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.

Introduction

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.
The sum total of the biochemical reactions that take place in an organism are called its metabolism. Metabolic reactions involve breaking down fuel molecules, such as sugars, and using the energy stored in them to do work. The processes that convert energy stored in food molecules into biological work are not very efficient, so heat is generated as a byproduct.
The higher an organism's metabolic rate—the amount of chemical fuel it burns in a given period of time—the more heat it will produce.
So, as an endotherm is exposed to colder external temperatures, it will increase its metabolic rate, burn more fuel, and produce extra heat to keep its body temperature constant.
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.
A graph of a mouse's internal temperature across different outside temperatures.
X axis: outside temperature in degrees Celsius, 0 to 40 degrees
Y axis: animal's internal temperature in degrees Celsius, 0 to 40 degrees
The mouse's body temperature stays close to 37 degrees Celsius across a range of temperatures approximately 5 degrees Celsius to 42 degrees celsius, with a downturn below 5 degrees Celsius and an upturn above 42 degrees Celsius. That is, it is a straight horizontal line at 37 degrees Celsius for most of the external temperature range.
A mouse is an endotherm; it generates metabolic heat to maintain internal body temperature.
Image credit: diagram based on data from Cannon and Nedergaard1^1, Figure 2, and on similar figure in Purves et al.2^2
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.
A graph of a snake's internal temperature across different outside temperatures.
X axis: outside temperature in degrees Celsius, 0 to 40 degrees
Y- axis: animal's internal temperature in degrees Celsius, 0 to 40 degrees
The snake's body temperature varies with external temperature, creating a line with a slope of one between about 5 degrees Celsius and 42 degrees Celsius.
A snake is an ectotherm; it's body temperature changes with the temperature of its environment.
Image credit: diagram based on theoretical graph from Meek3^3, Figure 1 and on Akin4^4, 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^{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.
Sun shining on a dog that is sitting on the ground. Radiation is being absorbed by and reflecting off the dog, convection is happening in the air around the dog, evaporation is occurring on the dog's surface, and heat conduction occurs between the dog and the ground.
  • Radiation: Radiation is the transfer of heat from a warmer object to a cooler one by infrared radiation, that is, without direct contact.
    Sure! You've experienced radiation if you've been warmed by heat from the sun, a fire, or a radiator in a building.
  • 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.
    Here are a couple of examples:
    Conduction: If you pick up an ice cube, you'll lose heat to the ice by means of conduction. If you walk barefoot on stone on a sunny day, on the other hand, you'll absorb heat from the stone by conduction.
    Convection: Wind helps move air away from your body by convection, bringing along new, cooler air and thus increasing the transfer of heat from skin to air. This makes us feel colder when it is windy out, something you may have experienced as wind chill.
  • 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.
Y axis: oxygen consumption rate
X axis: external temperature ranging from 0 to 40 degrees Celsius.
Blue curve A: decreases linearly from 5 degrees Celsius to about 28 degrees Celsius, is flat from 28 degrees Celsius to about 37 degrees Celsius, and increases linearly from 37 degrees Celsius to 40 degrees Celsius.
Red curve B: Increases slowly and more or less linearly from 5 degrees Celsius to 40 degrees Celsius.
The red curve remains below the blue curve at all points on graph.
Image credit: figure based on Hiebert and Noveral9^{9}, Figure 1
Which curve represents the endotherm, and which represents the ectotherm?
Choose 1 answer:
Choose 1 answer:

