How behavior, anatomy, and physiology help animals regulate body temperature.

Key points

  • Many animals regulate their body temperature through behavior, such as seeking sun or shade or huddling together for warmth.
  • Endotherms can alter metabolic heat production to maintain body temperature using both shivering and non-shivering thermogenesis.
  • Vasoconstriction—shrinking—and vasodilation—expansion—of blood vessels to the skin can alter an organism's exchange of heat with the environment.
  • A countercurrent heat exchanger is an arrangement of blood vessels in which heat flows from warmer to cooler blood, usually reducing heat loss.
  • Some animals use body insulation and evaporative mechanisms, such as sweating and panting, in body temperature regulation.

Introduction

Why do lizards sunbathe? Why do jackrabbits have huge ears? Why do dogs pant when they're hot? Animals have quite a few different ways to regulate body temperature! These thermoregulatory strategies let them live in different environments, including some that are pretty extreme.
Polar bears and penguins, for instance, maintain a high body temperature in their chilly homes at the poles, while kangaroo rats, iguanas, and rattlesnakes thrive in Death Valley, where summertime highs are over 100F100\,^\circ \text F (38C38\,^\circ \text C)1^1.
Let's take a closer look at some behavioral strategies, physiological processes, and anatomical features that help animals regulate body temperature.
Left, polar bear jumping between ice floes. Right, lizard in Death Valley.
Left, polar bear jumping between ice floes; right, lizard in Death Valley. Image credits: left, Polar bear jumping by Arturo de Frias Marques, CC BY-SA 4.0; right, Lézard à queue de zèbre by Jon Sullivan, public domain

Mechanisms of thermoregulation

As a refresher, animals can be divided into endotherms and ectotherms based on their temperature regulation.
  • Endotherms, such as birds and mammals, use metabolic heat to maintain a stable internal temperature, often one different from the environment.
  • Ectotherms, like lizards and snakes, do not use metabolic heat to maintain their body temperature but take on the temperature of the environment.
Both endotherms and ectotherms have adaptations—features that arose by natural selection—that help them maintain a healthy body temperature. These adaptations can be behavioral, anatomical, or physiological. Some adaptations increase heat production in endotherms when it’s cold. Others, in both endotherms and ectotherms, increase or decrease exchange of heat with the environment.
We will look at three broad categories of thermoregulatory mechanisms in this article:
  • Changing behavior
  • Increasing metabolic heat production
  • Controlling the exchange of heat with the environment

Behavioral strategies

How do you regulate your body temperature using behavior? On a hot day, you might go for a swim, drink some cold water, or sit in the shade. On a cold day, you might put on a coat, sit in a cozy corner, or eat a bowl of hot soup.
Nonhuman animals have similar types of behaviors. For instance, elephants spray themselves with water to cool down on a hot day, and many animals seek shade when they get too warm. On the other hand, lizards often bask on a hot rock to warm up, and penguin chicks huddle in a group to retain heat.
Some ectotherms are so good at using behavioral strategies for temperature regulation that they maintain a fairly stable body temperature, even though they don't use metabolic heat to do so.
Top left, iguana basking in the sun on a rock; top right, elephant spraying itself with water; bottom left, free-range chickens all sitting in the shade under a tarp in a field; bottom right, penguin chicks huddling together for warmth.
Examples of behavioral temperature regulation, from top left: basking in the sun, cooling off with water, seeking shade, and huddling for warmth. Image credits (from top left): Iguana by Skeeze, public domain; Elephant cooling off by Jean Beaufort, public domain; Chickens seeking shade by Geoffrey McKim, CC BY-SA 2.0; Penguin chicks huddling, by David Stanley, CC BY 2.0

Increasing heat production—thermogenesis

Endotherms have various ways of increasing metabolic heat production, or thermogenesis, in response to cold environments.
One way to produce metabolic heat is through muscle contraction—for example, if you shiver uncontrollably when you're very cold. Both deliberate movements—such as rubbing your hands together or going for a brisk walk—and shivering increase muscle activity and thus boost heat production.
Nonshivering thermogenesis provides another mechanism for heat production. This mechanism depends on specialized fat tissue known as brown fat, or brown adipose tissue. Some mammals, especially hibernators and baby animals, have lots of brown fat. Brown fat contains many mitochondria with special proteins that let them release energy from fuel molecules directly as heat instead of channeling it into formation of the energy carrier ATP.2^2
To learn more about how energy is released as heat in brown fat cells, have a look at the section on uncoupling proteins in the oxidative phosphorylation article.

