Animals - Thermal regulation

November 7, 2002

1. The chemical reactions that occur in living cells are temperature-dependent, and most animal species are adapted to live within a certain range of environmental temperatures.

2. In some animals the body temperature conforms to the ambient temperature of the environment, whereas other species regulate body temperature to optimize performance and survival.

• All animals produce body heat as a consequence of metabolic activity, and this heat is eventually lost to the environment. In ectothermic animals (e.g. a snake or lizard), this metabolic heat is minimal and the body temperature is always close to the ambient.
• In contrast, endothermic animals are capable of thermogenesis, i.e. they can generate excess body heat to specifically raise the body temperature above the ambient.
  1. Some endotherms only generate excess heat in certain tissues and at certain times. For instance, flying insects are ectothermal at rest, but when they prepare to fly they become endotherms by 'warming up' their flight muscles.
  2. Homeothermic animals are a subset of endotherms, (including both mammals and birds) that maintain a (nearly) constant body temperature, i.e. temperature varies little between different parts of the body or is the same at different ambient temperatures.
• Homeothermy is a kind of homeostasis, i.e. the animal varies its heat production (or heat loss) in order to maintain a fixed body temperature.

3. All cells release heat as a result of their ongoing chemical reactions. Endothermic animals have evolved means of harnessing these heat-producing reactions solely for the purpose of generating body heat.

• In a contracting muscle cell, the chemical energy stored in ATP is converted into the kinetic energy of movement. However, this conversion is imperfect, and some of the energy is lost as heat.
  1. When cold, mammals and birds generate body heat through the muscular activity of shivering. During shivering, muscle cells undergo asynchronous contractions. This asynchrony insures that there is little overt movement while the heat is being produced.
  2. Flying insects also use muscular contractions to warm-up their flight muscles. These are sustained isometric contractions, i.e. all the muscle cells contract, but opposing muscles contract simultaneously and cancel out each other's force ( no movement).
• Mammals and some birds are also capable of non-shivering thermogenesis.
  1. During normal glucose metabolism, roughly 40% of the chemical energy stored in glucose is used to synthesize ATP. About 55% is lost as heat (resulting in no movement).
  2. But during non-shivering thermogenesis, glucose metabolism is uncoupled from ATP synthesis. As a result, 95% of the chemical energy in glucose is converted into heat.
• Thermogenesis is energetically expensive, and requires extra nutrition. An inactive human expends 1500 kcal/day, whereas an inactive alligator of the same size only expends 60 kcal/day
• Because of this large energy expense, some endotherms (even homeotherms) 'take time off' and go through periods in which they let their body temperatures drop. This is what occurs in those mammal species that hibernate during the winter.

4. Many features of animal anatomy are determined by the constraints of thermal regulation.

• Since endotherms are generally hotter than their environment, their exterior body surface has evolutionary adaptations that retard the loss of body heat to the surrounding environment.
  1. The skin of mammals is covered by fur, and the skin of birds by feathers. Both structures evolved from the scales of reptilean ancestors, and they consist of dead skin cells which contain an especially hard form of the protein keratin.
  2. Skin and feathers trap a layer of stationary air that serves as a thermal insulator. [This is the same principle behind the use of foam or fiberglass insulation in houses.]
  3. Mammals and birds further insulate their skins to conserve body heat with a layer of subcutaneous fat in the underlying connective tissue.
  4. Water conducts heat much more rapidly than air, and wet fur loses its insulating layer of trapped air. To compensate, marine mammals have evolved a very thick layer of subcu-taneous fat called 'blubber'. [In fact, whales and walruses have completely lost their fur.]
• The surface area:volume ratio of endotherms also has a major impact on their thermal regulation. Heat production is generally proportional to an animal's tissue volume, whereas heat loss is proportional to the surface area over which the animal is in contact with its environment.
  1. Animals with a low surface area:volume ratio can conserve heat more readily, whereas animals with a high surface area:volume ratios can lose heat more readily.
  2. This relationship affects the morphology (= shape) of animals in different climates. Animal species in cold climates have evolved small extremities (reducing surface area and heat loss), whereas closely related species in hot climates have evolved larger extremities (increasing surface area and heat loss) relative to the size of the body as whole.
• Endotherms that are routinely exposed to cold environments often rely on countercurrent exchange to minimize heat loss from the blood in their extremities. This is seen in the legs of water fowl and the flippers of a dolphin [see Campbell, Fig. 44.5].
  1. Arteries carry warm blood coming from the heart. But when arteries enter a limb that extends into cold water, heat is lost from the blood and conducted out through the cold tissues towards the water.
  2. Countercurrent exchange permits the animal to recapture a significant fraction of this 'lost' heat. The anatomical basis for countercurrent exchange is that arteries lie at the center of the limb, and are surrounded by a weblike arrangement of veins.
  3. What this means is that much of the heat lost by the arterial blood is being absorbed by the (cold) venous blood. This heated venous blood is returned to the warm body before it can lose very much heat to its surroundings. [Note: this exchange is counter-current because arterial and venous blood are flowing in opposite directions as they exchange heat.]

5. In addition to thermogenesis, animals regulate their body temperture by influencing the gain or loss of heat at the body surfaces that interface with the environment.

• Many animals use voluntary behavioral adaptations to gain or lose heat. Examples include basking in the sun, bathing in hot water, or dousing oneself with water so as to be cooled by evaporation.
• Endotherms also regulate heat loss to their environment through a number of involuntary behaviors. An overview of this process is shown in Campbell's Fig. 40.09b.
  1. If suddenly exposed to a cold environment, a mammal vasoconstricts the 'cutaneous' blood vessels in the connective tissue layer of its skin. This reduces the amount of warm blood that gets close to the cold body surface, and reduces heat loss from that surface.
  2. Another mammalian adaptation to protect against cold climatic conditions is to raise the fur, which increases the layer of insulating air. [Each strand of fur is raised by a small bundle of smooth muscle at its base. Even though we are furless animals, we still have these same muscles at the base of our body hairs. The contraction of these muscles accounts for the phenomenon of 'goose bumps' when you get chilled.]
  3. If a mammal becomes over-heated it vasodilates its cutaneous blood vessels, increasing the flow of warm blood to the body surface and the rate of heat loss from that surface. This is why people become 'flushed' on a hot day or during exercise.
  4. Some (but not all) mammals have sweat glands, and can respond to over-heating by the release of sweat on the body surface. When the sweat evaporates, the body is cooled.

Learning Goals

1. What is thermogenesis, and how does it distinguish animal species that are ectothermic, endothermic, or homeothermic?

2. How is heat produced during shivering? How is heat produced during non-shivering thermogenesis?

3. How does each the following structures or behaviors influence heat exchange at the body's surface: fur; feathers; subcutaneous fat; vasodilation/vasoconstriction; sweating.

4. What is the relationship of arteries and veins in a goose's leg, and how does this anatomical relationship minimize heat loss?

5. Why does an animal species that is adapted to a cold environment tend to have smaller extremities than a related species that is adapted to a hot environment?