Principles of Animal Physiology, Second Edition

Thermal Physiology

Endothermy, the ability to generate and maintain elevated dominate Earth in later years. Fossils dating back to this body temperatures, has arisen several times in the evolu- period reveal the existence of several distinct mammalian- tionary history of animals. It goes hand in hand with the ca- like reptilian lineages. These animals differed from other pacity to produce heat through metabolism, and therefore reptiles by the morphology of the skull and the organiza- activity levels. Most modern birds and mammals have high tion of the teeth. Although most of these lineages disap- metabolic rates and are able to maintain their body tem- peared, one group of reptiles called cynodonts gave rise to peratures well above ambient temperature, often within true mammals. The earliest mammals retained the reptil- narrow thermal windows. While both are perceived as ian trait of egg laying, like the modern monotremes, “higher vertebrates,” birds and mammals arose from sep- echidna and platypus. By the early Cretaceous period (144 arate reptilian ancestors. Thus, endothermy arose inde- million years ago), mammals had diversified into several pendently at least twice. However, fossil evidence suggests lineages of marsupials and insectivores. When the di- that other extinct reptiles may also have been endotherms. nosaurs disappeared about 65 million years ago, at the end The fossil record of the animals in the paleontological pe- of the Cretaceous period, there was an explosion of mam- riod from 200 to 65 million years ago is particularly clear, malian diversification. New species of mammals began to showing definitive examples of the transitions from rep- occupy the environmental niches vacated by the dinosaurs. tiles to mammals and birds. It cannot be said for certain when endothermy arose in the The first mammals appeared approximately 200 mil- transition from mammalian-like reptiles to true mam- lion years ago, evolving from small, nocturnal reptiles that mals. However, it is likely that the cynodont reptiles were were only distantly related to the dinosaurs that would already endothermic. Unlike most other reptiles of the day,

From Chapter 13 of Principles of Animal Physiology, Second Edition. Christopher D. Moyes, Patricia M. Schulte. Copyright © 2008 by Pearson Education, Inc. Published by Pearson Benjamin Cummings. All rights reserved. 660 Thermal Physiology

Asymmetrical fossilized feather.

symmetrical feathers would be useless in flight, they must have arisen in these dinosaurs for other benefits, such as insulation. Although these other lineages of feathered reptiles became extinct, they were likely also endothermic Archaeopteryx. animals. Many researchers believe that endothermy arose in other, nonfeathered dinosaur lineages as well. The largest cynodonts possessed a bony, secondary palate in the roof dinosaurs were simply too big to shed metabolic heat, and of the mouth that would have allowed them to breathe therefore remained warm-bodied. Many smaller dinosaurs while chewing. This anatomical arrangement is a charac- may also have been endothermic. Multiple lines of evi- teristic of endotherms because they must maintain unin- dence support the notion that these animals had the high terrupted respiration to sustain high metabolic rates. metabolic rates necessary for an endothermic animal. Cynodonts also appear to have possessed hair, which Bone structure and posture suggest rapid rates of locomo- could have helped insulate their bodies. tion, which in modern animals require high metabolic Birds, the other group of modern endotherms, also rates that are possible only in warm-bodied animals. Just arose from reptiles, although much later than mammals as in modern endotherms, many dinosaurs had relatively and from different reptilian ancestors. Around the time di- large brains associated with superior sensory processing. nosaurs were declining, several reptilian lineages had al- Since brain tissue has a high energy demand, a large ready evolved featherlike body coverings. In one group, the brain can have an important influence on the whole body theropod dinosaurs such as Archaeopteryx, the feathers metabolic rate. Other theories have been raised to sup- were similar in structure to those of modern birds. Their port arguments that dinosaurs were endotherms. How- feathers were asymmetrical, a trait that is necessary to be ever, no argument is definitive because of the limitations useful in feathered flight. In contrast, the other feathered in using the properties of modern animals as guidelines reptiles of the era, such as Protarchaeopteryx robusta and in predicting the physiological features of these long- Caudipteryx zoui, had symmetrical feathers. Since these extinct animals. 2

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Overview reach greater extremes than do water temperatures. Recall that thermal energy influences chemical in- Many ecosystems exhibit spatial variation in teractions in ways that affect macromolecular temperature. Underground refuges are buffered structure and biochemical reactions. Conse- from thermal extremes on the surface. The TA in quently, temperature has pervasive effects on all alpine regions varies as a result of altitudinal gra- physiological processes. As a result of these tem- dients arising over only a few kilometers. Large perature effects, every animal displays a thermal bodies of water, such as lakes and oceans, can vary strategy: a combination of behavioral, biochemi- in TA with depth. Deep-ocean (bathypelagic) tem- cal, and physiological responses that ensure body peratures are often close to 4°C, whereas midwa- ter (mesopelagic) and surface water (epipelagic) temperature (TB) is within an acceptable limit. The most important environmental influence on the temperatures can be much warmer and more vari- thermal strategy (though not the only one) is am- able. Large temperate lakes may be nearly uniform in temperature, or have sharp demarcations (ther- bient temperature (TA). Animals must survive the moclines) between top and bottom water, some- highest and lowest TA in their niche (thermal ex- times differing more than 10°C in less than a meter tremes), as well as the change in TA (thermal change). of depth. Animals inhabit most thermal niches on the Ecosystems can also change in temperature planet (Figure 1). The hottest environments ex- temporally. Terrestrial and aquatic ecosystems in ploited by animals are the regions near thermal the tropics tend to have a relatively constant TA, vents, such as the hydrothermal vents of the deep but polar and temperate zones experience sea- sea, volcanoes, and geysers. The coldest places sonal and daily cycles of cold and heat. Air tem- inhabited by animals are the alpine and polar re- peratures can change more rapidly than water gions. The animals that survive in the extremes temperatures, sometimes more than 20°C in a sin- of heat and cold are impressive, but the ability to gle day. Intertidal animals may experience the tolerate changing temperature is every bit as heat of a summer day mere seconds before the challenging physiologically. Environmental tem- cold ocean washes over them. Many animals in- peratures are most variable in terrestrial ecosys- corporate behavior into their thermal strategy, but tems; air temperatures change more rapidly and animals must also cope with the effects of temperature on biochemistry and physiology.

Hot springs Alpine (extreme cold) (high TA) Hot desert (daily variation) Temperate (seasonal variations)

Intertidal (rapid variation in TA)

Lakes (thermal Epipelagic stratification, Subterranean (variable TA) winter freezing) refuges (moderate and stable TA) Mesopelagic (stable TA)

Bathypelagic (cold, stable TA)

Hydrothermal vent (>100°C) Figure 1 Thermal niches in the temperate zone

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Heat Exchange and Thermal Strategies

The most important physiological parameter in an animal’s thermal physiology is body temperature (TB). An animal’s thermal strategy serves to control the transfer of energy between the animal and the environment. Some animals tolerate wide Radiant (direct changes in TB and the effects of these solar) changes on many physiological Radiant Dust processes. Others must use a combi- (reflected solar) nation of physiological and behavioral means to ensure that TB remains Radiant nearly constant. As in other physio- Radiant (reflected solar) logical systems, both strategies—tolerance and regulation—have costs and benefits. The physiological mech- Conduction and convection (air) anisms that impart a constant TB use energy. When TB is allowed to vary, important physiological processes such as development become sensi- Conduction Radiant tive to environmental changes. Al- Conduction though TA has the most obvious impact on animal thermal biology, other routes of heat exchange are also important in many contexts. Figure 2 Sources and sinks for thermal energy The body temperature of an animal is influenced by heat exchange with the environment. This snake is warmed by radiant energy from the sun, as well as thermal energy radiated from its Controlling Heat Fluxes surroundings. The animal exchanges thermal energy through objects and fluids in contact with its external surface (conduction). Movement of the air enhances the An animal’s TB is a reflection of the efficiency of thermal exchange by convection. The animal itself radiates thermal thermal energy held within the mole- energy to the surrounding air. cules of the body. Thermal energy can move from the animal to the environment, or from of the animal and TB will remain constant. If the the environment to the animal, depending on tem- flow of thermal energy into the animal exceeds the perature gradients. Metabolism—the sum of all bio- heat loss, TB will increase. Each of these routes of chemical reactions occurring within the body—is thermal energy exchange depends on the thermal the main source of thermal energy in the heat bal- properties of the environment as well as the physi- ance equation of most animals. However, other im- cal properties and physiology of the animal. portant sources and sinks for thermal energy also affect an animal’s thermal budget (Figure 2). The • Conduction is the transfer of thermal en- thermal balance equation takes into consideration ergy from one region of an object or fluid to all of the routes through which thermal energy, ab- another. Animals can be cooled when ther- breviated as H, can enter or exit the body: mal energy is conducted away from the body, or can be warmed as they absorb heat ⌬H ϭ⌬H ϩ⌬H ϩ⌬H total metabolism conduction convection from conductive objects. ϩ⌬H ϩ⌬H radiation evaporation • Convection is the transfer of thermal en- ⌬ ϭ If the equation above sums to zero ( Htotal 0), ergy between an object (the animal in this there will be no net change in the thermal energy case) and an external fluid that is moving.