Controlling the loss and gain of heat

Animals also have body structures and physiological responses that control how much heat they exchange with the environment:
  • Circulatory mechanisms, such as altering blood flow patterns
  • Insulation, such as fur, fat, or feathers
  • Evaporative mechanisms, such as panting and sweating

Circulatory mechanisms

The body's surface is the main site for heat exchange with the environment. Controlling the flow of blood to the skin is an important way to control the rate of heat loss to—or gain from—the surroundings.

Vasoconstriction and vasodilation

In endotherms, warm blood from the body’s core typically loses heat to the environment as it passes near the skin. Shrinking the diameter of blood vessels that supply the skin, a process known as vasoconstriction, reduces blood flow and helps retain heat.
A bed of capillaries near the surface of the skin is fed by a blood vessel that can be vasoconstricted—narrowed—or vasodilated—expanded—to control flow of blood through the capillaries. When it is cold, this blood vessel is vasoconstricted, and the blood coming from the heart does not enter the capillary bed, instead traveling through an alternative "shunt" blood vessel that lets it bypass the skin surface. Thus, the blood returning to the heart has not lost much heat.
Image credit: based on similar diagrams in Gillam3^3
On the other hand, when an endotherm needs to get rid of heat—say, after running hard to escape a predator—these blood vessels get wider, or dilate. This process is called vasodilation. Vasodilation increases blood flow to the skin and helps the animal lose some of its extra heat to the environment.
A bed of capillaries near the surface of the skin is fed by a blood vessel that can be vasoconstricted—narrowed—or vasodilated—expanded—to control flow of blood through the capillaries. When it is hot, this blood vessel is vasodilated, and the blood coming from the heart enters the capillary bed, avoiding an alternative "shunt" blood vessel that would let it bypass the skin surface. As it travels close to the skin, the blood loses heat to the cooler environment and is thus cooled by the time it exits the capillary bed on its way back to the heart.
Image credit: based on similar diagrams in Gillam3^3
Furry mammals often have special networks of blood vessels for heat exchange located in areas of bare skin. For example, jackrabbits have large ears with an extensive network of blood vessels that allow rapid heat loss. This adaptation helps them live in hot desert environments.4^4
Image of jackrabbit in desert and zoomed-in close-up of rabbit's ear, showing network of blood vessels
Image credit: modified from Black-tailed jackrabbit by K. Schneider, CC BY-NC 2.0
Some ectotherms also regulate blood flow to the skin as a way to conserve heat. For instance, iguanas reduce blood flow to the skin when they go swimming in cold water to help retain the heat they soaked up while on land.5,6^{5,6}

Countercurrent heat exchange

Many birds and mammals have countercurrent heat exchangers, circulatory adaptations that allow heat to be transferred from blood vessels containing warmer blood to those containing cooler blood. To see how this works, let's look at an example.
In the leg of a wading bird, the artery that runs down the leg carries warm blood from the body. The artery is positioned right alongside a vein that carries cold blood up from the foot. The descending, warm blood passes much of its heat to the ascending, cold blood by conduction. This means that less heat will be lost in the foot due to the reduced temperature difference between the cooled blood and the surroundings and that the blood moving back into the body's core will be relatively warm, keeping the core from getting cold.7^7
Diagram of blood vessel arrangement in the leg of a wading bird
  1. Warm arterial blood from the body's core travels down the leg in an artery.
  2. Arterial blood passes heat to cold venous blood coming back from the foot.
  3. Arterial blood is now cooler and will lose less heat to the environment as it travels through the foot.
  4. Cold venous blood ascending from the foot is warmed before it returns to the body's core.
Image credit: modified from Counter current exchange in birds by Ekann, CC BY-SA 4.0; the modified image is licensed under a CC BY-SA 4.0 license

