Animals

Warm-Blooded vs. Cold-Blooded: What's The Difference?

Some animals are warm-blooded, while others are cold-blooded. What sets them apart, and what advantages does each kind have over the other?

Most people who took a science class when they were in elementary school will remember one thing when it comes to how animals regulate their body temperature. There are warm-blooded animals (most mammals and birds) and there are cold-blooded animals (fish, amphibians, reptiles). That's a good way to understand a basic distinction, but it turns out, there are more specific terms we can apply to dividing the animal kingdom.

Science defines an animal's thermoregulation with two characteristics: endothermic or ectothermic; homoeothermic or poikilothermic. Most mammals are categorized as endothermic homeotherms, meaning they produce their own heat and maintain a constant internal body temperature. Ectothermic poikilotherms, on the other hand, adjust their body temperature depending on their environment.

Our understanding of animals and body temperature is evolving. For example, it was long believed that dinosaurs were cold-blooded animals, but a study published in Science found they likely utilized a combination of internal processes and environmental factors to fluctuate their overall body temperature-making them mesotherms. There are also outliers worth noting in these various taxonomies. The yellow fin tuna, for example, is a poikilotherm. The tuna species has evolved to keep its core body temperature slightly higher than the surrounding water, which makes it easier to catch prey.

Internal body regulation is important because it dictates an animal's muscle movements. Mammals are ready to spring into action right away because they keep their muscles warm. Ectothermic animals, however, have to move slowly if they are in a cold environment. For mammals, keeping muscles warm and loose requires great amounts of energy-hence why we eat so much. Reptiles, on the other hand, can go for much longer periods of time in between meals.

Learn More:
Animal Body Types - Basics (minerva.union.edu)
"Endotherms are animals that primarily produces its own heat. Ectotherms are animals that primarily gains heat through the environment."

Dinosaurs neither warm-blooded nor cold-blooded (Nature)
"Dinosaurs were neither sluggish like lizards nor high-energy like mammals, but something in between, a study suggests."

What Is a Cold Blooded Animal? (wisegeek.org)
"A cold blooded animal, or ectotherm, is one that does not have an internal mechanism for regulating its body temperature."

What Is the Difference between Warm-Blooded and Cold-Blooded Animals? (wisegeek.org)
"The terms "cold-blooded" and "warm-blooded" are misleading because cold-blooded animals' blood is not necessarily cold, it just varies based on the temperature of the environment."