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For example, warm air feels cooler when it has more molecules per unit volume, there is a flows over your skin than when the air is greater likelihood of a molecular collision that re- still. Most often, convection causes a loss of sults in a transfer of energy. thermal energy from animals. The Fourier equation describes how thermal • Radiation is a general term that refers to energy moves in a very simple system: heat trans- the emission of electromagnetic energy fer in a single dimension (from the heat source to from an object. An animal can absorb radi- heat sink) in a single uniform material. These same λ ⌬ ant heat emitted from the surroundings, but parameters ( , T, and L) apply in thermal biology, can also emit radiant heat from its own sur- but animals are much more complex systems. Con- face, a major form of heat loss. The infrared sider the influence of thermal conductance. Heat is radiation emitted from an object indicates conducted from the internal tissues, through other its surface temperature. tissues and fluids, and to the external surroundings, each with a characteristic thermal conductivity • Evaporation of water molecules from the (Table 1). The body surface layers may possess in- surface of an object absorbs thermal energy sulation that reduces conductive heat transfer. In- from the object. Thus, evaporative heat ex- sulation, such as fur and feathers, also increases change is almost always a heat loss from the the distance between the hottest point near the skin animal. and the coldest point in the bulk phase. The relative importance and even the direc- Calculations of heat flux are complicated by the tion of heat transfer from each of these parame- geometry of the environment and the animal. Heat ters differ among animals and conditions. The does not move from your body through a one- properties of the animal, including physical com- dimensional cylinder of air extending from your position and color, have a profound influence on skin, but rather is conducted in multiple dimensions the relative importance of these exchanges. from the source. Animal geometry also plays a role. A long, thin animal produces as much heat as a short, round animal of the same mass, but the differ- Water has a higher thermal ences in surface area affect heat exchange. Since conductivity than air conductive heat losses occur across the external sur- Conduction is difficult to quantify because of the faces, an animal can alter conductive heat exchange many factors that affect heat exchange. Let’s begin by engaging in activities that alter its effective sur- our discussion by considering how conduction is face area. For example, a penguin reduces heat loss involved in the transfer of thermal energy through from the foot by rolling back on its heels, using its tail a single material, such as a thin metal bar heated feathers for balance. Because its tail feathers are less at one end. The rate of heat transfer from the conductive than its feet, less heat is lost. Figure 2 warm end to the cool end (heat flux) is described shows a snake simultaneously exchanging heat with by Fourier’s law and the following equation: multiple surfaces. It loses heat via conduction across its upper surface while also exchanging heat through l¢T Q ϭ its lower surface in contact with the rock. L where heat flux (Q) depends upon the temperature Table 1 Thermal conductivity of materials. gradient (⌬T), the distance over which the gradi- Material Thermal conductivity ent extends (L), and the thermal conductivity (λ) (W/m per K) measured in watts per meter per kelvin (W/m per Air 0.02 K). Thermal conductivity is a specific property of a material. Those objects we think of as heat sinks Snow 0.10 have high thermal conductivity. For example, an Water 0.59 aluminum pot feels cold to the touch because it has Rock 1–3 a high thermal conductivity (210 W/m per K) and readily draws heat from your hand. Similarly, 5°C Ice 2.1 water feels cooler than 5°C air because water has Muscle 0.5 a thermal conductivity that is 25-fold higher than Fat 0.2 air (0.58 versus 0.024 W/m per K). Because water

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Convective heat exchange depends on fluid movements Imagine yourself immersed in a pool of water that is 10°C colder than your body. Almost immedi- ately, your body begins to lose thermal energy as it warms the water in the boundary layer by almost 10°C. Once the boundary layer is warmed, thermal energy is slowly conducted to the bulk phase of the water. When the heat exchanges reach steady state, the body loses thermal energy at the rate required to rewarm this boundary layer that slowly cools as it dissipates its thermal energy outward to the bulk phase. Much less energy is required to rewarm the boundary layer under these Figure 3 Heterogeneity of TB in the intertidal steady-state conditions than was required to heat zone Infrared photography can be used to compare the the boundary layer in the first place. Now consider body temperature (TB) of animals. In this image, the mussels how the gradients change when fluid is flowing are warmer than the starfish because they are better at over the body. The body rapidly loses thermal en- absorbing radiant energy. The starfish, with its greater surface area, may also be affected more by evaporative ergy warming a boundary layer that is immedi- cooling. ately replaced by another, colder boundary layer. (Photo courtesy of Dr. Brian Helmuth, University of South Carolina) Heat lost to a moving fluid, either air or water, is convective heat loss. The rate of convective heat In terrestrial systems, the ground warms dur- loss depends on the thermal gradient between the ing the day and then becomes an important source surface and the fluid, the rate of flow of the fluid of thermal energy in the form of conduction and over the surface, and its conductivity. radiant heat when the sun sets. Animals also lose thermal energy when they emit radiant heat. Thus, radiant heat may be a net gain or net loss Radiant energy warms some animals from animals. The relationship that describes radiation from a warm animal is described by the In biological systems, radiant heat exchange occurs Stefan-Boltzmann equation: through electromagnetic radiation in the long wave- ϭ σ 4 Ϫ 4 length, infrared range. Thus, if a red light (long P Ae (TB TA ) wavelength) and a blue light (short wavelength) of where P is the radiating power, A is its surface equal intensities are shown on your skin, the red area, e is the ability of the object to emit radiation, light will more effectively warm the surface. σ is the Stefan constant, and T the temperature of In the natural world, the most important the body (TB) or surroundings (TA) in kelvins. Ani- source of radiant heat is the sun. Photons from the mals can influence their radiant heat loss through sun excite the molecules in the atmosphere, the changing the nature of the surface (e) and the sur- soil, and the water, warming them by radiant heat. face area (A). Thus, when animals are warmed by conduction from air, water, or soil, the ultimate source of the heat is radiant energy. But animals can also be warmed directly by solar radiation, which many Evaporation induces heat losses species accentuate by the behavior known as Evaporative cooling arises when fluids draw ther- basking. White body coloration reflects photons in mal energy from the body surface as the water the visible range, and dark coloration absorbs the molecules make the transition from liquid to va- photons within this range of wavelengths. Animals por. The magnitude of the heat loss depends on the that bask to warm themselves often possess high volume of water and its heat of vaporization. It re- levels of black or brown pigments to help absorb quires more energy to evaporate water from salty thermal energy. As a result of diversity in color, an- sweat than from pure water because the solutes imals in the same area can have markedly differ- increase the heat of vaporization of water. The ef- ent temperatures (Figure 3). ficiency of evaporative cooling also depends on the

665 Thermal Physiology

partial pressure of water vapor in the air. If the air animal evolution. Bergmann’s rule states that ani- has a high humidity, then the water is less likely to mals living in cold environments tend to be larger evaporate. than animals in warmer environments. Allen’s Sweating is only one of the ways that animals rule states that animals in colder climates tend to employ evaporative cooling. When a hippopota- have shorter extremities than animals in warmer mus rolls in the mud of a wet riverbank, the cool climates. Thus, mammals or birds living in polar mud draws heat from the body (conduction). This regions or high altitudes tend to be larger and is an effective cooling strategy even if the mud is shorter legged than individuals of the same warm: thermal energy is absorbed from the body species from more temperate regions. These rules as the mud dries. Other animals cover their body of ecogeography apply to most of the mammals surfaces with water, such as an elephant that and birds studied to date, but have little relevance

sprays water onto its back or birds that splash in to animals that allow TB to change. a pool of water. Wet feathers also have a dimin- An animal regulates heat exchange by altering ished insulatory capacity, allowing more metabolic the posture of the body to minimize or maximize heat to be lost. Birds that live in hot environments the exposed surface area. Pythons will roll into a may soak the belly before returning to the nest, al- ball to conserve metabolic heat during digestion. lowing the eggs to benefit from evaporative cool- When the python, approximately cylindrical in ing. Kangaroos, which do not produce sweat, lick shape, rolls into a ball, its externally exposed sur- well-vascularized skin surfaces, which then cool face area decreases by about 85%, greatly reduc- as the saliva evaporates. ing heat loss. Not all evaporative cooling is positive. When Animals can also reduce effective surface area semiaquatic animals leave the water, they are by huddling with other animals. Naked mole rats typically left with wet body surfaces, causing (Figure 4) live in burrows at relatively constant body temperature to decrease due to evaporative temperatures and have a very limited ability to use cooling. metabolism to control their body temperature. If housed in groups, they huddle when temperatures drop below about 22°C. This allows them to main- Ratio of surface area to volume tain a relatively constant T near 22°C. However, a affects heat flux B solitary naked mole rat is unable to defend its TB The ratio of surface area to volume can influence all at low TA. When prevented from huddling, its TB aspects of the heat exchange equation: conduction, closely reflects TA, decreasing to as low as 12°C. convection, radiation, and evaporation. Variation From the perspective of the individual animal,

in the ratio is important in several contexts. A given huddling reduces heat by increasing TA, replacing animal may alter its exposed surface area to cold air with a warm neighbor. From the perspec- change heat flux. Dogs stretch out when hot to tive of the colony, huddling works as a thermoreg- maximize conductive heat loss to the ground, but ulatory strategy by reducing ratios of surface area roll up when cold to minimize conductive heat loss to volume. to the air. Ratios of surface area to volume also come into play when comparing animals of different body dimensions or body mass. The significance of body size, or more precisely, the ratio of surface area to mass, is appar- ent in many comparisons. An arctic wolf is about one-tenth the mass of a grizzly bear, but it has twice the ratio of surface area to volume. Although they live in similar niches, the arctic wolf incurs greater thermoregulatory costs because of its size. Similarly, when an animal grows, its body mass increases faster than its surface area. In general, larger animals lose heat more slowly and retain heat better than do small animals. The effects of body size and shape also manifest themselves in Figure 4 Naked mole rats

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Insulation reduces thermal exchange to shed fur in spring. Since much of hair is composed of dead cells, the cost of rebuilding the coat Internal and external insulation also reduce heat when temperatures cool is minor in comparison to losses. Marine mammals have a thick layer of adi- the metabolic costs the animal would incur trying pose tissue under the skin in the form of blubber. to cool itself using physiological mechanisms. This lipid layer disrupts the flow of thermal energy Mammals alter the nature of their fur coat season- from the core to the external surface of the animal. ally, producing a greater density of hairs. Some More commonly, animals use external insulation birds, such as the ptarmigan, produce specialized to reduce heat loss. Fur and feathers restrict the feathers with an additional shaft to increase the movement of molecules between the surface of the feather density. animal and the bulk phase of the environment. Though a main function of hair is thermal insu- Heat is lost from the animal in proportion to the lation, it can also serve other purposes. Male lions, thermal gradient (⌬T) at the surface of the animal. for example, possess a thick coat of fur around the Molecules of air or water in the insulation layer head region. Though it may provide the lion with are warmed by the animal and then trapped some defense in male-male encounters, it also cre- within the insulation. The overall temperature ates a thermal burden. Box 1, Evolution and Diver- gradient from the skin to the bulk phase is the sity: Lions’ Manes Are Hot! describes the same, but the distance is greater and the animal evolutionary and physiological trade-offs between loses less heat to conduction. The fur also impedes thermal biology and sexual selection. the flow of fluids over the surface of the skin, so there is less convective heat loss. The effectiveness of insulation depends on its thickness. When faced with cold temperatures, Thermal Strategies birds (or mammals) can change the orientation of Invertebrates are the most thermotolerant animals the feathers (or fur) to alter the volume of air in each thermal niche. The hottest deserts are pop- trapped within the coat. Similarly, animals that ulated by myriads of insects, but only a few verte- live in colder environments have thicker coats brates. Invertebrates can also tolerate the coldest with greater insulating capacity (Figure 5). Some temperatures, often by entering an inactive, dor- species change the thickness of the external insu- mant state. Once stabilized in this state of “sus- lation seasonally. Thick coats are a thermoregula- pended animation,” they can survive temperatures tory burden in the warm season, so it is beneficial far colder than even the coldest natural environments. In contrast, only a few vertebrates, such as the wood frog, can survive subzero body tempera- Arctic fox Wolf tures, frozen in underground refuges. The lay terms cold-blooded and warm-blooded fail to reflect the complexity of thermal strategies, Reindeer Grizzly bear which are properly described by two alternate sets of terms: poikilothermy versus homeothermy, or Rabbit ectothermy versus endothermy.

