Cold Fins, Warm Heart

Kris in Sydney, Australia has a question about whether tuna are warm blooded. I thought all fish were cold-blooded. Is my high school biology textbook out-of-date?

Thank you for the question, Kris. It's important to know the difference between cold and warm blood.

Humans are warm blooded (endothermic). This means that we generate heat in our bodies and can stay warm no matter the environment in which we find ourselves. Endothermy was a very important evolutionary adaptation that helped mammals survive during cold times.

Creatures that are cold blooded (ectothermic) do not have any internal mechanism for generating heat, and must rely on the environment to supply it for them. This is why lizards do not move fast on cold mornings, and must sit on a warm rock in the sun to increase their body temperature.

Similar animals have similar methods of generating heat: mammals are warm blooded, reptiles are cold blooded. Generally speaking, fish are cold blooded; however, some fish are different.

There are about 30 species of fish that have some degree of ability to control their body temperature -- tuna among them. This affords them great advantages in hunting for food, as they do not need to stay in water of a specific temperature, as would other fish. It also allows tuna to swim very fast -- some have been clocked at up to 40 miles per hour over short distances -- allowing them to hunt faster prey, or avoid slower predators.

The illustration is by famed naturalist Charles Harper.