29 Chapter 29: Thermoregulation
Lisa Limeri
Learning Objectives
By the end of this section, students will be able to…
- Describe how endotherms and ectotherms regulate their body temperature.
- Explain how environmental temperature has shaped the evolution of animal morphology.
Thermoregulation
There are some basic limits on survivable body temperature for most animals. At one end of the spectrum, water freezes at / to form ice. If ice crystals form inside a cell, they will 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 /
Disruption of the body’s ability to thermoregulate can lead to temperatures that are too low (hypothermia) or too high (hyperthermia). Slight temperature variations can be reversible with behavior changes and physiologic responses, while extreme variations can ultimately lead to organ failure, coma, and/or death.
Endotherms and Ectotherms
There are two main sources of heat that animals use to maintain their body temperatures. Some animals generate their own heat internally through their metabolism, termed endothermic. 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 (Fig 29.1). These organisms can maintain high levels of activity even when the external temperature is very low because they generate internal heat that keeps their cellular processes operating optimally. Endotherms are often called “warm-blooded.”
In contrast, animals that do not have internal control of their body temperature and rely on external sources of heat are called ectotherms. Although ectotherms do generate some metabolic heat—like all living things—ectotherms can’t increase this heat production to maintain a specific internal temperature. The body temperature of these organisms is generally similar to the temperature of the environment, although the individual organisms may do things that keep their bodies slightly below or above the environmental 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 (e.g., seeking sun, burrowing underground) to find environments whose temperature meets their needs. One challenge of ectotherms is that they are often unable to be active when environmental temperatures are low. The ectotherms have been called cold-blooded, a term that may not apply to an animal in the desert with a very warm body temperature. The advantage of ectothermy is that metabolic energy from food is not required to heat the body. For example, reptiles, which are ectotherms, can survive on about 10 percent of the calories required by a similarly sized endotherm.
Generally, endotherms have relatively stable body temperature since they generate their own heat and ectotherms generally have variable body temperature since their temperature is more influenced by environmental conditions which may vary (Figure 29.1). However, some ectotherms have relatively constant body temperatures due to the constant environmental temperatures in their habitats, like some deep sea fish species.
The division between endotherms and ectotherms is not perfect, there are some species that blur the lines between them. For instance, animals that hibernate 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!
Reading Question #1
The distinction between endotherms and ectotherms is…
A. The source of their body heat.
B. How they get rid of excess body heat.
C. Whether temperature is regulated through a positive or negative feedback loop.
D. Whether they hibernate or migrate during cold seasons.
Mechanisms of heat exchange
When the environment is not thermoneutral, the body uses four mechanisms of heat exchange to maintain homeostasis: radiation, evaporation, convection, and conduction (Figure 29.2). Each of these mechanisms relies on the property of heat to flow from a higher concentration to a lower concentration; therefore, each of the mechanisms of heat exchange varies in rate according to the temperature and conditions of the environment.
Radiation is the transfer of heat via infrared waves. This occurs between any two objects when their temperatures differ. A radiator can warm a room via radiant heat. On a sunny day, the radiation from the sun warms the skin. The same principle works from the body to the environment. About 60% of the heat lost by the human body is lost through radiation.
Evaporation is the transfer of heat by the evaporation of water. Because it takes a great deal of energy for a water molecule to change from a liquid to a gas, evaporating water (in the form of sweat) takes with it a great deal of energy from the skin. However, the rate at which evaporation occurs depends on relative humidity—more sweat evaporates in lower humidity environments. Sweating is the primary means of cooling the body during exercise, whereas at rest, about 20% of the heat lost by the human body occurs through evaporation.
Convection is the transfer of heat to the air surrounding the skin. The warmed air rises away from the body and is replaced by cooler air that is subsequently heated. Convection can also occur in water. When the water temperature is lower than the body’s temperature, the body loses heat by warming the water closest to the skin, which moves away to be replaced by cooler water. The convection currents created by the temperature changes continue to draw heat away from the body more quickly than the body can replace it, resulting in hypothermia. About 15% of the human body’s heat is lost through convection.
Conduction is the transfer of heat by two objects that are in direct contact with one another. It occurs when the skin comes in contact with a cold or warm object. For example, when holding a glass of ice water, the heat from your skin will warm the glass and in turn melt the ice. Alternatively, on a cold day, you might warm up by wrapping your cold hands around a hot mug of coffee. Only about 3% of the human body’s heat is lost through conduction.
Reading Question #2
A lizard basking in sunlight to raise its body temperature is gaining heat through…
A. Radiation
B. Evaporation
C. Convection
D. Conduction
Thermoregulatory Mechanisms
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 100 F/38C.
Both endotherms and ectotherms have adaptations 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 examine three broad categories of thermoregulatory mechanisms: 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 (Fig 29.3). 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 metabolic heat production
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, using metabolic waste heat as a heat source. When muscles are contracted, most of the energy from the ATP used in muscle actions is wasted energy that translates into heat. Severe cold elicits a shivering reflex that generates heat for the body.
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.
Reading Question #3
Brown fat’s primary role in thermoregulation is to…
A. insulate against heat loss in ectotherms.
B. generate metabolic heat in endotherms.
C. generate metabolic heat in ectotherms.
D. insulate against heat absorption in endotherms.
Controlling the exchange of heat with the environment
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. In endotherms, warm blood from the body’s core typically loses heat to the environment as it passes near the skin (Figure 29.4). Shrinking the diameter of blood vessels that supply the skin, called vasoconstriction, reduces blood flow and helps retain heat. In contrast, widening the blood vessels near the skin surface, called vasodilation, brings more blood and heat to the body surface, facilitating radiation and evaporative heat loss, which helps to cool the body.
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. This adaptation prevents the cold venous blood from cooling the heart and other internal organs. An example is a leg of a wading bird. 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 (Fig 29.6).
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.
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. In some species, such as dogs, evaporative cooling from panting combined with a countercurrent heat exchanger helps keep the brain from overheating.
Reading Question #4
Which of the following is a way of decreasing body temperature that only endotherms use?
A. Sweating for evaporative cooling.
B. Adjusting the timing of their daily activities.
C. Seek out or avoid direct sunlight.
D. Huddle in a group.
Thermoregulation in humans
In humans, the nervous system is important to thermoregulation, as illustrated in Figure 29.7. The processes of homeostasis and temperature control are centered in the hypothalamus of the advanced animal brain.
The hypothalamus maintains the set point for body temperature through reflexes that cause vasodilation and sweating when the body is too warm, or vasoconstriction and shivering when the body is too cold. It responds to chemicals from the body. When a bacterium is destroyed by phagocytic leukocytes (part of the immune system), chemicals called pyrogens are released into the blood. These pyrogens circulate to the hypothalamus and reset the thermostat. This allows the body’s temperature to increase and results in a fever. An increase in body temperature causes iron to be conserved, which reduces a nutrient needed by bacteria. An increase in body heat also increases the activity of the animal’s enzymes and protective cells while inhibiting the enzymes and activity of the invading microorganisms. Finally, heat itself may also kill the pathogen. A fever that was once thought to be a complication of an infection is now understood to be a normal defense mechanism.
Reading Question #5
What is the control center for thermoregulation in humans?
A. The hypothalamus
B. Sweat glands
C. Muscles that control vasoconstriction and vasodilation
D. Pyrogens
References
Adapted from
Clark, M.A., Douglas, M., and Choi, J. (2018). Biology 2e. OpenStax. Retrieved from https://openstax.org/books/biology-2e/