Dog Polar bear Marten

Fur insulation Poikilotherms and homeotherms differ in the stability of TB Squirrel The terms poikilothermy and homeothermy dis- Weasel tinguish animals on the basis of the stability of Shrew TB.Apoikilotherm is an animal with a variable African mammals TB—one that varies in response to environmental Fur thickness conditions. A homeotherm, in contrast, is an animal with a relatively constant T . Most Figure 5 Insulation There is a direct relationship B between the thickness of fur and its ability to act as homeotherms achieve a constant TB using physi- insulation. ological processes to regulate the rates of heat (Source: Modified from Wilmer et al., 2002) production and loss.

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BOX 1 EVOLUTION AND DIVERSITY Lions’ Manes Are Hot!

Lions live in prides, with one mature male slow in the winter. Individual hairs in humans grow for and a harem of females. The lionesses do the about five years, then remain static for another 12 parental care and all of the hunting, while the male guards weeks. At that point, a new hair from the same follicle over the pride, defending his position against any male is formed, pushing the old hair out. In principle, lions challenger. An unmated female will choose a male that she could grow longer hair by having hair grow faster or for perceives to have superior “quality,” in terms of reproduc- longer periods of time. The growth rate is determined tive potential. The concept of mate selection is often diffi- by the androgen steroid hormones: testosterone and cult to interpret in terms of animal physiology. How does a dihydrotestosterone (DHT). The enzyme 5␣-reductase female lion recognize the reproductive potential of a male converts testosterone to its more active DHT form. The lion from visible or behavioral traits? Recent studies sug- levels of the pigment melanin, which is produced di- gest that one feature females assess is the size and color rectly by the hair cells, determine the hair color. The of the male’s mane, the thick coat of hair that covers his process of pigmentation is also influenced by testos- neck and throat. Field observations suggest that female li- terone levels. Therefore, high androgen levels can ons tend to choose mates with a long, dark mane. This trait cause a mane to be long and dark. Since androgens may be a faithful signal of male quality because of the link also control both sex drive and aggression, mane prop- between mane properties and thermoregulation. erties may reflect the propensity for aggressive behav- Most African lions live in hot savanna conditions, iors that aid in mating and defense. where daytime temperatures can reach greater than An interesting variation in the story of the lion’s mane 45°C. For these animals, the challenge is to keep cool, so comes from the Tsavo lions. The Tsavo region of Africa is the thick mane of males would seem to be counterpro- much hotter than other lion habitats, such as the ductive in relation to thermal physiology. Studies using Serengeti. A thick mane would be an even greater hin- infrared cameras support the presumption that males drance to male Tsavo lions. The Tsavo lions gained notori- with dark, thick manes have a more difficult time shed- ety for two reasons. First, they were perceived to be more ding excess heat. Since the mane represents a ther- aggressive than other lions, even gaining a reputation as moregulatory burden, why might it be subject to sexual man-eaters. The white ghost lions of Tsavo, as they be- selection? Researchers have investigated several poten- came known, were also remarkable in that they lacked a tial explanations for this seemingly maladaptive trait. mane. Juvenile males throughout Africa are maneless, There is little doubt that females use mane appear- and it had long been thought that only the young males ance in mate choice, but it is less clear why the thicker, were being spotted in the Tsavo. Researchers lured lions darker manes are most desirable. The simplest theory is from the dense vegetation and discovered that even the that the thick mane helps protect the vulnerable neck re- dominant males lacked manes. For these animals, the gion of the male during the violent fights for dominance. mane would be an insurmountable burden, due to both For the female, a well-protected male is better able to hotter temperatures and dense vegetation. Thus, in the protect the pride from invaders. From the male’s per- Tsavo lions, evolution has led to a loss of the mane. spective, the benefits of additional protection exceed the The physiologist might ask how these lions become additional costs associated with thermoregulation. It is maneless. Although some testosterone is needed for difficult to establish if long manes provide significant pro- hair growth, excessive testosterone causes hair loss. It tection in a fight. An alternate explanation for the under- is not yet known if these males have higher testosterone lying reason for long manes is the handicap hypothesis. levels than their Serengeti relatives. However, there is Much like the tail of a peacock, a thicker mane may adver- also a change in the social structure of prides that im- tise to females that the male has a robust physiology ca- plicates testosterone. In the Serengeti lions, a pride pable of coping with the additional physiological burden. possesses as many as four mature males, but most Independent of the underlying reason why a mane is Tsavo prides have only a single mature male. Perhaps a trait under sexual selection, a physiologist might ask high levels of testosterone both cause hair loss and in- how the properties of the mane of a dominant male are crease the aggressive nature of the Tsavo males, alter- altered and how this reflects his dominance. The an- ing the social structure of the pride. swer may lie in an understanding of the cellular basis of hair growth. Hair length is controlled by the cells of Reference the hair follicle, and its growth rate changes in relation q West, P. M., and C. Packer. 2002. Sexual selection, temperature, to environmental conditions: fast in the summer and and the lion’s mane. Science 297: 1339–1343.

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The distinction between poikilo- ENDOTHERMYMammals Mammals HOMEOTHERMY therm and homeotherm depends on (torpid) both the properties of the animal and Birds the nature of the environment. An animal could maintain a constant T Birds B (torpid) by living in an environment with a Large flying insects Monotremes Fish (polar) constant TA. For example, polar fish live in waters that are constantly Invertebrates cold, and by definition are (aquatic, polar) homeotherms. However, their closest Large reptiles Fish (tropical) relatives live in oceans with temperatures that vary seasonally, and are Naked mole rat Invertebrates therefore poikilotherms. Similarly, a (soil, temperate) goldfish in an indoor aquarium might Invertebrates never experience a change in TB, but (aquatic, tropical) if it were moved outside to a pond its Amphibians T would vary. The goldfish could ar- B Reptiles Fish guably be called a homeotherm or a POIKILOTHERMY ECTOTHERMY poikilotherm, depending on the situ- Invertebrates ation. Since these terms depend Figure 6 Thermal strategies Most animals can be classified as homeotherm or more on the nature of the environ- poikilotherm, or alternately, ectotherm or endotherm. This figure illustrates the many ment than the animal’s physiology, species whose thermal strategies combine elements of multiple strategies. For example, the terms are not always useful in de- monotremes are less homeothermic and less endothermic than other mammals. scribing thermal strategies. Heterotherms exhibit temporal Ectotherms and endotherms differ or regional endothermy in the source of body thermal energy Just how constant does TB have to be for an animal The terms ectotherm and endotherm distinguish to be considered a homeotherm? In actuality, most animals by the physiological mechanisms that animals experience some variation in temperature, determine TB. The environment determines the either spatially or temporally. Many endothermic TB of an ectotherm. If the ectotherm shows a animals place greater priority on maintaining cer- variable TB, then it might also be called a poi- tain anatomical regions within very narrow thermal kilotherm. An endotherm is an animal that gen- ranges. Typically, homeotherms maintain the cen- erates internal heat to maintain a high TB. tral nervous system and internal organs at a more Endotherms regulate TB within a narrow range, constant temperature, while allowing the periphery but it need not be constant. to vary. The temperature of these deep, internal re- Both this and the preceding approach to clas- gions is often called the core temperature. Humans, sifying thermal strategies work effectively for most for example, maintain a near-constant core temper- animals. Most birds and mammals can be classi- ature. However, regions of the human body can ex- fied as homeotherms, because TB is stable, and perience temperatures much lower than the core also as endotherms, because metabolic heat ele- TB. In the cold, humans change blood flow to allow vates TB. However, many animals are best de- hands and feet to cool to conserve internal heat. scribed by a combination of terms (Figure 6). For Males alter the position of the scrotum to keep example, the polar fish we described earlier in this spermatogenic tissue from overheating. However, chapter are homeothermic ectotherms; TB is con- human core TB can also change under some cir- stant but determined by TA. Monotremes, like cumstances. TB can change in females during the other mammals, are endotherms, but maintain a reproductive cycle. It can rise several degrees as a lower TB that is more variable. Proper use of these result of a fever. In comparison to other animals, terms requires an understanding of the physiolog- these are relatively minor regional and temporal dif- ical capacities of the animal as well as an aware- ferences in TB, and a human is considered an en- ness of the thermal properties of its environment. dothermic homeotherm.

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In contrast to humans, many other mammals elevate TB much above TA. However, a regional het- and some birds can undergo dramatic, prolonged erotherm can retain heat in certain regions of the

changes in TB. When exposed to cold nighttime body. Billfish, such as marlin and swordfish, are ec-

temperatures, TB may decrease by several de- totherms but are also able to warm specific regions grees (Figure 7). Hibernating mammals, such as of the body. Their heater organs produce enough

ground squirrels and bats, allow TB to drop for heat near the eye and optic nerves to improve visual the winter months. Although these animals allow clarity when they dive deep into cold waters. Large their bodies to cool, they are still considered en- pelagic fish possess countercurrent heat exchangers dotherms because they produce and retain meta- to conserve the heat of digestion within the body

bolic heat to maintain TB above TA. However, core. Tuna and lamnid sharks are able to retain myo- these endothermic animals are more precisely genic heat within the muscle. Warming of the red described as temporal heterotherms, to reflect muscle increases metabolic capacity and may im-

the variability in TB over time. Some ectothermic prove contractile performance during swimming. animals also fit the description of temporal het- Thermal gradients occur within the bodies of many erotherms. Many large snakes, such as pythons, animals, but these regional heterotherms have spe- wind their bodies into a ball after they have in- cific physiological mechanisms to produce and re- gested their prey. This helps the snake retain the tain heat regionally. metabolic heat produced by digestion. Temporal Although most insects are ectotherms, some heterothermy is a strategy that has different species are regional heterotherms, others temporal benefits for endotherms and ectotherms. It al- heterotherms, and some species are both, depend- lows an endotherm to conserve energy in cold ing on the time of year. The largest of flying insects, temperatures by reducing the costs of thermoreg- such as bumblebees, large moths, and cicadas, have ulation. It provides an ectotherm with a period of a very high metabolic rate in the flight muscles. Tho- accelerated metabolism to speed digestion, nutri- racic temperature in a large flying insect can in- ent assimilation, and biosynthesis. crease by more than 10°C, even while other regions

Most ectotherms rapidly lose their metabolic of the body remain near TA. Interestingly, these ani- heat to the environment, and consequently cannot mals are also able to modulate heat production. Prior to flight they initiate thermogenic pathways to warm the thorax. When flight commences, they can alter heat exchange to maintain near-constant thoracic TA ° 42 20 C temperatures during flight, even when TA is variable 0°C (Figure 8). Social insects use huddling as a means of –20°C controlling the temperature of the colony. Honeybees survive the cold winters by forming tightly crowded 38 clusters. An individual bee in the colony is uniformly warm or uniformly cold, depending upon its position

( ° C) in the cluster. The clusters act like the body of a re- B T gional heterothermic animal. The “core” body heat of the colony is generated by the bees that are located 34 near the center of the cluster. The outermost bees (mantle bees) act as insulation.

