Cold-blooded: What’s it mean?

What is the first thing that comes to mind about reptiles and amphibians? Probably it is the term “cold-blooded.” Most of us have learned that one component of being a reptile or amphibian is being “cold-blooded.” And what are mammals and birds? Why, “warm-blooded” of course. If you’ll indulge me for a few moments (okay, if you’ve read my blogs before, then you know maybe more than a few), let’s do a CliffsNotes version (who remembers those?)  of what “cold-blooded” and “warm-blooded” mean and why us science-y types are not fond of those terms.

Most reptiles and amphibians (as well as most fish and invertebrates) are examples of ectothermic animals. First off, the origin of the word. Ecto means “outer” or “outside” and therm means “heat.” Therefore, ectothermic animals are those that rely on the environment to maintain body temperature. Now, let’s compare that to most birds and mammals, which are generally considered to be endothermic. We already know what therm means, shall we guess what endo means? If you guessed “internal” or “inside,” you’d be right! Most mammals and birds maintain fairly stable body temperatures despite the environmental conditions around them.

So what is wrong with “warm-blooded” and “cold-blooded?” Well, not that I recommend trying, but if you cut open a snake, icicles don’t drop out and similarly you know that if you, as a mammal, get a cut, hot lava doesn’t flow out. These terms just don’t really work.  The term “cold-blooded” implies that these animals are in a never-ending struggle to stay warm. That really isn’t correct. Many species do like it hot, with some monitor lizards basking at temperatures of 120–150 F. I’d certainly call that some warm blood! When at their ideal body temperature (also called thermal optimum), they have metabolisms that function near or exceeding the level of many birds! On the other side of the spectrum, there are many salamanders, which tend to operate best in temperatures in the 50–60 Fahrenheit range or even lower. Some, like spotted salamanders (Ambystoma maculatum) can sometimes be observed swimming or even breeding under the surfaces of frozen ponds. Ectotherms simply rely on the temperature of their environment (read, environment, not just the air) to reach their thermal optimum, whether it be high in the case of the monitor lizards or low in the case of the salamanders. And of course, they can get too hot and too cold just like we can. But instead of sweating, panting, or shivering, they have to move from place to place.

When not at their thermal optimum, their metabolisms don’t function at full capacity. If too hot, they seek places to cool down. If too cold, they may seek a sunny spot to warm up. In colder months, most temperate species seek a place with a lower than optimum yet fairly stable temperature, allowing their metabolisms to slow down so they require little or no food, and wait for better conditions to come back (like spring). Ectotherms can do this because they aren’t having to constantly stoke that internal furnace to maintain that thermal optimum. Most mammals and birds have to slog through those rough months because they can’t simply power the furnace down until things get better. But what about hibernating bears, you may ask. Well, sadly we don’t have time to cover all the oddballs here, of which there are many on both sides!

And of course, this short(ish) discussion is streamlined for the sake of time and space! But hopefully I’ve at least stoked your mental furnace a bit! I hope to see you visiting all of the wonderful ectotherms and endotherms that reside at Zoo Atlanta!
Robert Hill
Assistant Curator of Herpetology