Summary

Heat Transfer between Animals and Their Environments

• In addition to making heat metabolically, animals exchange heat with their environments by conduction, convection, evaporation, and thermal radiation. An animal’s body temperature depends on heat gains and losses; it is constant only if the sum total of gains equals the sum total of losses.
• Conduction and convection have in common the property that when heat moves through a material substance by either mechanism, the atoms and molecules of the substance participate in the transfer of heat. Conduction occurs when a material substance is macroscopically motionless. Convection is heat transfer brought about by flow of a material substance (e.g., by wind). Convection is much faster than conduction.
• Evaporation is a potentially potent mechanism for heat transfer because the change of state of water from a liquid to a gas absorbs a great deal of heat per gram of water. The heat is absorbed from the surface where evaporation occurs and is carried away with the water vapor.
• Thermal-radiation heat transfer occurs by means of beams of radiant energy that all objects emit and that travel between objects at the speed of light. Because of thermal-radiation heat transfer, objects can exchange heat at a distance. In most instances of thermal-radiation heat transfer in the biosphere, the heat transfer occurs at invisible infrared wavelengths; because all objects are nearly black at such wavelengths, visible color plays little role, and the net transfer of heat is from the object with higher surface temperature to the one with lower surface temperature. Visible color, however, is a major factor in how well objects absorb the visible and near-visible wavelengths of solar radiation.

Poikilothermy (Ectothermy)

• Poikilotherms, also called ectotherms, are animals in which body temperature (T_B) is determined by equilibration with external thermal conditions. They often thermoregulate. Their mechanism of thermoregulation is behavioral; a poikilotherm controls its T_B by positioning its body in environments that will bring its T_B to the set-point (“preferred”) level.
• The resting metabolic rate of a poikilotherm is usually an approximately exponential function of its T_B. The Q₁₀ is typically 2–3. The metabolism–temperature curves of poikilotherms are often plotted on semilogarithmic coordinates because exponential functions are straight on such coordinates.
• From the viewpoint of metabolic rate, when poikilotherms acclimate to cold or acclimatize to low-temperature environments in nature, their most common response is partial compensation. Partial compensation returns an animal’s metabolic rate toward the level that prevailed prior to the change in environment, and thus it blunts the effect of environmental change. The most common known mechanism of partial compensation is for cells to change their concentrations of key, rate-limiting enzymes.
• Different species of poikilotherms that have long evolutionary histories of living at different body temperatures frequently display evolved physiological differences that suit them to function best at their respective body temperatures. Species of lizards sprint fastest at their respective preferred body temperatures, and polar species of fish function at higher rates in frigid waters than temperate-zone species can. The important mechanisms of evolutionary adaptation to different body temperatures include molecular specialization: Species with evolutionary histories in different environments often synthesize different molecular forms of protein molecules and different cell-membrane phospholipids. The evolution of structurally distinct protein forms and phospholipids conserves functional properties of the molecules; as a consequence, species living in different thermal environments are similar to each other in their enzyme–substrate affinities and membrane-lipid fluidities.
• When exposed to threat of freezing, some poikilotherms actually freeze and are freezing-tolerant; freezing must be limited to the extracellular body fluids, however. Other poikilotherms are freezing-intolerant and exploit one of three strategies—behavioral avoidance, antifreeze production, or supercooling—to avoid freezing. Antifreezes lower the freezing point. Stabilization of supercooling permits animals to remain unfrozen while at temperatures below their freezing points.

Homeothermy in Mammals and Birds

• Homeothermy—thermoregulation by physiological means—is energetically expensive.
• The principal way that a mammal or bird thermoregulates in its thermoneutral zone is that it varies its body insulation to offset changes in the driving force for dry heat loss (T_B – T_A). Insulation can be modulated by changes in posture, cutaneous blood flow, the thickness of the relatively motionless air layer trapped by the pelage or plumage, and regional heterothermy.
• Below thermoneutrality, variation in the rate of metabolic heat production (thermogenesis) is the principal mechanism of thermoregulation. The two most prominent mechanisms of increasing heat production are shivering—found in both mammals and birds—and nonshivering thermogenesis (NST)—found mainly in placental mammals. The principal site of NST in mammals is brown adipose tissue, which, by expressing uncoupling protein 1, is able to employ uncoupling of oxidative phosphorylation to achieve very high rates of lipid oxidation with immediate heat release.
• Regional heterothermy, which is often exhibited when animals are at ambient temperatures below thermoneutrality, usually depends on countercurrent heat exchange. Close juxtaposition of arteries and veins short-circuits the flow of heat into appendages.
• Above thermoneutrality, species with long evolutionary histories in hot, dry environments typically use nonevaporative mechanisms—notably hyperthermia and cycling of body temperature—as first lines of defense. When active evaporative cooling occurs, the specific mechanisms usually employed to increase the rate of evaporation are sweating (only in certain mammals), panting (mammals and birds), and gular fluttering (only birds). Both hyperthermia and the effort involved in active evaporative cooling can cause metabolic rate to rise at ambient temperatures above thermoneutrality.
• Acclimatization to changing seasons is the norm and may involve one or more of three mechanisms: acclimatization of peak metabolic rate, acclimatization of metabolic endurance, and insulatory acclimatization.
• Controlled hypothermia permits animals to evade temporarily the high energy costs and water costs of homeothermy. During hibernation, estivation, and daily torpor, T_B is generally allowed to fall close to T_A within a species-specific range of T_A. Forms of shallow hypothermia also occur.

Warm-Bodied Fish

• Tunas, lamnid sharks, and billfish are distinguished from other fish by exhibiting endothermy in certain body regions, and the opah displays whole-body endothermy. The tissues that are endothermic in tunas and lamnids are (1) the red swimming muscles and (2) sometimes the stomach, other viscera, brain, and retinas. In billfish, only the brain and retinas are endothermic.
• A countercurrent vascular array that short-circuits outflow of heat from a tissue—thereby preventing the heat from reaching the gills—is required for the tissue to be endothermic.
• Ordinary metabolic heat production is the source of heat for endothermy in all cases except the billfish, which have specialized “heater” tissues derived from extraocular eye muscles.

Endothermy and Homeothermy in Insects

• Many solitary insects, especially those of medium to large size, display thoracic endothermy or homeothermy during flight or certain other sorts of activity. Warming of the flight muscles increases their power output. Often in these insects, a certain minimum flight-muscle temperature is required for flight.
• When insects are not flying, activation of the flight muscles in a nonflight mode—termed shivering—is the mechanism they employ to warm the thorax. Shivering is used for preflight warm-up. Nonflying insects also sometimes thermoregulate by modulation of shivering, as observed in bees working in their hives.
• When insects are flying, the best-known mechanism of thermoregulation is modulation of thoracic insulation, brought about by raising and lowering circulatory transport of heat out of the thorax.
• Colonies of social bees and wasps sometimes employ group efforts to maintain exquisitely stable hive temperatures.

Copyright 2016 Sinauer Associates