Animals have a characteristic degree 1600 h 2000 h 2400 h 0400 h 0800 h of thermotolerance Time of day Physiological strategies for coping with tempera- Figure 7 Short-term cooling in birds Many ture differ in ectotherms and endotherms. For ec- temperate birds allow their body temperatures to decrease when nighttime temperatures decrease. This strategy of totherms, a change in TA alters TB and directly temporal heterothermy saves metabolic energy. (Source: Modified from Reinertsen and Haftorn, 1986)

670 Thermal Physiology

50 Thermoneutral zone

40 Thermogenesis Active cooling MR

BMR TB

( ° C) 30

B Metabolic rate T Onset of Onset of Body temperature hypothermia hyperthermia

TB 20 LCT UCT Thorax Ambient Temperature Abdomen Figure 9 Zones of thermal effects of a resting 10 homeotherm Homeothermic endotherms maintain near- 10 20 30 40 constant body temperature over a wide range of ambient ° TA ( C) temperatures (purple line). Once ambient temperatures decrease below the lower critical temperature (LCT), the Figure 8 Insect heterotherms Many large insects animal must increase its metabolic rate (MR) to generate heat are able to conserve metabolic heat that arises when their to help maintain a constant TB. By extending the line explaining flight muscles are activated during flight. This warms the the metabolic rate below LCT to the x-axis, the body thorax while the rest of the body remains near ambient temperature (TB ) can be obtained as the intercept. Below a temperature, an example of regional heterothermy. certain point, the animal can no longer maintain a constant (Source: Based on Harrison et al., 1996) core temperature and hypothermia results. When ambient temperatures increase past the upper critical temperature (UCT), the animal increases metabolic rate to shed heat. At still higher temperatures, the animal can no longer defend its body temperature and hyperthermia results. changes the rates of many biological processes. In contrast, an endotherm responds to a change in TA by inducing a compensatory regulatory response. Despite the differences, both endotherms faced with a hypothermic challenge, animals may and ectotherms incur physiological costs and reduce TB to maintain homeostasis at metabolic consequences when environmental conditions rate. In general, these compensatory responses at change. high TA or low TA allow the animal to maintain a The effects of temperature can be defined in constant TB, but beyond a point, the animal cannot terms of its impact on animal function. An animal sustain a constant TB. typically spends most of its life in a range of temper- The concept of a thermoneutral zone does not atures that is optimal for physiological processes. apply to animals that alter TB, but ectotherms also The thermoneutral zone of a resting homeother- have ranges of TA (and TB) where growth and re- mic endotherm is the range of ambient tempera- production are optimal. Animals actively seek out tures where metabolic rate is minimal, which is their preferred temperature, a TA that is within its considered the basal metabolic rate, or BMR (Fig- range for optimal function. At low temperatures, ure 9). If temperatures rise to a point called the all developmental processes slow because the upper critical temperature (UCT), the metabolic lower TA reduces the rate of metabolic reactions. rate rises as the animal induces a physiological re- Higher temperatures damage molecules, cells, and sponse to prevent overheating. If the temperature tissues, jeopardizing an animal’s health. Re- falls below a lower critical temperature (LCT), searchers can assess the thermal tolerance of an the metabolic rate rises to increase heat produc- ectotherm or a poikilotherm by transferring an an- tion. For many animals, the TB can be predicted imal from its acclimation temperature to a chal- from the extrapolation of the line that describes the lenging temperature and assessing survival. The metabolic rate at temperatures below LCT. When incipient lethal temperature is the temperature

671 Thermal Physiology

that has a 50% probability of killing the fish within Acclimation to a high temperature tends to in- an identified period. The range of tolerance is the crease both the ULLT and ILLT. Likewise, acclima- difference between the incipient upper lethal tion to a low temperature reduces the upper and temperature (IULT) and the incipient lower lower lethal temperature points. lethal temperature (ILLT). For ectotherms and Animals differ in their ability to tolerate poikilotherms, the ability to tolerate temperature changing ambient temperature. A eurytherm

changes with acclimation history (Figure 10). can tolerate a wide range of TA, whereas a stenotherm can tolerate a narrow range of TA. Eurythermal endotherms/homeotherms possess a wide thermoneutral zone, maintaining a con- 30 stant TB over a wide range of TA; eurythermal ectotherms/poikilotherms display a large thermal tolerance polygon area, with well-separated IULT incipient lethal temperatures. 20 Differences in thermotolerance can be observed in comparisons of populations or species that have evolved in regions separated by latitude or altitude. The ability of an animal to tolerate a 10 lower TA than its competitor allows the tolerant animal to expand into a colder environmental Lethal temperature ( ° C) Lethal temperature niche. Many closely related animals have distinct ILLT differences in thermal preferences that contribute 0 to their geographical distributions. Latitudinal patterns are common in fish species in both ma- 0102030 Acclimation temperature (°C) rine and freshwater. Closely related species of bar- racuda, for example, live at specific latitudes along (a) Eurythermal fish the Pacific coast with a characteristic average TA. From north to south, one species gradually re- 30 places another once the average water temperature changes by only 3–8°C. There are also altitudinal patterns seen with terrestrial animals. Many bird species exist in high-altitude and low- 20 IULT altitude populations, each with physiological spe- cializations and morphological differences. The thermal environment resulting from the combination of altitude and latitude also determines the 10 range of many amphibians. Andean tree frogs

Lethal temperature ( ° C) Lethal temperature (Hyla andina) can be found at low elevation far ILLT from the equator, but closer to the equator they can live at higher altitudes. 0 The genetic basis of a difference in thermotol- 0102030erance is not always clear. We can often determine Acclimation temperature (°C) why levels or properties of a single protein differ in (b) Stenothermal fish two animals in relation to temperature. However, the underlying basis for complex differences in Figure 10 Temperature polygon Acclimation affects the incipient upper lethal temperature (IULT) and thermal physiology is more complex. For example, incipient lower lethal temperature (ILLT) for ectotherms and two species of Siberian hamsters, Phodopus camp- poikilotherms. The tolerance of an animal is reflected in the belli and P. sungorus, differ in thermal biology in area of the polygon created by joining the upper line (IULT, terms of morphology, insulation, behavior, and in red) and the lower line (ILLT, in blue). Analysis of a eurythermal fish (a) yields a larger polygon than that of a physiology. Although these are very closely related stenothermal fish (b). species, they last shared a common ancestor more

672 Thermal Physiology than 2 million years ago. A complex trait such as sessed using an Arrhenius plot (see Box 2, Mathe- fur density depends on multiple genes, many cell matical Underpinnings: Evaluating Thermal Effects types, and networks of genetic regulators. Further- on Physiological Processes Using Q10 and Arrhenius more, the two species may have many genetic dif- Plots). Researchers can use this approach to study ferences, but only some of these may influence how temperature affects the structure and function their thermal biology. of macromolecules, enzymatic reactions, and complex processes, such as metabolic rate.

2 CONCEPT CHECK Animals remodel membranes to maintain near-constant fluidity 1. What are the sources and sinks in an equation describing thermal balance? Recall what you’ve learned about the structure of 2. What is the difference between an endotherm cellular membranes and the importance of mem- and an ectotherm? brane fluidity. Van der Waals forces hold mem- 3. What is the difference between a homeotherm brane lipids together. Although the interactions and a poikilotherm? between phospholipids are strong, the membrane 4. What is the difference between a regional and a must also remain fluid enough to allow proteins to temporal heterotherm? rotate and diffuse laterally within the membrane. Low temperatures cause membrane lipids to so- lidify, which impairs protein movement. Con- Coping with a Changing versely, high temperatures liquefy the membrane, Body Temperature which can compromise its integrity and reduce its effectiveness as a permeability barrier. Cells regu- Although many ectotherms and poikilotherms live late the balance between the solid gel state and the in thermally stable environments—underground liquid sol state. burrows, tropical rain forests, the deep sea, or a Temperature exerts effects on membranes homeotherm’s intestine—others must cope with fre- through both protein function and phospholipid flu- quent and dramatic changes in TB. Because of the ef- idity. The effects on membrane protein function can fects of temperature on macromolecular function be assayed using kinetic analyses. For example, the and metabolism, ectotherms and poikilotherms Naϩ/Kϩ ATPase interacts with membrane lipids must either tolerate or compensate for the complex, during the transport process. The activity of the often deleterious, effects of changing temperature. transporter can be measured at a series of temperatures, then plotted in relation to temperature. A change in membrane fluidity typically results in a breakpoint in the Arrhenius plot of membrane pro- Macromolecular Structure tein function, as shown in Box 2. and Metabolism Membrane fluidity is measured in biological Of the four classes of macromolecules, only proteins membranes using a dye (diphenyl hexatriene) that and lipids are substantially affected by temperature changes in optical properties in relation to its free- over the normal range encountered by animals. dom to move within the membrane (Figure 11). Weak bonds (van der Waals forces, hydrogen When membranes from different species are com- bonds, and hydrophobic interactions) govern the pared, each exhibits a decrease in fluidity (mea- interactions within and between these macromole- sured as a change in optical properties) when the cules. Each type of bond has a characteristic re- membrane is cooled. Taking into consideration the sponse to temperature. Whereas hydrogen bonds differences in thermal niche, this analysis shows and van der Waals forces are disrupted at high tem- that animals produce membranes that exhibit the perature, hydrophobic interactions are stabilized at same fluidity at the natural temperature. This high temperature. Thus, the effects of temperature observation is analogous to the conservation of on macromolecular structures depend on the rela- Km seen in enzymes from animals in different tive importance of each type of bond. The effects of niches. The same pattern is seen when an temperature on a biological process can be as-

