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Many assume that all mammals were warm-blooded, and while the majority are, there are some that are heterothermic.

Most mammals are endothermic, which means they can regulate their body temperature. Some mammals, such as the Arctic ground squirrel, are classed as heterothermic, allowing their surrounding environments to affect their body temperature.

Please read on if you want to know more about why not all mammals are warm-blooded.

Arctic ground squirrel

Are All Mammals Warm-Blooded?

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

Mammals generate body heat in cooler climates, which helps them keep warm. When the environment around them is hotter than their body temperature, most can sweat to cool off.

To maintain a constant body temperature, mammals must eat plenty of food. Food is converted to energy to fuel temperature changes in the body.

Cold-blooded animals do not generate body heat themselves, and their body temperatures vary depending on the environments surrounding them.  

When the climate is hot, cold-blooded animals become warmer. In these situations, cold-blooded animals tend to slow down. They are more sluggish than when in cold environments. 

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

How does the Arctic fox survive? Find out in this article I have written

Are There Any Cold-blooded Mammals?

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

It is true to say all mammals are endothermic, which is essentially the same as warm-blooded. However, some mammals are not endothermic but heterothermic. 

These animals do not keep a constant body temperature, typical of most warm-blooded mammals. However, they are not genuinely cold-blooded like reptiles and amphibians. 

Heterothermic Mammals

Mammals that do not keep a constant body temperature are classed as heterothermic mammals. A heterothermic animal has a relatively constant body temperature but can change its body temperature to suit the environment.

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

The Arctic is one of the coldest regions globally, but the Arctic ground squirrels adjust their body temperatures to survive.

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

Mammals are less tolerant of freezing conditions. During winter some mammals use hibernation to get through the winter. 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 survive when their body temperatures became subzero.

Hibernation works for some animals. Find out more here

Arctic Ground Squirrel

A study published in the Journal of Anaesthesiology 6, 2016, demonstrated that squirrels could survive under subzero body temperatures.

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

Many animals cannot tolerate freezing conditions due to fluid in their cells.

The fluid expands if an animal’s body temperature drops below zero. After expanding, the water then freezes into ice. This expansion can be fatal because ice crystals will continue to grow as they form. Eventually, they will rupture the cells, resulting in death.

Apart from the Arctic ground squirrel, many other mammals can survive while having extreme body temperatures. 

Several small mammals, such as shrews, have evolved to reduce their temperatures lower than usual. This allows them to save energy, commonly referred to as daily torpor.

Newborn mammals cannot control their body temperatures in the first few days. The temperatures of many newborn mammals depend on the surrounding environment. As they grow older, they develop internal heat mechanisms.


Is There Evidence of Extinct Mammals That Were Ectothermic?

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

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

This is a suggestive study that 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 of proving this theory.

Myotragus balearicus was alive 3,000 years ago and inhabited the Balearic Islands with minimal food supplies. 

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

Unfortunately, many mammals have gone extinct, even in the last 100 years. Find out which animals are no longer around here.

Will There Be More Mammals Found With An Ectothermic-like State?

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

For this reason, even though it is a mammal, they do not need an internal heat mechanism. There may be further discoveries of heterothermic mammals in the future as further studies are undertaken.

For more information on how mammals survive winter, please click here

Advantages of the Ectothermic-like State

One of the primary reasons why some mammals adapt to 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 less food to survive.

Mammals must eat lots of food to produce energy, which is used to maintain constant body temperatures. To conserve most of their power, some mammals must take on a heterothermic state.

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

Have you ever wondered how mammals are classified? I have written this article which will help you.

References and Further Reading

“Mammals of North America” by Roland W. Kays and Don E. Wilson

“The Diversity of Mammals” by P. D. Gingerich

“Thermoregulation in Mammals and Birds” by Peter J. Butler

“The Biology of Marsupials” by M. Archer

“The Thermal Biology of Desert Organisms” by J. Diamond and T. Case

Frontier Scientists – Lab fringed Arctic Squirrels – Proteomic Profiling Reveals Adaptive Responses to Surgical Myocardial Ischemia-Reperfusion in Hibernating Arctic Ground Squirrels Compared to Rats – Extinct goat was cold-blooded