Endothermy vs Ectothermy

A polar bear in freezing temperatures
A polar bear in freezing temperatures, Credit: Alan Wilson

Have you ever poured sweat on a hot day? Or shivered in the cold? This is because of your body’s ability to control its own internal temperature, which is called endothermy.

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What are Endothermy and Ectothermy?

Birds and mammals (such as humans) are ‘warm-blooded’ animals called endotherms. Endotherms generate heat using their own bodies. Reptiles, amphibians, and fish are ‘cold-blooded’ or ectotherms. Instead of generating their own body heat, ectotherms absorb heat from their surroundings. Think of them as solar water heaters on rooftops that collect heat from sunlight to warm the water inside. 

The terms come from the Greek words ‘endo’ meaning inside, ‘ecto’ meaning outside, and ‘therm’ meaning heat.

warm-blooded human holding a cold-blooded snake
Thermal image of a warma warm-blooded human holding a cold-blooded snake. Things that appear blue are colder than the parts that appear red and yellow, Credit: Arno / Coen

How do ectothermy and endothermy work?

To maintain a constant internal body temperature, animals have to find ways to cool down when it is too hot and to warm up when they are cold. 

One way that warm-blooded animals create heat is by using the energy stored in the food that they eat. This happens inside their cells, in tiny energy ‘factories’ called mitochondria.

Think of an engine in a normal car. Energy stored in fuel is used to drive the car forward, charge the battery, and light up the lamps. In the process, the engine also gives off heat as a by-product.

Mitochondria are like the mini engines of the cells in our body. They use the molecules in our food as ‘fuel’ to make energy at a rapid rate. This energy powers all our activities, from moving around and breathing to healing wounds and running our hearts. Much like the engine in a car, these mitochondria in our cells also release heat as a by-product. 

When endotherms are very cold, they can make mitochondria run on a special kind of ‘fuel’ called Brown Adipose Tissue. This special ‘fuel’ helps produce extra warmth. Shivering also kicks the mitochondria into high gear, helping endotherms keep the cold at bay.

A dog panting on a hot day to expel heat from its body
A dog panting on a hot day to expel heat from its body

But heating up the body is only half the story! Sometimes endotherms need to get rid of too much heat. Sweating and panting help release heat from the body. The redirection of blood towards the surface of the skin serves the same purpose as radiator fins on an air-conditioner, helping us cool down faster.

Hair, blubber, and feathers act as a barrier between the inside and outside of the bodies of mammals and birds! These work like the insulation in a vacuum flask, allowing cold fruit juice to stay cool for many hours on a warm summer day and hot coffee to stay hot.

feather from a bird that insulates it from the cold
A down feather from a bird that insulates it from the cold. These feathers are so effective in beating the cold that we stuff them inside down jackets, Credit: Wouter Hagens

Most fish, reptiles, and snakes do not have any of these abilities. Ectotherms have mitochondria that can produce some heat but not enough for keeping their bodies at a constant temperature. They depend on their environment to change their body temperature for them. On a cold evening, they might seek out a warm rock out in the sun or search for shade when it is too hot outside.

A sunbathing lizard
A sunbathing lizard

Why is it important to maintain proper body temperature?

All life forms are dependent on chemical reactions within their cells (like the ones in the mitochondria). Animals in particular require these chemical reactions to take place at a high speed, to keep up with the needs of their bodies.

These reactions take place rapidly only across a certain set of temperatures. Too hot or too cold and animals will overheat or freeze because the reactions can no longer take place quickly enough.

How does Endothermy help Mammals and Birds?

Being endothermic is expensive for animals because they continuously lose some of their body heat to their surroundings. The energy they used to produce this heat could have been used for growth, movement or stored for use later on. In fact, resting endotherms can end up spending more than ten times the amount of energy as an ectotherm of the same size! If endothermy is so costly in terms of energy, how do endotherms thrive?

The answer lies in the chemistry of our bodies. All the important chemical reactions that take place inside an animal’s body make up its metabolism. The speed at which all these reactions take place is its metabolic rate.

As animals were beginning to evolve new ways to control their own body temperatures, they found that they were also able to increase their metabolic rates. This allowed them to use large amounts of energy over long periods of time, for activities like running long distances. Ectotherms do not have this ability because of their low metabolic rates. While they can jump and run short distances, they cannot do anything that needs a lot of energy for more than a few minutes at a time

Hummingbirds have some of the highest metabolic rates with respect to their body size, Credit: James Wainscoat

How did endothermy evolve?

For a long time, scientists thought that most dinosaurs were cold-blooded, and that birds and mammals were the first to be warm-blooded. As a result of this belief, we expected birds and mammals to have evolved their temperature-controlling powers separately from each other.

Recent studies looking at fossils have found, however, that many dinosaurs were not ectothermic as expected.  Instead, they belonged to a group of in-between animals that were part endothermic and part ectothermic.

Hummingbirds have some of the highest metabolic rates with respect to their body size
A 3D reconstruction of Dimetrodon grandis, a dinosaur suspected to be an early endotherm, Credit: Max Bellomio

This means that warm-blooded animals evolved far earlier than we previously knew. These older endotherms probably passed their temperature-controlling abilities to their younger relatives. It was through these shared relatives that the birds and mammals that we know today became warm-blooded.

Species that show both ecto- and endothermy are still around today! These are animals such as the great white shark and tuna. Although they have the ability to control their body temperature like an endotherm, they can also survive wide changes in their body temperature like a cold-blooded animal.

Even species that are known to be endotherms can go into states that are like ectothermy for some parts of the year. Bears that hibernate during the winter are an example. During hibernation, the metabolic rate and body temperature of bears drop to levels that only a cold-blooded creature would normally have.

So, the next time you find yourself sweating on a hot summer day, keep cool and thank evolution!


Endotherm: An organism that can control its own body temperature (also called warm-blooded)

Ectotherm: An organism that depends on its surroundings to control its body temperature (also called cold-blooded)

Metabolism: All the chemical reactions that take place in an organism’s body

Mitochondria: A cell organelle that is responsible for using oxygen from the air we breathe and molecules from the food we eat to make energy and heat.

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  • Pradyumna is a Biology undergraduate, with a taste for Ecology and Wildlife Biology. He has an appetite for understanding biological systems, and sharing the excitement of discovery through writing When he's not reading papers on his laptop, he's out mountain biking, bird watching and travelling the countryside. His other passions include drumming, photography and tinkering with vintage gizmos. Writing for Smore allows him to combine his fascination for the natural world with his passion for popular science communication.

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