673 BOX 2 MATHEMATICAL UNDERPINNINGS Evaluating Thermal Effects on Physiological Processes Using Q10 and Arrhenius Plots

For many physiological processes, a 60 Q10 = 1.6 10°C increase in temperature typically doubles 50 or triples the rate of the process. We can describe these 40 effects of temperature on reaction velocity mathemati- 30 Q10 = 2.4 cally by the Q10. The Q10 is essentially the ratio between reaction rates at two temperatures, adjusted for a 10°C 20 Q = 2.0 temperature difference. It is calculated as Reaction velocity 10 Ϫ K 10 T2 T1 ϭ 2 3 >1 24 10 Q10 320 325 330 335 340 345 350 c K1 d Temperature (1/K × 105) where the rates of a reaction (K) are compared at two temperatures (1 and 2). Thus, if a rate of 10 units/min 37 29 22 15 (K ) was observed at 15°C (T ), and a rate of 20 units/ min 1 1 Temperature (°C) (K2) at 25°C (T2), then

10 25Ϫ15 20 3 >1 24 Q ϭ ϭ 21 ϭ 2 lines show data where one line fits the data at low tem- 10 c 10 d peratures, but a different line fits the relationship at high

The Q10 for a process is the best way to express the in- temperatures. The point where the two lines cross is fluence of temperature on reaction rates, but a better ap- called the breakpoint. Since the slope differs between proach to exploring the mechanism of action is through the two lines, we can infer that different activation ener- an Arrhenius plot. In the late 1800s, the chemist Svante gies govern the reaction over each temperature range. In Arrhenius described a mathematical approach to explor- many cases, this is due to a mechanistic transition from ing the impact of temperature on macromolecular one state to another state. If the process under consid- processes. We now use his approach to study processes eration is membrane fluidity, for instance, the break- such as enzymatic reactions, diffusion of molecules, and point might reflect the transition from a liquid to a solid lipid membrane phase transitions. The sensitivity of a re- phase. If the process is an enzymatic reaction, the break-

action to temperature reflects the activation energy (Ea) of point might occur at a temperature where a critical bond the process. The Arrhenius equation describes the rela- is broken, converting the enzyme from an efficient cata- tionship between the activation energy, temperature, and lyst to a less-efficient catalyst or denatured enzyme. the rate of the process under study: The versatility of the Arrhenius plot allows re-

Ϫ searchers to describe the thermal behavior of any simple k ϭ Ae( Ea /RT ) or complex process. However, the reasons for particular More often, the Arrhenius equation is shown as relationships are more difficult to ascertain in complex systems. Thermal effects on membranes are often dif- ln(k) ϭ ln(A) Ϫ E /(RT ) a ficult to assess because of the considerable hetero- where k is a rate coefficient, R is the gas constant geneity of the membrane. Lipid rafts, for example, are (8.31447 × 10Ϫ3 kJ/K per mol), T is temperature (in de- cholesterol-rich regions of the cell membrane that of- grees Kelvin), A is called the pre-exponential factor, and ten accumulate distinct phospholipids. Temperature will

Ea is the activation energy (kJ/mol). have a different effect on the fluidity of these regions in Let’s say that a researcher was interested in how comparison to the bulk phase of the membrane. Simi- temperature influenced the rate of an enzymatic reac- larly, many integral membrane proteins accumulate dif- tion. She would vary temperature over a range of inter- ferent types of lipids. For example, the mitochondrial est and measure enzymatic rates. The data she enzyme binds cardiolipin molecules within the inner mi- collected could be plotted on a graph with axes chosen tochondrial membrane. Changes in the bulk phase of the from a rearrangement of the Arrhenius equation that membrane do not necessarily reflect changes in the lipid generates a linear equation (y ϭ mx ϩ b): membrane in direct contact with the proteins of interest. Even more complex processes, such as metabolic rate, ln(k)ϭϪE /R × (1/T ) ϩ ln(A) a are really the sum of many simple processes, each with Ϫ Plotting ln(k) versus 1/T gives a slope of Ea /R and a y their thermal sensitivity and unique Arrhenius equation. intercept of ln(A). Suggested Reading The accompanying figure illustrates two potential q Metz, J. R., E. H. van den Burg, S. E. Bonga, and G. Flik. 2003. outcomes from an Arrhenius plot. For the green line, the Regulation of branchial Na(ϩ)/K(ϩ)-ATPase in common carp data fall along a straight line. The slope of the line re- Cyprinus carpio L. acclimated to different temperatures. Journal flects the activation energy of the reaction. The purple of Experimental Biology 206: 2273–2280.

674 Thermal Physiology

bonds between fatty acid chains, the mem-

Bird brane is more fluid. For example, pure stearic acid (C18:0) becomes liquid only at tempera- Mammal tures above 69°C, whereas oleic acid (C18:1) is liquid at 12°C. The position of the double bond is also critical. A double bond near the mid- point of the fatty acid chain (as with oleic acid) is more effective than a double bond near the Tropical fish end of the fatty acid chain. Anisotropy ( Fluidity) 3. Phospholipid classes. The difference in the shape of the polar head groups alters the Cold-water fish ability of the phospholipids to interact at the surface of the membrane. Phosphatidyl- 04010 20 30 choline (PC) is more common in membranes Temperature (°C) of warm-acclimated cells, whereas phos- Figure 11 Membrane fluidity Membranes are phatidylethanolamine (PE) is more common in treated with a dye (diphenyl hexatriene) with optical cold-acclimated cells. The ratio of PC to PE de- properties that change in relation to membrane fluidity. creases in the cold acclimation and adaptation. Anisotropy is an optical property that reflects the ability of a dye to alter the behavior of plane polarized light. Anisotropy 4. Cholesterol content. A pure phospholipid bi- is inversely related to fluidity; at warmer temperatures, a layer is mostly fluid at high temperature and decrease in anisotropy reflects an increase in fluidity. mostly solid at low temperature. Cholesterol Animals that live in different environments produce added to fluid phospholipid bilayer has little membranes that possess a similar fluidity at their normal range of temperatures (indicated by the thickened portion of effect on fluidity. If the same membrane is the lines). cooled, cholesterol tends to prevent it from so- (Source: Adapted from Logue et al., 2000)

Low fluidity High fluidity animal is acclimated to different temperatures. Ec- tothermic animals reduce the deleterious effects of temperature by changing the composition of their membranes. In this process, called homeoviscous Shorter chain length adaptation, cells remodel membranes to preserve fluidity. Three mechanisms target phospholipids (Figure 12), and a fourth mechanism alters cholesterol content.

1. Fatty acid chain length. Phospholipids with Unsaturation short chain fatty acids cannot form as many interactions with adjacent fatty acids and therefore are highly mobile. The effectiveness of chain shortening depends upon the fatty acid PC PE position on the phospholipid. Due to the three- dimensional structure of a phosphoglyceride, Polar head a short chain fatty acid in position 1 makes a group greater contribution to enhancing fluidity than does the same fatty acid in position 2. Figure 12 Phospholipid properties and 2. Saturation. Double bonds create a kink in the membrane fluidity Cells change the fluidity of fatty acid chain that prevents effective bond membranes by altering the composition of membrane formation with other fatty acids. With fewer phospholipids.

675 Thermal Physiology

lidifying. Put another way, cholesterol tends to Phospholipids are synthesized de novo within the make a membrane more fluid when external endoplasmic reticulum, then packaged into vesi- conditions otherwise encourage a transition to cles that fuse with cellular membranes. a gel phase.

Cells use two general pathways to modify Temperature changes enzyme kinetics membrane composition in response to tempera- Temperature affects protein structure and function ture: in situ modification and de novo synthesis. in complex ways. Changes in temperature alter the Both pathways require cells to modify the proper- number of bonds that form within and between ties of the fatty acids within the fatty acid pool us- molecules. Even minor changes in protein struc- ing suites of enzymes that elongate, shorten, ture can have important effects on protein func- saturate, and desaturate fatty acids. Since these tion. In enzymes, for example, these structural enzymes begin with fatty acids derived from the effects manifest as changes in catalytic properties. diet, the nature of the diet also affects the profile First, changes in weak bonds can alter the three- of fatty acids within the membrane. dimensional structure of the enzyme. For instance, Enzymes alter the structure of individual phos- warm temperatures could break bonds that are pholipids directly within the membrane (Figure 13). necessary to fold the protein in a way that forms First, phospholipase A removes an acyl chain from the active site. Second, temperature can alter the membrane phospholipids to form a lysophospho- ionization state of critical amino acids within the lipid. Next, lysophospholipid acyltransferase uses a active site. For instance, the amino acid histidine is more appropriate fatty acid (in the form of fatty acyl important in many active sites, and changes in his- CoA) to rebuild the phospholipid. tidine protonation state can alter enzyme substrate

More commonly, membranes are remodeled affinity. Any increase or decrease in Km could be by endocytosis and exocytosis (see Figure 14). The disruptive. Third, temperature can alter the ability old membrane is removed using endocytosis. of the enzyme to undergo the structural changes necessary for catalysis. Enzymes must be rigid enough to maintain the proper conformation, but flexible enough to undertake conformational

COOH changes during catalysis. Thus, temperature can affect enzyme kinetics through effects on maximal

velocity (Vmax) or affinities for substrates (Km), allosteric activators (K ), and inhibitors (K ). When Fatty acid a i animals experience a change in T , they may either CoA B tolerate the effects on enzyme kinetics or alter meta- ATP Acyl CoA synthase bolic regulation to compensate. Biochemical reactions are accelerated by COOH CoA higher temperature and reduced at lower temperature. Recall that the rate of a chemical reaction depends on the proportion of molecules within the system that possess energy equal to Fatty acid Fatty acyl CoA or greater than the activation energy (Ea). As temperature increases, the average kinetic energy of the substrates increases and a greater Phospholipase Lysophospholipid acyltransferase proportion of molecules has sufficient energy to be converted to products, causing the enzyme ve- Phospholipid Lysophospholipid locity to increase. For most enzymes working Figure 13 Phospholipid remodeling Cells can over a biologically relevant range of tempera- remodel the phospholipids directly within membranes by tures, an increase of 10°C results in a two- to removing a fatty acid. A phospholipid is rebuilt by threefold increase in reaction velocity. Recall lysophospholipid acyltransferase, which attaches another fatty acid produced by the cell. The fatty acid must first be from Box 2 that this implies a Q10 value of 2–3. activated by the esterification of coenzyme A. Q10 can be calculated for simple reactions

