area 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!).
So here we all are, face to face with one of the very recent biological and cultural shaping forces of our species! May we never run out of chocolate, cheese, and brandy!