We can use what we know about how endotherms and ectotherms maintain body temperature to figure out which line corresponds to which animal.
To start with, let's "translate" the Y axis of the graph. Oxygen consumption is a common measure of metabolic rate, because O2\text O_2 gas is used up when fuel molecules are broken down during cellular respiration. The faster an organism is using up oxygen, the higher its metabolic rate. We could equally well measure CO2\text {CO}_2 production or heat production to determine metabolic rate.
So, which curve represents the endotherm? Endotherms increase their metabolic rate as temperature drops, producing more heat and thus keeping their internal temperature up. The only curve that goes up as temperature goes down is the upper curve—the blue curve A\text{A}—so this must be the endotherm.
Y axis: oxygen consumption rate
X axis: external temperature ranging from 0 to 40 degrees Celsius.
Blue curve A: decreases linearly from 5 degrees Celsius to about 28 degrees Celsius, is flat from 28 degrees Celsius to about 37 degrees Celsius, and increases linearly from 37 degrees Celsius to 40 degrees Celsius. As temperature drops, oxygen consumption—indicating metabolic rate—rises, so we know that the organism represented by this curve must be an endotherm.
Red curve B: Increases slowly and more or less linearly from 5 degrees Celsius to 40 degrees Celsius.
The red curve remains below the blue curve at all points on graph.
Image credit: based on Hiebert and Noveral12^{12}, Figure 1, and Purves et al.2^2
The flat part of the curve corresponds to the thermoneutral zone—the range of external temperatures over which the endotherm doesn't have to expend extra energy above its basal, or resting, metabolic rate to maintain body temperature. The increase in metabolic rate at higher temperatures represents the expenditure of energy to try and cool the body, and/or the heating of tissue as cooling systems fail.9^{9}
The remaining curve—the red curve B\text B—must be our ectotherm. Not only is the ectotherm's metabolic rate consistently lower than that of the endotherm, but it also drops as external temperature decreases—the opposite pattern from the endotherm. That's because biochemical reactions tend to slow down at low temperatures, such as those of an ectotherm's body when external temperature decreases.
Y axis: oxygen consumption rate
X axis: external temperature ranging from 0 to 40 degrees Celsius.
Blue curve A: decreases linearly from 5 degrees Celsius to about 28 degrees Celsius, is flat from 28 degrees Celsius to about 37 degrees Celsius, and increases linearly from 37 degrees Celsius to 40 degrees Celsius.
Red curve B: Increases slowly and more or less linearly from 5 degrees Celsius to 40 degrees Celsius. The red curve remains below the blue curve at all points on graph. As temperature drops, oxygen consumption—indicating metabolic rate—also drops, so this organism must be an ectotherm.
Image credit: figure based on Hiebert and Noveral12^{12}, Figure 1, and Purves et al.2^2
This article is licensed under a CC BY-NC-SA 4.0 license.

Works cited

  1. Barbara Cannon and Jan Nedergaard, "Nonshivering Thermogenesis and Its Adequate Measurement in Metabolic Studies," Experimental Biology 214 (2011): 242-253, http://dx.doi.org/10.1242/jeb.050989.
  2. Purves, William K., David Sadava, Gordon H. Orians, and H. Craig Heller, "41.7 Endotherms and Ectotherms," in Life: The Science of Biology, 7th ed. (Sunderland: Sinauer Associates, 2003), 788.
  3. Roger Meek, "Reptiles, Thermoregulation, and the Environment," British Chelonia Group, accessed July 9, 2016, http://www.britishcheloniagroup.org.uk/testudo/v4/v4n2thermoreg.
  4. Jonathan A. Akin, "Homeostatic Processes for Thermoregulation," Nature Education Knowledge 3, no. 10 (2011): 7, http://www.nature.com/scitable/knowledge/library/homeostatic-processes-for-thermoregulation-23592046.
  5. David E. Sadava, David M. Hillis, H. Craig Heller, and May Berenbaum, "Physiology, Homeostasis, and Temperature Regulation," in Life: The Science of Biology, 9th ed. (Sunderland: Sinauer Associates, 2009), 839.
  6. John W. Kimball, "The Transport of Heat," Kimball’s Biology Pages, last modified June 25, 2014, http://www.biology-pages.info/H/HeatTransport.html.
  7. Jane B. Reece, Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, and Robert B. Jackson, "Homeostatic Processes for Thermoregulation Involve Form, Function, and Behavior," In Campbell Biology, 10th ed. (San Francisco: Pearson, 2011), 881.
  8. Worthington Biochemical Corporation, "Temperature Effects," Introduction to Enzymes, accessed June 25, 2016, http://www.worthington-biochem.com/introbiochem/tempeffects.html.
  9. Sara M. Hiebert and Jocelyne Noveral, "Are Chicken Embryos Endotherms or Ectotherms? A Laboratory Exercise Integrating Concepts in Thermoregulation and Metabolism," Advances in Physiology Education 31, no. 1 (2007): 97-109, http://dx.doi.org/10.1152/advan.00035.2006.

