Someone recently asked us what it means to be “cold-blooded.”  For most reptile keepers, the definition is pretty simple; anything that can’t maintain its own body temperature and must rely on the external environment for heat.  It’s why we all keep our snakes on heat tape and use thermostats to control temperature to ensure an optimal environment.   As simple as that sounds, we thought we would perhaps take it one or two levels deeper to really get at the differences between the “warm-blooded” and “cold-blooded” creatures that inhabit our planet.  Read on if interested.

To start with, using “cold-blooded” as a phrase to describe reptiles is somewhat misguided; their blood, after all, is not cold!  What the phrase actually refers to is a) the inability of most reptiles to produce enough metabolic heat to keep their bodies warm independently from the temperature in the external environment and b) the inability of reptiles to retain what little metabolic heat they can produce within their bodies.

What is metabolic heat?  Metabolic heat is the heat given off by the biochemical processes that keep most living creatures alive.  In most living organisms these biochemical processes comprise the metabolic system.  While the biology behind the metabolic system can become quite complicated, for our purposes we’ll just think about metabolism as the process by which living creatures convert food into energy and heat (the heat comes from cellular and mitochondrial activity, but we’ll leave this for another post).

Both mammals, which are supposedly “warm-blooded,” and reptiles, which are supposedly “cold-blooded,” produce metabolic heat.  The difference is that mammals produce far more metabolic heat (over 10x more) and are also far, far better at retaining that heat (with insulating feathers, fur, fat, etc.).   This allows mammals to keep their body temperature roughly constant at optimal levels to carry out their life processes, independent of the ambient temperature.   We humans do that by, among other methods, sweating when it’s hot and shivering when it’s cold; essentially a process of internal thermoregulation.

Reptiles can’t regulate their body heat in the same manner as mammals, but this doesn’t mean they don’t regulate their body temperature.  All living creatures need to maintain their body temperature in a range that allows them to optimally carry out their life activities. Reptiles do this by taking advantage of their physical surroundings to ensure that their body temperatures remain in that ideal range, a form of external thermoregulation.  They will bask in the sun and lay on warm surfaces to increases their body temperature, while taking advantage of shade, water, and cool burrows to cool down. 

In the case of snakes, their bodies are actually perfectly designed for external thermoregulation.  Their long slender bodies, with the associated high body surface to mass ratio, make for rapid heat loss or gain, which makes their internal body temperatures very responsive to the physical environment they choose.  In addition, they can further regulate the rate of heat gain or loss by stretching out to maximize the area of their bodies that are exposed, or alternatively coiling up to minimize the amount of body surface area exposed.

Through external thermoregulation, reptiles ensure that their internal body temperature remains at its optimal level, which in many cases is actually quite warm.  We’ve read that for snakes the average internal body temperature is 30 degrees Celsius, or 86 degrees Fahrenheit, colder than humans but not that cold!

For the record, cold-blooded creatures are known as ectotherms, while warm-blooded creatures are known as endotherms.  The origin of the word ectotherm is Greek, with “Ecto” essentially meaning “out” and “therm” meaning “hot,” implying that an ectotherm gets its heat from without.  “Endo” basically means “within,” implying that endotherms get their heat from within.  That’s our quick summary of what it actually means to be “cold-blooded,” hope it was helpful.

HVH