The difference between endotherms and ectotherms. How to read graphs related to endotherms and ectotherms.

Key points

  • Most animals need to maintain their core body temperature within a relatively narrow range.
  • Endotherms use internally generated heat to maintain body temperature. Their body temperature tends to stay steady regardless of environment.
  • Ectotherms depend mainly on external heat sources, and their body temperature changes with the temperature of the environment.
  • Animals exchange heat with their environment through radiation, conduction—sometimes aided by convection—and evaporation.


What’s it like outside today? If it’s winter where you are, it might be pretty cold. If it’s summer, it might be pretty hot. Either way, odds are that your core body temperature is right around 98.6F98.6\,^\circ \text F/37C37\,^\circ \text C. As we saw in the article on homeostasis, mechanisms like shivering and sweating kick in when your body gets too cold or too hot, keeping your internal temperature steady.
Not all organisms keep their body temperature in as narrow a range as we humans do, but virtually every animal on the planet has to regulate body temperature to some degree—if only to keep the water in its cells from turning to ice or to avoid denaturing its metabolic enzymes with heat.
Broadly speaking, animals can be divided into two groups based on how they regulate body temperature: endotherms and ectotherms. Let's take a closer look at the difference between these two groups.

Endotherms and ectotherms

People, polar bears, penguins, and prairie dogs, like most other birds and mammals, are endotherms. Iguanas and rattlesnakes, like most other reptiles—along with most fishes, amphibians, and invertebrates—are ectotherms.
Endotherms generate most of the heat they need internally. When it's cold out, they increase metabolic heat production to keep their body temperature constant. Because of this, the internal body temperature of an endotherm is more or less independent of the temperature of the environment.
This pattern is shown on the graph below: the mouse maintains a steady body temperature close to 37C37\,^\circ \text C across a wide range of external temperatures.
For ectotherms, on the other hand, body temperature mainly depends on external heat sources. That is, ectotherm body temperature rises and falls along with the temperature of the surrounding environment. Although ectotherms do generate some metabolic heat—like all living things—ectotherms can't increase this heat production to maintain a specific internal temperature.
Most ectotherms do regulate their body temperature to some degree, though. They just don't do it by producing heat. Instead, they use other strategies, such as behavior—seeking sun, shade, etc.—to find environments whose temperature meets their needs.
Some species blur the line between endotherms and ectotherms. Animals that hibernate, for instance, are endothermic when they are active but resemble ectotherms when they are hibernating. Large fish like tuna and sharks generate and conserve enough heat to raise their body temperature above that of the surrounding water, but unlike a true endotherm, they don't maintain a specific body temperature. Even some insects can use metabolic heat to increase body temperature by contracting their flight muscles! 5,6,7^{5,6,7}
One other important point: as a general rule, endotherms have considerably higher metabolic rates than ectotherms. That's because they have to burn large quantities of fuel—food—to maintain their internal body temperature.

Why regulate temperature?

There are some basic limits on survivable body temperature for most animals. At one end of the spectrum, water freezes at 32F32\,^\circ \text F/ 0C0\,^\circ \text C to form ice. If ice crystals form inside a cell, they'll generally rupture its membranes. At the other end of the spectrum, enzymes and other proteins in cells often start to lose shape and function, or denature, at temperatures above 104F104\,^\circ \text F/ 40C40\,^\circ \text C.8^8
Why do many organisms—including you and me—keep their body temperature in a narrower range than this? The rate of chemical reactions changes with temperature, both because temperature affects the rate of collisions between molecules and because the enzymes that control the reactions may be temperature-sensitive. Reactions tend to go faster with higher temperature, up to a point, beyond which their rate drops sharply as their enzymes denature.
Each species has its own network of metabolic reactions and set of enzymes optimized for a particular temperature range. By keeping body temperature in that target range, organisms ensure that their metabolic reactions run properly.

Temperature balance

For both endotherms and ectotherms, body temperature depends on the balance between heat generated by the organism and heat exchanged with—lost to or gained from—the environment.
Heat always moves from warmer to cooler objects, as described in the Second Law of Thermodynamics.
There are three main ways that an organism can exchange heat with its environment: radiation, conduction—along with convection—and evaporation.
  • Radiation: Radiation is the transfer of heat from a warmer object to a cooler one by infrared radiation, that is, without direct contact.
  • Conduction: Heat can be transferred between two objects in direct contact by means of conduction. Conduction of heat between your skin and nearby air or water is aided by convection, in which heat is transferred through movement of air or liquid.
  • Evaporation: Vaporization of water from a surface leads to loss of heat—for example, when sweat evaporates from your skin. To learn why this is the case, take a look at the Why does sweating cool you down? video.
How do organisms control heat production and heat exchange to maintain a healthy internal temperature? We'll answer just that question in the next article on temperature regulation strategies.

Check your understanding: graphs of metabolic rate

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