676 Thermal Physiology

such as an enzymatic step, or com- Extracellular fluid plex processes such as metabolic ER GolgiEndosome Vesicle rate. Consider how temperature affects the Vmax of lactate dehydroge- Endocytosis nase (LDH) in the muscle of a desert lizard as it experiences daily transitions in T . Over the course of a single B Cytoplasm day, the total number of LDH enzyme molecules does not change apprecia- bly, but their catalytic activity changes with temperature. LDH maximal ac- Exocytosis tivity (Vmax) typically doubles when temperature is increased by 10°C Vesicle (that is, its Q10 is 2). Let’s begin at a TB Plasma of 40°C, and assume that the muscle membrane of the lizard has 400 units (U) of LDH Figure 14 Membrane remodeling Cell membranes are constantly per gram of tissue (One U of enzyme remodeled by endocytosis and exocytosis. When temperature decreases, the cell can convert 1 µmol of substrate to produces vesicles possessing phospholipids with fatty acids that are shorter and more ϭ unsaturated than those in the cell membrane. Over time, the cycles of endocytosis product each minute.) With Q10 2, a and exocytosis remove undesirable phospholipids, replacing them with more desirable decrease in temperature from 40°C to phospholipids. 30°C causes the LDH enzymes to operate at only one-half the velocity, giv- ing a Vmax of 200 U/g. Similarly, at 20°C the LDH Vmax example, it may increase substrate concentrations is 100 U/g, and at 10°C only 50 U/g. Over the course or stimulate the enzyme with allosteric regulators. of a single day, from the midday heat to the cool evening, a desert lizard may have to cope with an eightfold change in its LDH V activity as a result Evolution may lead to changes max in enzyme kinetics of changes in TB. It is easy to imagine how an eightfold reduc- When animals are exposed to suboptimal tempera- tion in LDH capacity might severely impair the ca- tures for generations, there is the possibility of evo- pacity to produce ATP by glycolysis. How does an lutionary changes in the genes encoding enzymes. animal cope with such dramatic reductions in the We draw again on research with LDH for examples rates of enzymatic reactions? The simple answer of evolutionary changes that cause differences in is that the rates of ATP synthesis decline in paral- enzyme kinetics as well as enzyme synthesis. lel with the rates of ATP utilization, with each step Mutations may lead to structural changes in exhibiting a Q10 ranging from 2 to 3. Put another the enzyme that impart a favorable difference in way, the animal can tolerate lower rates of muscle enzyme kinetics. Lowering temperature increases ATP production because it slows down and needs the affinity of LDH for its substrate pyruvate. Evolu- less ATP for muscle activity. However, it is impor- tion has led to a fine-tuning of enzyme properties ϭ tant to recognize that Q10 2 is quite different such that subtle structural differences allow each ϭ from Q10 3. If a 10°C decrease in temperature species to possess a similar Km at its respective caused ATP supply to decrease threefold when ATP normal TA. This strategy, called conservation demands decreased only twofold, the tissue would of Km, is commonly seen when we compare the be depleted of ATP within seconds or minutes. Su- effects of temperature on the enzyme kinetics of perimposed on the Q10 effects are numerous layers different animals. of metabolic regulation that ensure that energy Alternately, evolution may lead to mutations in metabolism remains in homeostasis. If an individ- the promoter for an enzyme, causing a change in ual enzyme is more sensitive to temperature than the level of gene expression of an otherwise un- are other enzymes in the pathway, the cell has sev- changed enzyme. Killifish live along the eastern eral options to increase flux through that step. For coast of North America from Newfoundland to

677 Thermal Physiology

Florida. Within the population as a whole, there Surprisingly little is known about the hor- are different alleles of the LDH-B gene. One allele mones and signaling pathways that cause an ec- predominates in northern populations, while an- totherm to remodel its tissues during acclimation other allele predominates in southern popula- and acclimatization. Cold-sensing and warm- tions. Intermediate populations have both alleles. sensing neurons are important for detecting tem- These alleles have differences in enzyme proper- perature, but the links to gene expression are not ties and differ in the level of gene expression. The well known. In some cases, seasonal changes in northern allele is expressed at twofold higher lev- physiology that mitigate the effects of temperature els than the southern allele, due to mutations in are triggered by changes in the photoperiod. the promoter. The northern fish produce more LDH enzyme molecules, which compensates for the debilitating effects of temperature on enzy- Life at High and Low Body matic activity that would occur as a result of living Temperatures in the colder waters. Animals that can tolerate extreme temperatures can invade and colonize niches that are underex- Ectotherms can remodel tissues ploited by their competitors. Ectothermic animals in response to long-term changes exposed to thermal challenges must possess in temperature mechanisms to mitigate the effects of temperature Many ectothermic animals remodel their cellular on macromolecular structure and metabolism. In contrast, endothermic animals survive thermal machinery to mitigate the effects of variation in TB. In the laboratory, where the researcher changes extremes using complex regulatory pathways to maintain a constant T . Their existence at ex- only TA, this remodeling process is called thermal B acclimation. In the natural world, seasonal transi- tremes is a testament to their physiological capac- tions in temperature are accompanied by other ity to resist the effects of TA. environmental changes and the response of the animal to complex seasonal changes is called acclimatization. In winter, photoperiods get shorter, Some enzymes display cold adaptation food may be less abundant, and oxygen levels may Earlier in this chapter we discussed how relatively

change. The complexity of these seasonal environ- subtle differences in TA can lead to evolutionary mental changes makes it difficult to link remodel- changes in enzyme structure and gene expression. ing with the temperature. On one hand, there is However, the need for enzymatic structural modi- uncertainty about the trigger for the remodeling fication is much more pronounced at thermal ex- process; is the change initiated by changes in tem- tremes, particularly at the subzero temperatures perature, or by some other factor, such as pho- encountered in polar seas. Psychrotrophs are or- toperiod? On the other hand, it is not always clear ganisms that thrive in the extreme cold, in con- that the remodeling itself serves to compensate trast to mesotrophs that live at more moderate specifically for temperature. temperatures. Animal psychrotrophs, including Temperature-dependent remodeling involves polar invertebrates and fish, remain active at body combinations of quantitative and qualitative temperatures near the point of freezing. Many strategies. Low temperature may increase the psychrotrophic organisms possess cold-adapted number of mitochondria in muscle, or trigger the proteins that function optimally at very low tem- hypertrophic growth of the heart. This is an exam- peratures. Although these enzymes are more sta- ple of a quantitative strategy; there is simply more ble in the cold, they are rapidly inactivated at of the same machinery. Muscles can also alter the slightly higher temperatures. types of proteins they use to build the contractile The catalytic and structural differences be- machinery. For instance, animals express differ- tween enzymes of psychrotrophs and mesotrophs ent myosin isoforms in winter and summer—an example of a qualitative strategy.

678 Thermal Physiology can be traced to the weak bonds that stabilize enzyme structure. Enzymes undergo pronounced Stress proteins are induced changes in three-dimensional shape during the at thermal extremes catalytic cycle, known as protein breathing. During Many proteins are best suited to function over nar- these transitions in folding, weak bonds break and row ranges of temperature that span the biologi- form. When temperatures decrease, most of these cal range of the animal. During the normal weak bonds are strengthened, stabilizing the pro- structural change that occurs when a protein tein in a form that occupies a smaller volume. In this breathes, the protein is vulnerable to further conformation, it is much harder for the protein to changes in structure. Occasionally, the protein can breathe, and consequently enzymes in the cold are unfold or misfold into a nonfunctional conforma- less efficient. The psychrotroph enzyme has fewer tion. This denatured protein must be repaired or weak bonds stabilizing its structure; it occupies a cleared from the cell before it disrupts other cellu- larger volume and has an easier time breathing dur- lar functions. Denaturation is a normal process, ing catalysis. The reduced stability allows it to func- and cells are able to detect and remove denatured tion better in the cold, but makes it vulnerable to proteins using pathways of protein quality control. temperature-dependent unfolding. In comparison These pathways function throughout the lifetime to mesotroph enzymes, cold-adapted enzymes are of a cell, but become even more important during more efficient enzymes at low temperatures, but in- times of thermal stress when denatured proteins ferior enzymes at high temperatures. can accumulate and kill the cell. Unique loss-of-function mutations also occur Heat shock proteins (Hsp’s) are molecular in polar animals. Many Antarctic fish have lost the chaperones that use the energy of ATP to catalyze ability to express functional oxygen-binding pro- protein folding after translation. Chaperones can teins, such as hemoglobin and myoglobin. These also help refold proteins that have become dena- fish can survive without these oxygen carriers be- tured as a result of thermal stress. Many cells ex- cause they have low metabolic rates and the sur- posed to extreme temperatures undergo a heat rounding polar waters are rich in oxygen. shock response, which leads to a dramatic in- There are many such examples of thermal crease in the levels of specific proteins that help adaptations of individual selected genes in polar repair damaged proteins. During a heat shock, the animals. However, more controversial is the ques- cell undertakes a rapid increase in the synthesis of tion of whether or not polar animals have a funda- several critical Hsp’s. The cell can halt the tran- mentally different organization of metabolism as a scription and translation of other genes, sparing result of evolution in the extreme cold. Early stud- biosynthetic resources for Hsp synthesis. It stimu- ies suggested that polar animals had metabolic lates the expression of the Hsp genes by activating a rates that were much higher than the metabolic heat shock factor (HSF), a transcription factor that rates of temperate animals measured near 0°C. binds to the heat shock elements in the promoters of These observations were used to support a theory genes for heat shock proteins. Although there is still that became known as metabolic cold adaptation. some uncertainty about the exact mechanism of acti- It was proposed that thousands of years in the ex- vation of HSF, the trigger for the process is thought to treme cold led to evolutionary changes that pro- involve damaged protein (Figure 15). In the absence vided these polar animals with an ability to elevate of thermal stress, most of the cellular HSF is bound to their metabolic rate. Even with years of study it re- Hsp70 as inactive monomers. When the cell is mains unclear whether metabolic cold adaptation stressed, the chaperones are lured away from HSF by is a real phenomenon. The earliest studies were damaged proteins. The released HSF can then form based on comparisons of goldfish and arctic cod. trimers, which in turn bind the heat shock element Now that more species have been analyzed using on the Hsp genes, activating them. Once the damaged more sophisticated technologies, it seems less likely proteins are repaired, Hsp70 is free to bind HSF that metabolic cold adaptation occurs as a general monomers and reverse the transcriptional activation. phenomenon. Nonetheless, many studies have The Hsp response is central to the ability of identified evolutionary differences and physiologi- ectothermic animals to survive brief periods of cal peculiarities in some polar animals.

