IN COLD BLOOD

Last week I hiked along a pallid limestone trail by the light of the moon. I was just beyond the 7,000 foot lower elevation limit for Douglas-fir (Pseudotsuga menziesii), and hiking in comfort mandated the donning of a second sweatshirt. The air temperature was just a few degrees above freezing, so I was more than a bit surprised to see a snake crawling along the stony soil. It was a big Rubber Boa (Charina bottae), about 30 inches long. I have hiked in these mountains all of my life, and have caught only a handful of these interesting snakes. Thinking over my previous captures, it occurs to me that they all took place during abnormal cold snaps. I never realized it before, but it appears to me that the ideal body temperature for Rubber Boas must be extremely low.

It's a gross oversimplification to describe animals as either “warm-blooded” or “cold-blooded,” since there are a multitude of techniques creatures use to thermoregulate. When driving through the country in the morning or evening, it's common to see lizards and snakes stretched out on the warm pavement, often permanently. This is the sort of behavior we normally think of when considering reptile thermoregulation, but for animals like the Rubber Boa, life is more of an exercise in avoiding heat than seeking it. A number of reptiles function best at low temperatures. A little rattler called the Midget Faded Rattlesnake (Crotalus viridis concolor) is rather common in the eastern part of Utah, and is also only seen on very cool nights. In Florida, Indigo Snakes (Drymarchon corais) spend the summers deep underground, emerging only when cold weather drives the other resident snakes beneath the surface. The relictual Tuataras (Sphenodon punctatus) of New Zealand are the most cold-blooded reptiles of all, with an optimum body temperature of around 10ºC (50ºF). Amphibians tend to have lower optimum body temperatures than reptiles.
Reptiles, amphibians, and other animals that are poikilothermic (meaning their body temperature varies with the ambient temperature) can raise their temperature in ways more sophisticated than simply basking on a rock. Muscle contractions release heat, and movement is often used in thermoregulation. Sphinx moths (family Sphingidae) vibrate their wings on cool evenings in a pre-launch warm-up. As an animal increases in size, its surface area-to-volume ratio decreases, and heat loss slows down. A boxy, half-ton Leatherback Turtle (Dermochelys coriacea) can increase its body temperature significantly simply by swimming. This is assisted by heat exchangers in the reptile's circulatory system. Such devices are also present in some sharks, and are most highly-developed in tuna of the genera Thunnus and Katsuwonus. These fish can maintain a temperature in the swimming muscles of their upper tails about 14ºC (25ºF) higher than the ambient water temperature. The veins and arteries inside the muscles branch into tiny intertwining vessels. As it passes through the body, heat diffuses from the muscle-warmed blood in the veins into the cold, oxygenated arterial blood.

We tend to think of poikilothermy as being limiting, but it's not without its advantages. Our constantly-burning metabolic furnaces are expensive to run. A lizard or snake can devote practically all of its energy to the business of foraging and reproducing, and has no need for all the extra food required for metabolic thermoregulation. Because of this, poikilotherms tend to me more plentiful; a given parcel of land can support far fewer weasels, for instance, than carnivorous reptiles of similar size.
One of the downsides of poikilothermy is the fact that internal chemicals often react differently at different temperatures. Reptiles tend to need lots of redundant chemical systems to function properly at different temperatures. Aside from this, though, the advantages of a steady internal temperature are often exaggerated. I can't speak for other homeotherms, but I probably work at least as hard at keeping myself surrounded by a favorable ambient temperature as your average Rubber Boa does, and it's not a bit unusual to find me stretched out, enjoying the heat of a nice, flat rock.
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upper: RETICULATED PYTHON & MASKED FINFOOT (1999) acrylic 20" x 30"
center: Rubber Boa Photograph by CPOBvK, May 16, 2006
lower: BASKING BAJA CALIFORNIA COLLARED LIZARD (2003) acrylic 9" x 12"