BY HANNES THUM
How the cold creeps in. At this time of year, one of those crossroads of evolution that I think of often is the divergence of “cold-blooded” versus “warm-blooded” creatures. As nighttime temperatures quickly drop in the Valley and as snow begins to threaten on the horizon, I sometimes ponder the extreme vulnerability of a warm-blooded creature (like me) that relies on keeping my core temperature at around 98°F when the air temperature, just inches away from my core, is much lower than that.
A cold-blooded animal’s temperature range is wide, and more or less matches the external environment (it’s a misunderstanding to think that they are always cold); a warm-blooded animal’s temperature must fight to keep their internal temperature constant no matter what the external environment is doing.
A cold-blooded rattlesnake, then, will be warmer (and more active) when it is out in the sun on a hot day, and cooler (and more sluggish) at night when the temperatures get colder. Folks who like to walk their dogs out Croy Canyon in the summertime will know this.
Evolutionarily, the snakes kept the ability to let their internal temperatures vary. We warm-blooded humans, on the other hand, traded that ability out for something else (as did most mammals and birds): the ability to be fully active in all climates.
We go through a huge amount of effort to make sure our internal core temperature stays at around 98°F for our entire lives, be it a hot summer day or a bitterly cold winter night. Creatures like us do not tolerate well changes in our internal temperature. Just a few degrees up or down can kill us.
In winter, it can be quite a feat for warm-blooded creatures to stay warm. Every cell in our bodies (there are trillions of them) acts like a tiny woodstove. As the cells break down the food that we eat, they are constantly producing excess heat that warms us from the inside out. We add oxygen (through our lungs) and fuel (blood sugar) to the stoves, and heat and carbon dioxide are produced.
In some ways, the process in our warm-blooded cells is a massive waste of energy. A cold-blooded creature is much more efficient about using food to directly power its cells or to build new body tissues because they don’t need to create heat to survive. The vast majority of food calories that we warm-blooded creatures eat is converted to thermal energy, meaning we have to eat much more food than a similarly-sized cold-blooded animal.
All of this effort just to keep our core warm, no matter what.
The physical boundary between our heart (the core of our core, so to speak) and the external environment is slim, indeed. A few inches, maybe. The line between life and death runs extraordinarily thin in the winter, but we were long ago committed to this warm-blooded life. So, we stoke the fires as best we can.