679 Thermal Physiology

Cytoplasm 1 1 Complex of HSF and Hsp's under Hsp70 unstressed conditions. 2 9 3 2 Heat stress causes complex to dissociate. 4 HSF monomers 3 Hsp70 binds to denatured proteins. 8 Denatured HSF protein trimer 4 HSF monomers associate into trimers.

5 Trimers move into the nucleus and bind to the promoter of genes with heat 5 shock element (HSE).

6 Hsp70 gene transcription increases.

Hsp70 gene 7 Poly A+ mRNA is exported to the DNA 6 cytoplasm. 7 8 Poly A+ mRNA is translated to form HSE more Hsp70.

AAA 9 The increase in Hsp70 levels allows the complex to form again, stopping + Nucleus Poly A mRNA transcriptional activation.

Figure 15 Heat shock response

extreme temperature that often occur within their low their tissues to freeze and even encourage ice natural environments. For most species, the Hsp to form in the body. Animals that avoid freezing response is induced at temperatures only a few de- use behavioral and physiological mechanisms to grees above the typical thermal range. This pow- prevent ice crystal formation and growth. To un- erful protective process may be central to the derstand why ice is so dangerous, let’s consider evolution of thermal sensitivities and thermal what happens to water molecules as temperatures ranges (see Box 3, Methods and Model Systems: decrease. Heat Shock Proteins in Drosophila). Interestingly, The freezing point of pure water is 0°C. This is some species have lost their ability to mount a heat the temperature at which ice could form if enough shock response. Antarctic fish have lived for thou- water molecules cluster together to begin an ice sands of years at Ϫ1.96°C. At some point, the crystal. Below the freezing point, water is on the species experienced genetic changes that dis- verge of freezing, awaiting an event that triggers ice rupted the capacity to invoke a heat shock re- formation. When water is below its freezing point, sponse. Since the Antarctic waters remain very but not yet frozen, it is considered supercooled. constant in temperature, these mutations have no Pure water, left undisturbed, can be supercooled to deleterious consequences to the animals. How- almost Ϫ40°C before ice forms spontaneously. The ever, when taken out of their natural environment, trigger for ice formation is a cluster of water mol- these fish rapidly succumb to temperatures only a ecules that act as a seed for an ice crystal. Alterna- few degrees above 0°C. tively, a macromolecule in solution can act as a nucleator, seeding ice crystal formation. Once the ice formation begins, water molecules bind to each Ice nucleators control ice crystal growth face of the growing crystal to create a complex in freeze-tolerant animals three-dimensional structure. Ectotherms that live at freezing temperatures use Ice crystals forming within a tissue have two two strategies to survive the cold: freeze-tolerance deleterious effects. First, since ice crystals have and freeze-avoidance. Freeze-tolerant animals al- points and sharp edges, the growing ice crystal can

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BOX 3 METHODS AND MODEL SYSTEMS Heat Shock Proteins in Drosophila

The best-studied Hsp’s are from the tant in thermotolerance, but that other physiological Hsp70 family. Each subcellular compartment factors may also play roles. has Hsp70 proteins that help fold proteins into the These studies also showed that flies with a robust heat proper conformation and target misfolded proteins for shock response also had lower fecundity. In other words, degradation. Mitochondria have Grp75, the cytoplasm superior thermotolerance comes with an evolutionary has Hsc70, and the endoplasmic reticulum has Bip. The cost. In a thermostable environment, flies with lower lev- namesake of the family, Hsp70, is produced by cells els of Hsp70 could outcompete flies with higher Hsp70 mainly under stressful conditions. When temperatures levels because of their greater fecundity. However, at rise to dangerous levels, cells dramatically induce syn- more thermally challenging conditions, the lower fecun- thesis of Hsp70. It is produced in the cytoplasm, where dity is offset by the greater thermotolerance. These labo- it refolds proteins that have been denatured in response ratory studies also reflected the nature of the evolution of to elevated temperatures. The ability to mount a heat thermotolerance in the natural world. Wild populations of shock response is central to the thermotolerance of an- Drosophila from around the globe exhibit a wide range in imals. Genetically modified cells that lack an ability to thermotolerance. In most populations the natural ability induce Hsp70 are very sensitive to thermal stress. Since to survive thermal stress correlates with the levels of this gene is essential for thermotolerance, many re- Hsp70 gene expression. For example, flies in Evolution searchers have studied whether variation in Hsp70 gene Canyon in Israel occur in separate populations that oc- expression is central to the differences in thermotoler- cupy the north- and south-facing slopes. The flies that live ance among animals. on the south-facing slopes, which are hotter and drier, Many studies of thermotolerance and Hsp70 have have a stronger heat shock response. The flies on the been performed on the fruit fly, Drosophila. If Hsp70 north-facing slope have a weaker heat shock response helps an animal survive heat stress, then it might be due to a disruption of the promoter of one of the Hsp70 reasonable to hypothesize that an animal could benefit genes. Since these two slopes are only hundreds of me- from greater expression of Hsp70. Laboratory studies ters apart, individual flies probably move between the two have provided important insight into the links between populations. Thus, natural selection acts to ensure that Hsp70 gene expression, thermotolerance, and evolu- the allelic differences between populations are retained. tion. In one study, lines of flies were manipulated to pos- Similar studies on other Drosophila populations showed sess extra copies of Hsp70 genes. Larvae from these that flies with lower thermotolerance usually possessed flies had greater thermotolerance, demonstrating the mutations that disrupted their ability to express one or importance of Hsp70 to thermotolerance. In other ex- more copies of Hsp70 genes. In most of these cases, the periments, lines of flies were exposed to high tempera- animals with higher Hsp70 inducibility and thermotoler- tures for generations to see if natural variations in the ance also showed reductions in fecundity. Hsp70 genes within a population could be subject to These studies show that, though critical for thermo- natural selection. Within only a few generations the av- tolerance, Hsp70 can have deleterious effects. Further- erage thermotolerance of the flies increased. These more, they illustrate why genetic variations in thermotolerant flies were able to induce Hsp70 to populations are essential for the survival of a species. higher levels than thermosensitive flies. Surprisingly, References the difference in Hsp70 gene expression between ther- q Feder, M. E., and G. E. Hofmann. 1999. Heat-shock proteins, mo- motolerant and thermosensitive flies was never more lecular chaperones, and the stress response: Evolutionary and than 15%. These data argue that Hsp70 may be impor- ecological physiology. Annual Review of Physiology 61: 243–282.

pierce membranes, killing the cell. Second, ice crys- Still, many ectotherms survive freezing. Intertidal tal growth removes surrounding water, causing hy- bivalves living in northern tidal flats can freeze perosmotic stress. If ice forms outside cells, then when exposed to cold air temperatures, then thaw water is drawn out of cells, causing a hypertonic when the warmer water returns at high tide. Sev- stress that shrinks the cell, perhaps even killing it. eral terrestrial vertebrates can also survive freezing.

681 Thermal Physiology

A wood frog in the north temperate zone enters the possess antifreeze macromolecules—typically pro- leaf litter in late fall, in preparation for overwinter- teins or glycoproteins—that reduce the freezing ing. When temperatures drop below freezing, the point of body fluids by noncolligative actions. They animal supercools but ice does not form. At still disrupt ice crystal formation by binding to the sur- lower temperatures, the animal begins to freeze. face of small ice crystals to prevent their growth First to freeze are the frog’s fingers and toes. The (Figure 16). body core begins to freeze shortly thereafter. The first antifreeze protein, or AFP, was dis- Freeze-tolerant animals usually produce ice nu- covered in an Antarctic fish about 30 years ago by cleators to control the location and kinetics of ice Dr. Art DeVries. Since then, AFPs have been found crystal growth. Ice is the most damaging when it in many distantly related taxa of fish, as well as in- forms inside cells, so freeze-tolerant animals secrete sects and plants. Four classes of AFPs are distin- nucleators out of the cell. This restricts ice formation guished by their structure: types I, II, and III, as to the extracellular fluids, such as hemolymph, and well as antifreeze glycoproteins, or AFGPs. Inter- allows the intracellular space to remain liquid. estingly, each of the classes of AFPs has arisen Many different types of molecules can act as nucle- multiple times in evolution. In fish, AFPs arose less ators in animals: calcium salts, membrane phos- than 20 million years ago. This coincides with re- pholipids, and long chain alcohols. However, it is not cent (in geological terms) sea level glaciation, always clear that these ice nucleators are actually which probably represented a strong selective necessary or helpful to freeze-tolerance strategies. pressure on the local marine species. The phyloge- For example, the wood frog has an ice nucleator netic distribution of AFPs suggests an intriguing that triggers ice formation at about Ϫ7°C. The same evolutionary history. ice nucleator is also found in the tissues of frogs that AFPs provide good examples of parallel evolu- cannot survive freezing. It may induce the formation tion. For example, AFP II appears in herring, of ice, but it does not necessarily provide the wood salmon, and sea ravens, fish from three separate frog with its freeze-tolerance. Some nucleators may orders. This suggests that AFPs arose multiple simply be present for other functions and have no times in these lineages but well after the modern adaptive role in freeze-tolerance. species diverged. These AFP II genes may have Because ice formation draws water from the arisen from similar genes independently in each cells, freeze-tolerant animals also produce intra- lineage. The structure of AFP II suggests the an- cellular solutes to counter the movement of water. cestral gene was a Ca2ϩ-dependent lectin, a pro- Large glycogen reserves of the liver are broken tein that binds sugars. In structural models, the down and converted to compatible solutes consist- ing of organic polyols, such as trehalose and glyc- erol. Compatible solutes have two main beneficial effects. First, by increasing the osmotic pressure within the cells, they reduce the movement of water and cell shrinkage. Second, the solutes help stabilize macromolecular structure. AFP

