Thermoregulation

Body-temperature regulation by animals is essential for life. The maintenance of life relies on the sum of all chemical reactions or metabolic activity in an organism. These reactions are facilitated by catalysts, substances not directly involved in a reaction as either a product or reagent but essential for accelerating the process or allowing the reaction to proceed under conditions compatible with life. For example, a reaction that, in a test tube, might require exceedingly high temperatures will proceed, if catalyzed, at normal body temperatures. Biological catalysts are complex proteins called enzymes. These are fragile molecules and are quite temperature-sensitive. If exposed to excessively high or low temperatures, they will be denatured and lose their functional properties.

Homeostasis is the maintenance of a constant internal environment, one suitable for proper enzymatic activity. Homeostatic mechanisms involve three components: a sensor (or receptor) that reacts to changes in environmental conditions, a coordinator (or integrator) that responds to information from the sensor, and one or more effectors (activated by the coordinator), which elicit appropriate, regulatory responses.

Temperature sensors are scattered throughout the bodies of most animals, but those specifically associated with temperature regulation in vertebrates (animals with backbones) are found in the hypothalamic region of the brain. Coordinators are found within the brain (or its equivalent in simpler animals), again in the hypothalamus of more advanced types. Effectors may be any structure capable of affecting temperature.

Animals generally function at temperatures between 4 and 40 degrees Celsius. Peak metabolic efficiencies, however, exist over a much narrower range, called the optimum temperature. This temperature varies by the animal and its habitat. Optimum temperatures often approach lethal limits, the highest temperature an animal can tolerate. This necessitates precise control of temperature to avoid exceeding those limits. Within lower temperature ranges, some animals can alter metabolic requirements to adapt to changing temperatures without sacrificing efficiency. This process, which involves complex biochemical and cellular adjustments, is called “temperature compensation.” Animals that utilize metabolic mechanisms to maintain constant, relatively high body temperatures are often referred to as being “warm-blooded.” Others, whose body temperatures are not regulated or are regulated primarily by behavioral means, are called “cold-blooded.” That these terms are imprecise and irrelevant becomes obvious when one considers that the temperature of a desert-dwelling “cold-blooded” lizard or insect may often exceed that of any bird or mammal. On the other hand, the core temperature of some hibernating mammals may be reduced to being anything but “warm.”

Most invertebrates (animals lacking backbones), as well as many fishes, amphibians, and some reptiles, do not regulate body temperatures; they are called poikilotherms. They monitor environmental conditions, attempt to seek out areas where temperatures are suitable, and avoid those where they are not. Their temperatures are essentially identical to environmental temperatures. If excessively high temperatures are unavoidable for more than short periods, death may occur. Low temperatures are seldom fatal (unless below freezing) but will result in diminution of metabolic functions, causing the animal to become torpid, or inactive. Since these animals are vulnerable, they will seek shelter, which is why insects, for example, are rarely encountered during colder months.

Ectotherms

Animals that regulate body temperatures fall into two categories. Those that utilize environmental sources of heat are called ectotherms (animals that “heat” their bodies using external sources). Those that utilize physiological temperature control mechanisms are called endotherms (animals that “heat” their bodies using internal sources). Since endotherms (birds and mammals) strive to keep temperatures constant, they may also be called homeotherms (animals that maintain constant temperatures). All regulators must invest considerable energy in the process. To minimize that expenditure, they utilize microhabitats in which regulatory mechanisms are not necessary. Ectotherms use behavior, enhanced by physical or physiological mechanisms, to take advantage of environmental conditions. A principal source of heat for most ectotherms is sunlight; temperature regulators that rely on the sun are called heliotherms (animals that “heat” their bodies using the sun). Lizards from temperate zones (areas with moderate and/or seasonal climates) are the most efficient ectotherms and may serve as models to illustrate the process. Tropical species, which live in constant, warm environments, tend to be poor regulators.

Sunlight and heat may be assimilated directly by basking lizards or indirectly by convection from sun-heated surfaces. Basking occurs when an animal exposes itself to sunlight by seeking unshaded perches. Position and posture are critical. Lizards will orient themselves to expose the greatest amount of surface to the sun. This involves a position in which the animal is broad-side to the sun. Surface area is further enhanced by flattening the body dorsoventrally (top to bottom). Similarly, animals may absorb heat from the substrate. Lizards flatten themselves against a warm surface to maximize the area through which heat is assimilated. Area is critical in elevating temperatures, either by basking or convection, but does not increase proportionately with volume as animals increase in size. Thus, large ectotherms require disproportionately more energy and time to raise their temperatures than animals with similar proportions but smaller dimensions. This explains why the first animals to emerge in the spring or early morning tend to be small. Also, since dark colors absorb more radiation (heat and light), cold animals will stimulate pigment cells and are invariably much darker than those at optimum temperatures. That these mechanisms work effectively is illustrated by observations of active lizards at near-freezing temperatures at high elevations in the Andes of South America. When these lizards are captured, body temperatures of 31 degrees Celsius are recorded. In another study, lizards active at -4 degrees Celsius have been found to have body temperatures above 10 degrees Celsius. Some investigators have observed lizards, buried in sand during the night, emerging slowly, exposing only their heads. Since many lizards have large blood sinuses in their heads, it has been suggested that they can raise their body temperatures while minimizing exposure to predators. It is unlikely that this is effective, as heat gained would be rapidly lost to the substrate by convection. Only if the ground were warmer than air and only until body temperature reached that of the ground would this mechanism be operative.