Insulation

Another way to minimize heat loss to the environment is through insulation. Birds use feathers, and most mammals use hair or fur, to trap a layer of air next to the skin and reduce heat transfer to the environment. Marine mammals like whales use blubber, a thick layer of fat, as a heavy-duty form of insulation.
In cold weather, birds fluff their feathers and animals raise their fur to thicken the insulating layer. The same response in people—goosebumps—is not so effective because of our limited body hair. So, most of us wear a sweater!
Left, a pigeon fluffs its feathers for warmth; right, human goosebumps are an attempt to increase insulation by trapping air near the skin—but are not very effective due to lack of hair!
Left, a pigeon fluffs its feathers for warmth; right, human goosebumps are an attempt to increase insulation by trapping air near the skin—but are not very effective due to lack of hair! Image credits: left, Parrow cold big bird by Mike Sandoval, public domain; right, Goose bumps, by Ildar Sagdejev, CC BY-SA 3.0

Evaporative mechanisms

Land animals often lose water from their skin, mouth, and nose by evaporation into the air. Evaporation removes heat and can act as a cooling mechanism.
For instance, many mammals can activate mechanisms like sweating and panting to increase evaporative cooling in response to high body temperature.
  • In sweating, glands in the skin release water containing various ions—the "electrolytes" we replenish with sports drinks. Only mammals sweat.
  • In panting, an animal breathes rapidly and shallowly with its mouth open to increase evaporation from the surfaces of the mouth. Both mammals and birds pant, or at least use similar breathing strategies to cool down.8^8
In some species, such as dogs, evaporative cooling from panting combined with a countercurrent heat exchanger helps keep the brain from overheating!9^9
Although panting and sweating are effective cooling mechanisms, these active processes have the unwanted side effect of increasing the metabolic rate, and thereby heat production.
In addition, panting and sweating cause the animal to lose water and can result in dehydration—always make sure your dog has lots of water available on a hot day! Sweating also depletes the body of electrolytes, which must be replaced to avoid an imbalance.
Left, wolf panting to lose heat; right, beads of sweat on a human arm.
Left, wolf panting to lose heat; right, beads of sweat on a human arm. Image credits: left, Panting wolf by Mark Dumont, CC BY-NC 2.0; right, Photo of sweating at Wilson Trail Stage 1 by Minghong, CC BY-SA 3.0
This article is licensed under a CC BY-NC-SA 4.0 license.

Works cited

  1. "Death Valley," Wikipedia, last modified June 30, 2016, https://en.wikipedia.org/wiki/Death_Valley.
  2. J. M. Berg, J. L. Tymoczko, and L. Stryer, "Regulated Uncoupling Leads to Generation of Heat," in Biochemistry, 5th ed. (New York: W. H. Freeman, 2002), http://www.ncbi.nlm.nih.gov/books/NBK22448/#_A2563_.
  3. Paul Gillam, "Thermoregulation: A* Understanding for iGCSE Biology," PMG Biology, last modified February 22, 2015, https://pmgbiology.com/tag/thermoregulation/.
  4. Ashley Meyers, "Large Ears Used to Cool Off: Jackrabbit," Ask Nature, accessed July 9, 2016, http://www.asknature.org/strategy/a250478ba7f69e68c71405d931c91d62.
  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), 842.
  6. 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), 879.
  7. Jennifer Fee, "Complex Duck Feet," Beyond Penguins and Polar Bears, accessed July 9, 2016, http://beyondpenguins.ehe.osu.edu/issue/arctic-and-anarctic-birds/how-do-birds-stay-warm#Complex.
  8. Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye, "Temperature Regulation and Behavior," Stanford Birds, accessed June 24, 2016, https://web.stanford.edu/group/stanfordbirds/text/essays/Temperature_Regulation.html.
  9. M. A. Baker and L. W. Chapman, "Rapid Brain Cooling in Exercising Dogs," Science 195, no. 4280 (1977): 781, http://dx.doi.org/10.1126/science.836587.

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Recommended viewing

Green, Hank. "Why Don't Penguins' Feet Freeze?" SciShow. Last modified July 23, 2016. https://www.youtube.com/watch?v=Nztud0JFStM&feature=youtu.be.
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