The term “cold-blooded” is one that gets thrown around often, most commonly to describe the body temperatures of reptiles and amphibians. But what does it really mean? How can reptiles living in some of the hottest places on the planet be considered “cold-blooded” animals, while the most northerly reaches of the arctic circle lack these “cold-blooded” reptiles and amphibians entirely? Read on to find out!
The proper scientific term for “cold-blooded” is ectothermic. Ectothermic animals, often called ectotherms, are unable to internally control their own body temperatures. In other words, their bodies remain at the same temperature as their immediate surroundings. The term DOES NOT imply that an animal is cold to the touch. Depending on an ectotherm’s surroundings, it might be warm or cold to the touch. On the opposite end of the spectrum are endotherms (“warm-blooded” animals), which convert energy from the food they eat into body heat. These animals are able to maintain near constant body temperatures, even when the temperature of their surroundings change. Some of you who have taken a biology class or two might have come across two other, very odd terms. The terms are poikilotherms and homeotherms. Poikilotherms are animals whose body temperatures change drastically on a regular basis. Homeotherms experience almost no regular changes in body temperature. These terms, as far as I have seen, have largely fallen out of use, I suspect because they are not very helpful in determining evolutionary relationships. For example, consider two closely related snakes, one living in the tropics, and the other living in a temperate area. The one in the tropics would be considered a homeotherm simply because the tropics maintain a relatively constant temperature all year long, and thus, so would the snake living there. The snake living in the temperate area, on the other hand, would be considered a poikilotherm, since it’s habitat changes it’s own body temperature significantly. These two terms are quite confusing and not very helpful, so it’s probably better to just forget about them!
Naturally, after learning that ectotherms cannot control their own body temperatures internally, you might be curious about why ectotherms even exist. Why haven’t they been out-competed by their “warm-blooded” adversaries? The answer is actually quite simple; generating your own body heat wastes A TON of energy! “Warm-blooded” animals must eat an enormous number of calories in order to grow, develop, sustain activity, and still have enough energy left over to constantly heat their bodies. Ectotherms, on the other hand, simply need to modify their behaviors to keep their body temperatures stable. If they need to warm up, they might rest in the sun. If they need to cool off, they might hide under rocks or logs, go for a swim, or wait until night to become active. Because ectotherms do not devote any of the calories they eat to stabilizing their body temperatures, they are able to grow and develop without eating much of anything! They can exist in huge numbers within a relatively small area, and there will still be enough food available for all of them to survive.
Now, let’s discuss why body temperature is so important in the first place. Chemical reactions within an animal’s body control quite literally everything that happens to the animal. These reactions often happen more quickly at higher temperatures, but they are more directly controlled by proteins called enzymes. Enzymes are incredibly sensitive to temperature; even a temperature change of a few degrees Fahrenheit (up or down) can cause an enzyme to permanently breakdown, or “denature”. Denatured enzymes mean a dead animal! “Warm-blooded” animals rarely have to worry about their enzymes, since they internally control their body temperatures. However, ectotherms often experience extreme changes in body temperature, despite their best efforts to keep it constant. They are able to protect their enzymes from harm by binding certain chemicals to them, but the enzymes still do not perform very well at extreme temperatures. This is why reptiles and amphibians are hardly able to move when they get too cold, yet are able to stay alive! “Warm-blooded” animals are often unable to withstand even minor changes in body temperature. A couple of degrees too hot or too cold, and they will often die!
So there it is! Everything you need to know about “cold-blooded” animals, aka ectotherms! These animals are truly remarkable to behold. Please join me on my adventures into the wilderness areas of Los Angeles to see wild reptiles and amphibians safely and easily! Check out the “Events” page on this site to see a calendar of outings and sign up. See you out there!