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Endotherms & ectotherms

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 98.6F/37C. 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 37C across a wide range of external temperatures.
Image credit: diagram based on data from Cannon and Nedergaard1, Figure 2, and on similar figure in Purves et al.2
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.
Image credit: diagram based on theoretical graph from Meek3, Figure 1 and on Akin4, Figure 1
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
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 32F/ 0C 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 104F/ 40C.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

The graph below shows metabolic rate as a function of external temperature for two animals: an endotherm and an ectotherm.
Image credit: figure based on Hiebert and Noveral9, Figure 1
Which curve represents the endotherm, and which represents the ectotherm?
Choose 1 answer:

Want to join the conversation?

  • blobby green style avatar for user maahtaab
    I have two questions.
    First, is the thermoneutral zone for humans around 24 Celsius or 37?
    And second, why do we feel hot when the environment temperature is 37 Celsius, when 37 is the temperature we want our body to have?
    (4 votes)
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    • winston baby style avatar for user Ivana - Science trainee
      Thermoneutral zone for humans is around 24, not 37.
      The thermoneutral zone is defined as the range of ambient temperatures where the body can maintain its core temperature solely through regulating dry heat loss, i.e., skin blood flow.

      This paper explains

      Why not 37? Because imagine even if we were nude at that temperature (anything above 30 is too much).
      Also while it is plausible for our basal metabolic rate, not suitable for any kind of work.

      Have you noticed that slightest amount of work even at 27 makes you sweat?

      We feel hot because we are not static objects which just conduct temperature. We also produce temperature in our core, plus have a skin which is an insulator.

      So the reason why you feel hot at 37 is mainly that you have skin whose surface temperature is lower than 37 so your body surface is colder and starts absorbing environmental temperature.
      (5 votes)
  • blobby green style avatar for user MT
    I have a question
    Are tuna endotherm or ectotherm? I have ever studied that tuna is Homeotherm in ectotherm. Now I feel confused.
    (4 votes)
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    • leafers sapling style avatar for user Nine.Naijai
      I found this online.

      "An endotherm is able to regulate its body
      temperature via metabolic processes, these are commonly known warm blooded animals.
      An ectotherm's body temperature is dictated by the environment surrounding it, the animals are commonly and incorrectly known as cold blooded.
      Poikilotherms are animals that do not require a fixed body temperature, their temperatures can fluctuate with little to no adverse effects to their overall health. Most terrestrial
      ectotherm's are poikilotherms, such as snakes and many lizards, also the naked mole rat is considered to be the only mammal poikilotherm. Homeotherms are animals that maintain a constant body temperature. All endotherms are homeothermic, but some ectotherms, like desert lizards, are so good at maintaining their body temperature with behavioral means that they are
      considered homeothermic."

      source: https://faculty.mtsac.edu/trevell/bio2/bio2resources/thermo
      (2 votes)
  • leafers seed style avatar for user Taylor, Vivian
    Why would one expect the Q10 value of an ectotherm to be around 2?
    (3 votes)
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    • winston baby style avatar for user Ivana - Science trainee
      If Q10 is 2 it means an increase in the surrounding temperature with an increase in 10 ◦C, and usually resulted in a doubling of the reaction rate.

      The Q10 values can be determined from the Arrhenius plots. (In chemical kinetics, an Arrhenius plot displays the logarithm of a reaction rate constant, (, ordinate axis) plotted against reciprocal of the temperature (, abscissa).)

      So I found the paper where Researchers have experimented on Pseudomonas sp. (ectotherm, relying upon external temperature) and phenol degradation was referent variable to detect metabolism of Pseudomonas.


      Again, your questions as why is that so?
      And I cannot tell you for sure why the rate usually is 2, it was experimentally obtained and concluded.

      Probably it has to do something with Laws of Thermodynamics and logic.

      Think about it, if it was 3, 4, or 7, what would it mean in BIological terms?! That metabolism would be super fast, and at 30*C, the organism may just burn out in half an hour. And it does not make sense.
      (2 votes)
  • duskpin ultimate style avatar for user Samuel Lee
    Why would endotherms use more oxygen at lower temperatures and ectotherms use more oxygen at higher temperatures?
    (4 votes)
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    • winston baby style avatar for user Ivana - Science trainee
      Because endotherms have fixed body temperature. And that temperature is obviously higher than any environmental unless during hot summers or jumping into Geysers.

      The temperature tends to leave body (Law of Thermodynamics) so energy must be used to generate new heat from brown fatty tissue.

      Ectotherms need to adapt their body temperatures to the envirnomantal. Let's say they already have 18C body temperature. Thean temperature strats raising. They cannot just 'absorb it from the atmosphere' they have also to generate energy in order to follow the external temperature. Why? because they also have protective layers such as skin, scales, etc. (which might act as isolators).

      ONE MORE THING: Work at the higher temperatures require higher energy inputs and you are way easily tired and less likely to do the same amount of work.
      (0 votes)
  • hopper cool style avatar for user Lee Wright
    Is the graph also supposed to demonstrate that ectotherms have minimal oxygen consumption regardless of ambient temperature? Or is that just an artifact?
    (3 votes)
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  • piceratops seed style avatar for user drunkgraw
    Which between a unicellular organism, ectotherm and endotherm has the highest BMR and which one is the lowest?
    (2 votes)
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    • female robot grace style avatar for user tyersome
      This is one of those questions that seem reasonable, but are so dependent on the specific organisms that they are probably impossible to get a useful answer for.

      For example, unicellular organisms are incredibly diverse (contain organisms from all three domains of life and from multiple eukaryotic kingdoms) — they will not have similar metabolic rates.
      Furthermore, unicellular organisms can't even all be grown at the same temperature — how would you meaningfully compare the metabolic rate of a marine bacterium that lives in arctic waters (prefers 4°C) vs. the rate for an archaebacterium that grows only on a "black smoker " (undersea thermal vent) and prefers 98°C (can grow at up to 122°C):

      Another example, is what kind of endotherm — a blue whale? an elephant? a shrew? a hummingbird? an ostrich?


      Well, hopefully you found some of that rant interesting or at least mildly entertaining ...
      (3 votes)
  • primosaur ultimate style avatar for user FrenchHornCat
    Why do the body temperatures of endotherms vary? Does it involves the percentage of ATP energy which becomes heat, or does it involve the amount of energy different animals have to consume? Or is it unrelated to both of those?
    (3 votes)
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  • blobby green style avatar for user 22cgoodwin
    Why does temperature affect the breathing and heart rates of ectothermic organisms?
    (2 votes)
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    • winston baby style avatar for user Ivana - Science trainee
      Because temperature per se affects any kind of work. At higher temperatures, the working machine (or int his case heart) has to work harder and easier gets overheated.

      The same way temperature affects Ebdithersm, it also affects Ectitherms, especially because they rely on the temperature. Because temperature directly correlates with metabolism rate.

      The greater the temperature faster the metabolism and vice versa.

      Here is one interesting paper depicting how also Ectotherms may have energy costs in their metabolisms because sudden increases and decreases impose some kind of stress on bodies. They are physiologically dependant on external temperature and their viability is reduced at extremes.

      (2 votes)
  • blobby green style avatar for user maahtaab
    Is the the temperature in thermoneutral zone, the temperature we use when calculating BMR?
    (2 votes)
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  • blobby green style avatar for user Cameron siler
    Do endotherms respirate faster than ectotherms?
    (2 votes)
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