Are Mammals Cold-Blooded?

The term Ectothermic is sometimes used interchangably with the term Cold-blooded. However, ectothermic comes from the greek meaning outside (ectos) and heat (thermos), i.e. describing animals that get heat from their surroundings. Cold-blooded however, isn’t really a great term as creatures that only use their surroundings to control their temperature could just as well have blood that is too hot as too cold. Reptiles are a perfect example of an ectothermic animal, basking in the sun to raise their temperature, and seeking shade when it all gets too hot!

Mammals are warm-blooded, i.e. their temperature is controlled almost regardless of the external temperature – they use internal metabolic reactions to maintain body temperature. However, most mammals will still use their surroundings to help manage their temperature. I’m sure you have done it yourself – turn up the heating, have a cold shower, that sort of thing! So to a certain extent we are all ectothermic.

However, some mammals rely on the outside temperature more than others. For example the Hyrax struggles to regulate its body temperature, despite a thick layer of fur. In the morning they are lethargic as they are still cold. They are often seen lazing about on an exposed boulder warming up in the sun before they are able to start feeding! Similarly they can overheat easily too, so they have to be careful to control how long they are out and about. At night they can be seen lying on top of each other (stacking) to help stay warm.

Researchers have found one mammal that is believed to exhibit ectothermic characteristics, a sheep like creature that went extinct about 5000 years ago. Bones of the Myotragus Balearicus show that its growth rate was related to the food available, similar to reptiles, and not continuous as in warm blooded mammals. However, this study probably doesn’t show enough evidence to class this animal as cold-blooded. A good counter argument can be found here.

So, there you go. There aren’t any cold-blooded animals, and there probably never were. However all mammals do use their surroundings as well as their internal metabolism to control their body temperature.

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