Are All Mammals Warm-Blooded?


Arctic Fox

I had always assumed that all mammals were warm-blooded, but through some research, I found out this is not necessarily true.

Most mammals are endothermic, which means they can regulate their body temperature. Some mammals are classed as heterothermic, allowing their surrounding environments to affect their self-regulating body temperature.

I wanted to look into this more, so I have done some research. If you want to know more about why not all mammals are warm-blooded, then please read on.

Alpine chipmunk sitting on a rock near Yosemite National Park in California.

Are All Mammals Warm-Blooded?

All mammals are considered to be warm-blooded. Warm-blooded animals regulate their body temperatures. This helps them to survive in harsh environments. By regulating their body temperature, changing conditions in the environment around them can be dealt with. 

Mammals generate body heat when in a cooler climate, which helps them to keep warm. Likewise, when the environment around them is hotter, they have a mechanism to ensure they can cool off.

To maintain constant body temperature, mammals have to eat plenty of food. The considerable amount of food they eat is used to fuel their changing body temperatures.

Cold-blooded animals do not generate body heat themselves. For this reason, their body temperatures vary depending on their environments.  

When the climate is hot, cold-blooded animals will adapt to their body temperatures, becoming warmer. In these situations, cold-blooded animals tend to slow down. They are more sluggish when in cold environments. 

The muscle activities of these animals depend on chemical reactions. These chemical reactions happen faster under warm conditions than when it is cold.

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Are There Any Cold-blooded Mammals?

Although most mammals are warm-blooded, there does exist a few species of cold-blooded mammals. 

Least weasel on the snow

The world is huge and filled with all species of animals. With research, you can sometimes come across some animals that have defied all odds. 

It is true to say all mammals are endothermic, which is essentially the same as warm-blooded. 

However, discoveries point out there are some mammals that are not entirely endothermic. 

These animals do not keep constant body temperature, typical of most warm-blooded mammals.

These exceptions to the rule are not truly cold-blooded like reptiles and amphibians. 

Woodland caribou

Heterothermic Mammals

These mammals are classed as heterothermic mammals. This means that they have a varied approach when it comes to their body temperatures. 

A heterothermic animal has fairly constant body temperature but can change their body temperature to suit the conditions in the environment.

Biologists have found evidence of heterothermic species. One of these species is the Arctic ground squirrel. These little mammals inhabit the regions of the Arctic. 

Photo of white rabbit

The Arctic is one of the coldest regions in the world, and it was found that the Arctic ground squirrels chill their body temperatures to survive.

Brian Barnes, Director of the Institute of Arctic Biology and Professor of Zoophysiology, is the person behind these findings. He took the time to study Arctic ground squirrels when hibernating. 

Mammals are less tolerant to freezing conditions. During winter, some mammals use hibernation to get through the inter. Other mammals, such as the Arctic fox, have thick fur to help keep them warm. 

Until this research by Brian Barnes, mammals were not known to be able to survive when their body temperatures became subzero.

Arctic Ground Squirrel

However, the arctic ground squirrel has proven otherwise. The zoologist conducted a study published in the Journal of Anaesthesiology 6, 2016. The study proved that the squirrel is capable of surviving under subzero body temperatures.

Many animals cannot tolerate freezing conditions. Most animals and living organisms have fluid or water in their cells.  

If the body temperatures drop below zero, water in the cells expands. After expanding, the water then freezes into ice. This expansion is bad news for the living organism because ice crystals will continue to expand as they form. 

Eventually, they will rupture the cells, resulting in the death of the organism.

The Arctic ground squirrel can drop their body temperatures to below zero and still survive under such extreme conditions. Body temperatures of the Arctic ground squirrel have been monitored as low as -2.9c.

Other than the Arctic ground squirrel, there are many more mammals capable of surviving whilst having extreme body temperatures. 

Snowshoe hare
Snowshoe Hare

Several small mammals have evolved a way in which they reduce their temperatures to lower than usual. It is a way for these animals to save energy, commonly referred to as daily torpor.

Newborn mammals cannot always control their body temperatures in the first few days. The temperatures of many newborn mammals depend on that of the surrounding environment. These mammals develop internal heat mechanisms later on.

Is There Evidence of Extinct Mammals That Were Ectothermic?

There is no direct evidence that points to an extinct ectothermic mammal species. However, there are some claims an extinct miniature goat known as Myotragus balearicus was ectothermic. The goat is said to have wholly depended on external heat from its surroundings.

The reason behind these claims is in the bone microstructure. This small goat had the same growth and development as other mammals, but the growth occurred at a rate and speed similar to ectothermic animals. 

This is a suggestive study the researchers conducted in 2009. There is not enough evidence to back these claims up, hence why it is a suggestive conclusion. As the animal is extinct, there is no chance to prove this theory.

Arctic Fox

The Myotragus balearicus was alive 3,000 years ago. They inhabited the Balearic Islands, where it is believed there were minimal food supplies. 

Researchers believe the harsh environments contributed to their ectothermic state. This may have been a way to conserve energy. These findings are unclear because no further studies have been completed.

Will There Be More Mammals Found With An ectothermic-like state?

In the coming years, there may be more mammals with fluctuating temperatures. This depends on the kind of environment mammals inhabit. For instance, the naked mole-rat lives in a place with relatively consistent temperatures. 

Cute chipmunk

For this reason, even though it is a mammal, they do not need an internal heat mechanism. It is possible that there will be further discoveries of heterothermic mammals in the future.

Advantages of the Ectothermic-like State

One of the major reasons why some mammals adapt a near ectothermic state is to conserve energy. This is the advantage cold-blooded animals have over warm-blooded animals. Ectothermic animals need less energy, and therefore less food to survive.

Mammals have to eat lots of food to produce energy. This is because energy is required to maintain constant body temperatures. 

Some mammals have to take on a heterothermic state so that they conserve the majority of their energy.

Mammals such as bears, gophers, and bats hibernate in winter. Scientifically, hibernation is a prolonged dormancy. During hibernation, these animals drop their body temperatures to as low as 50C. Hibernation works significantly to preserve their energy in these cold temperatures.

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Bryan Harding

Bryan has spent his whole life around animals. While loving all animals, Bryan is especially fond of mammals and has studied and worked with them around the world. Not only does Bryan share his knowledge and experience with our readers, but he also serves as owner, editor, and publisher of North American Mammals.

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