Biologia - program rozszerzony
How behavior, anatomy, and physiology help animals regulate body temperature.
- 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.
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 ().
Let's take a closer look at some behavioral strategies, physiological processes, and anatomical features that help animals regulate body temperature.
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
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.
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.
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
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.
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.
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.
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.
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.
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!
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.
In some species, such as dogs, evaporative cooling from panting combined with a countercurrent heat exchanger helps keep the brain from overheating!