In ectotherms, cooling is a much more difficult proposition. Without access to a source of “cold,” ectotherms can do little more than minimize heat absorption. Coloration is lighter to increase reflection, orientation is toward the sun, posture involves lateral (side-to-side) compression, and animals will “tiptoe,” lifting themselves away from warm substrates. If these are inadequate, animals must seek shelter. Many desert-dwelling lizards exhibit activity cycles that peak twice each day (morning and evening) to avoid cold nights and hot midday periods.

Endotherms

Endotherms use physiological effectors to raise or lower temperatures. If cold, they will generate heat (thermogenesis) by rapid muscular contractions (shivering) or increased oxidation of fats. Simultaneously, devices minimizing heat loss will be implemented. These include lowered ventilation (breathing) rates; since inhaled air is warmed during passage through the respiratory tract, heat is lost with each expiration. Also, superficial blood vessels narrow (vasoconstriction), reducing flow of warm blood to the skin, from which heat is lost by convection. Attempts to insulate skin are illustrated by “goose bumps.” Though ineffective in sparsely haired humans, this reaction to cold is quite effective in mammals with thick body hair or fur. Muscles attached to hair follicles contract and draw hairs into an upright position, and the ends droop, trapping dead air between matted ends and skin. A fine undercoat in many species enhances the process. Dead air is an excellent barrier to heat flow. A similar device affecting feathers exists in birds.

When hot, endotherms keep muscular activity to a minimum, increase ventilation rates (panting), and expand superficial blood vessels (vasodilation). Rates of heat dissipation in some mammals are enhanced by sweating. Sweating and panting rely on evaporative cooling, the same principle involved in using radiators to prevent hot automobile engines from overheating. Endotherms adapted to hot climates produce concentrated urine and dry feces to conserve water, since much is lost in cooling.

Many of these mechanisms are surface-area related. Consequently, endotherms in hot climates, especially large species with relatively poor surface-to-volume ratios, often possess structures, such as elephant’s ears, to increase area through which heat may be dissipated. On the other hand, endotherms occupying cold habitats are designed to minimize exposed surfaces. For example, arctic hares have short ears and limbs compared to the otherwise similar jackrabbits of warmer climes. In addition, cold-adapted endotherms may decrease rates of heat loss from poorly insulated appendages by means of countercurrent mechanisms. Heat from blood in arteries flowing into a limb is passed to venous blood returning to the body. This minimizes the amount of heat carried into a limb, whose surface-to-volume ratio is very high. It also functions to warm the returning blood, which prevents cooling of the body core. The appendages themselves are very cold; portions may even be at below-freezing temperatures. Actual freezing is prevented by special fats in the extremities.

Studying Thermoregulation

Specific methods vary according to the subject, approach, and discipline in question. Anatomy (study of structure), using both micro- and macroscopic methods, often centers on surface-related phenomena. For example, studies investigating the vascularization (blood supply) of whale flukes, whose physiology is difficult to study, have indicated that these are quite capable of dissipating heat and have led to the knowledge that these animals, even in cold water, because of their large size and poor surface-to-volume ratios, have potential problems with overheating. The role of blubber was reevaluated in this light and is now recognized as being one of fat storage with little to do with insulation. Furthermore, with new technologies in electron microscopy, anatomists have been able to describe, often for the first time, the complex structural components of organs (and even cells) that are active in thermoregulation.

Physiological studies of function are of two major types. One involves measurements of activity under different thermal regimes; for example, patterns of locomotion or digestion (involving specifically neural and muscular or neural, muscular, and glandular entities, respectively) may be observed at different temperatures. Often, these include observations of performance on treadmills or of rates at which food items are processed in controlled laboratory settings. On a different scale, metabolic activity itself might be linked to temperature by measuring rates of oxygen consumption in special metabolic chambers or utilization rates of products necessary for particular chemical reactions. These types of investigations have led to the determination of optimum and lethal temperatures in many species.

A second type of physiological study deals with actual thermoregulation. The ability to monitor body temperatures continuously, even in small animals, by means of radiotelemetry has made possible a whole series of experiments in which animals’ thermal responses to induced or natural conditions can be evaluated. Investigations of this type have provided insights into, for example, adaptive hypothermia (significantly reduced body temperatures) in small endotherms such as bats and hummingbirds. These species drastically reduce their core temperatures when inactive to conserve energy otherwise rapidly lost as heat through their relatively large surface areas.

Since laboratory work often fails to simulate natural conditions adequately, observations of animals in nature have been instituted. These seek to evaluate thermoregulation in the contexts of ethology (the study of behavior) and ecology (the study of organisms’ relationships with their environments). These types of studies frequently entail prolonged observations until patterns of behavior or habitat use emerge and can be quantified and evaluated. The use of rapid-reading thermometers or implanted radiothermisters facilitates understanding of the often-subtle modifications in thermoregulatory behavior or microhabitat use characteristic of many animals. Relating recorded temperatures to changes in posture, position, orientation, activity level, and ambient temperatures of substrate and air has, for example, led to an appreciation of how efficiently some ectotherms regulate temperature and the complexity of the mechanisms involved.