References

Akin, Jonathan A. "Homeostatic Processes for Thermoregulation." Nature Education Knowledge 3, no. 10 (2011): 7. http://www.nature.com/scitable/knowledge/library/homeostatic-processes-for-thermoregulation-23592046.
Brenglemann, G. "Temperature Regulation." In Physiology and Biophysics, edited by T. C. Ruch and H. D. Patton. Vol. III, 105-135. 20th ed. Philadelphia: Saunders, 1973.
Campbell, Kirsten. "How Does Temperature Affect Metabolism?" Accessed June 24, 2016. http://sciencing.com/temperature-affect-metabolism-22581.html
Cannon, Barbara and Jan Nedergaard. "Nonshivering Thermogenesis and Its Adequate Measurement in Metabolic Studies." Experimental Biology 214 (2011): 242-253. http://dx.doi.org/10.1242/jeb.050989.
Castellini, Michael. "Thermoregulation." In Encyclopedia of Marine Mammals, edited by William F. Perrin, Bernd Würsig, and J. G. M. Thewissen, 1166-1170. 2nd ed. Burlington: Academic Press, 2009.
"Convection." Biology-Forums.com. Last modified October 10, 2011. biology-forums.com/definitions/index.php?title=Convection.
Frappell, P. and K. Cummings. "Sources of Heat." In Encyclopedia of Ecology, 1885-1888. Amsterdam: Elsevier, 2008.
Ganong, W. F. "Temperature Regulation." In Review of Medical Physiology, 177-181. 9th ed. Los Altos: Lange Medical Publications, 1979.
Gillam, Paul. "Thermoregulation: A* Understanding for iGCSE Biology." PMG Biology. Last modified February 22, 2015. https://pmgbiology.com/tag/thermoregulation/.
Heindl, Alex L. "Rattlesnakes." Accessed June 24, 2016. http://www.alongtheway.org/rattlesnakes/basics.html.
Hiebert, Sara M. and Jocelyne Noveral. "Are Chicken Embryos Endotherms or Ectotherms? A Laboratory Exercise Integrating Concepts in Thermoregulation and Metabolism." Advances in Physiology Education 31, no. 1 (2007): 97-109. http://dx.doi.org/10.1152/advan.00035.2006.
"Heat Transfer." HyperPhysics. Accessed June 24, 2016. http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/heatra.html.
Kimball, John W. "The Transport of Heat." Kimball’s Biology Pages. Last modified June 25, 2014. http://www.biology-pages.info/H/HeatTransport.html.
McNab, Brian Keith. "The Impact of Endothermic Hibernation." The Physiological Ecology of Vertebrates: A View From Energetics, 383-385. Ithaca: Comstock Publishing Associates, 2002.
Meek, Roger. "Reptiles, Thermoregulation, and the Environment." British Chelonia Group. Accessed July 9, 2016. http://www.britishcheloniagroup.org.uk/testudo/v4/v4n2thermoreg.
"Mesotherm." Wikipedia. Last modified February 17, 2016. https://en.wikipedia.org/wiki/Mesotherm.
Nishiura, James. "Effect of Temperature on Enzyme Activity." Accessed May 27, 2016. http://academic.brooklyn.cuny.edu/biology/bio4fv/page/enz_act.htm.
OpenStax. "Energy and Heat Balance." OpenStax CNX. Last modified June 19, 2013. http://cnx.org/contents/x9vl-NZc@3/Energy-and-Heat-Balance.
OpenStax. "Homeostasis." OpenStax CNX. Last modified June 26, 2013. https://cnx.org/contents/BP24ZReh@7/Homeostasis.
Oswald, Nick. "Why Do Enzymes Have Optimal Temperatures?" Bitesize Bio. Last modified June 21, 2016. http://bitesizebio.com/120/why-do-enzymes-have-optimal-temperatures/.
Purves, William K., David Sadava, Gordon H. Orians, and H. Craig Heller. "41.7 Endotherms and Ectotherms." In Life: The Science of Biology, 788. 7th ed. Sunderland: Sinauer Associates, 2003.
Raven, Peter H., George B. Johnson, Kenneth A. Mason, Jonathan B. Losos, and Susan R. Singer. "Regulating Body Temperature." In Biology, 877-882. 10th ed., AP ed. New York: McGraw-Hill, 2014.
Reece, Jane B., Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, and Robert B. Jackson. "Homeostatic Processes for Thermoregulation Involve Form, Function, and Behavior." In Campbell Biology, 878-883. 10th ed. San Francisco: Pearson, 2011.
Ricklefs, Robert E. and Gary L. Miller. "Adaptations Match the Temperature Optima of Organisms to the Temperature of the Environment." In Ecology, 86-88. 4th ed. New York: W. H. Freeman and Company, 2000.
Sadava, David E., David M. Hillis, H. Craig Heller, and May Berenbaum. "Physiology, Homeostasis, and Temperature Regulation." In Life: The Science of Biology, 832-848. 9th ed. Sunderland: Sinauer Associates, 2009.
Starr, Cecie and Ralph Taggart. "Heat Gains and Losses." In Biology: The Unity and Diversity of Life, 749. 11th ed. Belmont: Thomson/Brooks-Cole, 2006.
Starr, Cecie and Ralph Taggart. "Temperature Regulation in Mammals." In Biology: The Unity and Diversity of Life, 750-751. 11th ed. Belmont: Thomson/Brooks-Cole, 2006.
"Thermoregulation." Wikipedia. Last modified June 14, 2016. https://en.wikipedia.org/wiki/Thermoregulation.
"Wind Chill." Wikipedia. Last modified March 15, 2016. https://en.wikipedia.org/wiki/Wind_chill.
Worthington Biochemical Corporation. "Temperature Effects." Introduction to Enzymes. Accessed June 25, 2016. http://www.worthington-biochem.com/introbiochem/tempeffects.html.
Loading