Applications of Thermoregulation Research

Long restricted by concepts of “warm-blooded” versus “cold-blooded” animals, investigators did not begin in-depth explorations of thermoregulation until the twentieth century. Most early efforts grew out of medical studies dealing with dynamics of human temperature regulation, especially in the context of pathological states associated with fever or trauma-induced hypothermia. Monitoring these conditions led to an appreciation of how complex temperature regulation is and how many of the body’s systems are involved. These studies, in turn, led to investigations of similar mechanisms in animals. Initially, most dealt with laboratory animals, but pioneering investigations into thermoregulation by animals in natural habitats soon opened whole new vistas. These studies were subsequently extended to “cold-blooded” species, which in turn led to an appreciation of how effective behavioral temperature regulation could be. In the 1970s, suggestions that at least some dinosaurs may have been homeotherms stimulated further interest in this field of study.

Most heat exchange with the environment occurs through skin or respiratory systems; muscular systems generate heat as a by-product of contraction; digestive and urinary systems regulate elimination of wastes, which influences retention or loss of heat-bearing water; cardiovascular systems transport heat; and nervous and endocrine systems regulate the entire complex. In addition, all cells require a proper thermal environment and may affect heat production by altering rates of oxidative metabolism. Therefore, a more complete understanding of thermoregulation has enhanced scientists’ awareness of both normal and pathological functions in most body systems. Specific medical applications of these studies include induced hypothermia during surgery-related trauma and treatment of accident-related hypothermia using mechanisms first observed under natural conditions in animals.

Studies of temperature-regulating mechanisms, both behavioral and physiological, have also provided insights into relationships between animals and their environments. Thermoregulatory needs have been used to explain behavioral and ecological phenomena for which causative agents were previously unknown. From a practical perspective, this knowledge is useful in developing management tools to sustain disrupted or endangered ecosystems. Appropriate techniques must be developed with a thorough knowledge of the dynamics in any given system, and this must be based on biological criteria rather than human perceptions. For example, reforested areas have often been managed as crops, with all the attendant problems of monocultures (areas cultivated for plants of only one species). Among these is the lack of biodiversity (variety of life-forms). When efforts began to take into consideration microhabitat requirements, often related to temperature regulation, varieties of plants—many with little or no commercial value in themselves—were planted. This resulted in managed areas becoming capable of supporting many different species.

Finally, a more complete knowledge of structures related to thermoregulation has been applied by paleontologists (scientists who study fossils) to the study of dinosaurs. Long thought to be “sluggish,” lizardlike ectotherms, dinosaurs are now thought by many investigators to have been more like mammals and birds in their physiological capabilities. This image is more in tune with their domination of the earth for some hundred million years.

Principal Terms

Convection: A transfer of heat from one substance to another with which it is in contact

Countercurrent Mechanism: A heat exchange system in which heat is passed from fluid moving in

Ectotherm: An animal that regulates its body temperature using external (environmental) sources of heat or means of cooling

Endotherm: An animal that regulates its body temperature using internal (physiological) sources of heat or means of cooling

Heliotherm: An animal that uses heat from the sun to regulate its body temperature

Homeostasis: The maintenance by an animal of a constant internal environment

Homeotherm: An animal that strives to maintain a constant body temperature independent of that of its environment

Optimum Temperature: The narrow temperature range within which the metabolic activity of an animal is most efficient

Poikilotherm: An animal that does not regulate its body temperature, which will be the same as that of its environment

Thermogenesis: The generation of heat in endotherms by shivering or increased oxidation of fats

Bibliography

Avery, Roger A. Lizards: A Study in Thermoregulation. University Park Press, 1979.

Bakker, Robert T. The Dinosaur Heresies: New Theories Unlocking the Mystery of the Dinosaurs and Their Extinction. Kensington, 1986.

Dukes, H. H. Dukes’ Physiology of Domestic Animals. 13th ed. Wiley Blackwell, 2015.

Gans, Carl, and F. Harvey Pough, eds. Physiology C: Physiological Ecology. Vol. 12 in Biology of the Reptilia. Academic Press, 1982.

Hickman, Cleveland P., Larry S. Roberts, and Frances M. Hickman. Integrated Principles of Zoology. 19th ed. McGraw Hill, 2023.

Johnston, Ian A., and Albert F. Bennett, eds. Animals and Temperature: Phenotypic and Evolutionary Adaptation. Cambridge UP, 1996.

Mota-Rojas, Daniel. "Thermoregulation Strategies in Domestic Animals." SCE, 19 Apr. 2022, encyclopedia.pub/entry/17753. Accessed 5 July 2023.

Schmidt-Nielsen, Knut. Desert Animals: Physiological Problems of Heat and Water. Dover, 1979.

Schmidt-Nielsen, Kurt. How Animals Work. Cambridge UP, 1972.