Antifreeze proteins can prevent Ice crystal Water intracellular ice formation molecules Freeze-avoidance is the second strategy animals use to survive extreme cold. In a car, antifreeze ele- vates the osmotic concentration of the radiator fluid. Solutes in general depress the freezing point of a so- Figure 16 Antifreeze proteins Antifreeze proteins lution, preventing ice formation at subzero temper- bind to the surface of ice crystals to prevent their growth. atures. Freezing point depression is one of the They bind along the face of the ice crystal, where the protein forms weak bonds with water molecules immobilized in the colligative properties of solutes. The solutes in ani- ice crystal. Because ice growth is very orderly, the presence mal tissues reduce the freezing point of water, but of the bound protein prevents ice crystal growth. generally not lower than about Ϫ2°C. Some animals (Source: Modified from Davies et al. 2002)

682 Thermal Physiology interaction of a lectin with the hydroxyl groups of Thermogenesis sugars is similar to the interaction of AFP with the hydroxyl group of a water molecule. Heat production is an inevitable consequence of The evolutionary origins of AFGP are also un- being alive. An endotherm warms its body using usual in terms of protein evolution. The ancestral heat that arises as a by-product of other metabolic gene was probably a gene for pancreatic trypsino- processes, primarily energy metabolism, diges- gen. A region between the first intron and second tion, and muscle activity. All animals—endotherms exon was duplicated not just once but more than and ectotherms—generate heat during these 40 times. The resulting gene possessed multiple, processes, but only the endotherms possess the tandem sequences that resulted in a repeating physiological adaptations that enable them to re- Thr-Ala-Ala motif necessary to prevent ice crystal tain enough metabolic heat to elevate TB above TA. growth. In most cases of gene duplication and di- In addition to the pathways that produce heat vergence, the resulting gene has properties similar as a by-product, endotherms possess specific ther- to those of the ancestral gene, with relatively subtle mogenic pathways with the main purpose of heat differences in function. In the case of AFGP, the re- production. Thermogenic pathways rely on futile sultant gene has a totally distinct function. AFGPs cycling, in which chemical potential energy is spent have no protease activity, and trypsinogen has no to generate heat. Most futile cycles involve cycling antifreeze activity. of ATP hydrolysis and ATP synthesis. Heat is released in ATP hydrolysis (ATP → ADP ϩ phosphate), but a great deal more heat is produced when the 2 CONCEPT CHECK cell uses intermediary metabolism to regenerate the ATP. Endotherms can enhance heat production 5. Compare and contrast the homeoviscous either by increasing the rate of ATP turnover or by adaptation and conservation of Km in relation to temperature effects on macromolecules. reducing the efficiency of ATP production. In both 6. How can an animal alter membrane fluidity? cases, most of the metabolic heat arises directly or 7. Distinguish between freeze-tolerant and freeze- indirectly from mitochondrial oxidative phospho- avoidance strategies. rylation.

Maintaining a Constant Shivering thermogenesis results from Body Temperature unsynchronized muscle contractions Muscle plays a critical role in the thermal budget of Endothermy is so inextricably intertwined with a endotherms. Because muscle is the most abundant high metabolic rate that it is not known which trait tissue in birds and mammals, it produces consider- arose first. High TB allows metabolic processes able heat, even at rest. Locomotion enhances the such as growth, development, digestion, and rate of muscle heat production. However, many biosynthesis to operate at faster rates, and the birds and mammals can also use skeletal muscle to higher metabolic rate in turn produces more heat. generate heat by shivering thermogenesis. As in The ability to become warm bodied requires meta- normal contraction, motor neurons from the spine bolic pathways to produce heat (thermogenesis) release neurotransmitters at the motor end plate, as well as physiological mechanisms to retain but during shivering the pattern of excitation is dif- heat. Most endotherms are also homeotherms and ferent. The smallest neurons—those innervating the committed to maintaining a constant TB. To do so, slow fibers—are recruited first, followed by the they must control both thermogenesis and heat larger neurons that innervate fast muscle. As a re- exchange. In cold environments, endotherms sult, individual myofibers contract but the motor stimulate thermogenesis and reduce heat loss. In units are uncoordinated and the whole muscle un- hot environments they increase heat loss, but may dergoes no gross movement. Shivering thermogen- also reduce thermogenesis. To control TB, animals esis is a strategy that works for short periods of cold must be able to sense both environmental temper- exposure, but it is not useful for prolonged cold ature and body core temperature. stress. The mechanics of shivering prevent an animal from using its locomotor muscles to hunt prey

683 Thermal Physiology

or escape predators. Furthermore, if shivering per- Takeoff sists, or repeats frequently, the muscles are rapidly 30 depleted of nutrients and they become exhausted, just as they would after high-intensity exercise.

Warm TA Heat is produced in metabolic futile cycles 20 Shivering thermogenesis is unique to birds and mammals; however, other animals also use muscle to generate heat. Large flying insects, such as Cold TA bumblebees and some moths, can generate 10 Thorax temperature ( ° C) Thorax temperature enough heat to warm the thoracic flight muscles, which improves flight muscle performance in terms of energy production, excitation-contraction coupling, and cross-bridge cycling. The high meta- 0 bolic rate during flight generates abundant heat, 0 10 20 30 enough to warm the flight muscles by several de- Time (min) grees. Remarkably, these insects are even able to Figure 17 Thermogenesis in insect flight warm their flight musculature prior to takeoff. muscle Many large flying insects can undertake a Three distinct mechanisms allow insects to preflight warm-up, using metabolic futile cycles and muscle warm the thorax prior to flight. These same ther- activity to elevate thoracic temperatures to a threshold mogenic pathways also allow social insects to work temperature required for flight. (Source: Modified from Heinrich, 1987) collectively to warm the hive. The first mechanism is a metabolic futile cycle in carbohydrate metabolism. Within the flight muscle, two opposing en- bution of ions across the membrane. Cells use zymes are activated simultaneously: the glycolytic chemical energy, usually in the form of ATP, to cre- enzyme phosphofructokinase and the gluco- ate these gradients. Consequently, any process neogenic enzyme fructose-1,6-bisphosphatase. that dissipates ion gradients will cause the cell to The metabolic cycle causes ATP hydrolysis and heat use chemical energy to reestablish the gradient. production, but without changes in the levels of the Ion gradients collapse for two main reasons. other substrates and products. A second warming First, many specific membrane proteins use elec- mechanism relies on muscle contraction. Two sets trochemical energy to drive other processes such of antagonistic flight muscles power wing move- as metabolite transport and biosynthesis. For ex- ments during flight. Bumblebees can induce both ample, many cells transport glucose and amino sets of muscles to contract simultaneously prior to acids into the cell using Naϩ-dependent cotrans- flight, so that energy is expended without produc- porters, causing the cell to use Naϩ/Kϩ ATPase to tive movement. The third mechanism for heat gen- pump the Naϩ back out of the cell. The mitochon-

eration is actual wing movement. The insect moves drial F1F0 ATPase is another transporter that dis- its wings fast enough to buzz, but controls the fre- sipates ion gradients, in this case the proton quency and orientation of the wings to avoid gener- motive force. Heat is produced when the mito- ating lift. Collectively, these thermogenic pathways chondrial electron transport system oxidizes re- allow the flight muscle to warm up prior to takeoff. ducing equivalents to regenerate the proton There appears to be a critical thoracic temperature gradient. that must be achieved before the insect will attempt The second pathway of ion gradient dissipation

to fly (Figure 17). At high TA, less of a preflight is ion leak, in which ion movements are not coupled warm-up is necessary to reach the threshold. to any other transport process. Since no biological membrane is completely impermeable, some ions leak across the bilayer or through gaps between Membrane leakiness enhances proteins and phospholipids. Ion-pumping mem- thermogenesis brane proteins produce heat as a by-product, and a Most cellular membranes maintain an electro- high proportion of the resting heat production, as chemical gradient arising from differential distri- much as 50% in some tissues, is due to the costs of

684 Thermal Physiology maintaining ion gradients. Any process that in- BAT is particularly important for thermogenesis in creases the need for ion pumping will also increase small mammals and newborns of larger animals, thermogenesis. Typically, an endotherm has a rest- particularly those that live in cold environments. ing metabolic rate that is as much as 10-fold BAT growth and thermogenesis is under the con- greater than that of an ectotherm of the same size trol of the sympathetic nervous system. Norepi- and TB. The higher metabolic rate is due in part to nephrine released from these nerves causes BAT to membrane leakiness; endotherm plasma mem- grow in cell number (hyperplasia) and cell size (hy- branes and mitochondrial membranes are inher- pertrophy). Undifferentiated precursor cells are in- ently leakier than those of ectotherms. Endotherms duced to proliferate and then later differentiate generate more heat to maintain ion gradients into BAT. Triglyceride is synthesized and mito- across leakier membranes. chondria proliferate. At this same time, the cells begin to express thermogenin, which causes the tissue to increase the rate of mitochondrial respi- Thermogenin enhances ration and consequently heat production. BAT heat mitochondrial proton leak production is often called nonshivering thermo- Mammals possess a unique way of generating heat genesis (NST); while the other pathways we have in specialized deposits of brown adipose tissue discussed also differ from shivering, NST is a term (BAT), typically located near the back and shoulder usually reserved for BAT-mediated thermogenesis. region (Figure 18). The brown adipocytes differ In the absence of thermogenin, the processes of from white adipocytes in important respects. They oxidation of reducing equivalents and phosphoryla- have much higher levels of mitochondria and ex- tion of ATP are coupled by their shared dependence press the gene encoding the protein thermogenin. on the proton motive force. When thermogenin is inserted into the inner mitochondrial membrane, it accentuates mitochondrial proton leak and dissipates the proton motive force. Since oxidation is no longer coupled to phosphorylation, thermogenin is said to cause uncoupling. In the presence of thermogenin, oxidation and proton pumping continue at high rates but with low rates of ATP synthesis. The way in which thermogenin induces uncoupling is not yet certain. One theory suggests that Brown thermogenin acts as a proton ionophore. It picks up adipose tissue protons from the cytoplasm and carries them into (BAT) the mitochondria, dissipating the proton gradient. An alternative theory suggests that thermogenin dissipates the proton gradient by causing the futile cycling of fatty acids. Thermogenin carries an ionized fatty acid (R-COOϪ) from the mitochondrial side of the inner membrane and flips it across the bilayer to face the cytoplasm. Because of the higher proton concentration (lower pH), the ionized fatty acid is rapidly protonated (R-COOH). In this neutral form it readily flops back into the inner leaflet of the bilayer, Brown adipocytes where it ionizes again. The complete “flip-flop” cycle Arteriole causes a proton to be translocated across the inner Mitochondria mitochondrial membrane. Still other explanations Nucleus for UCP (uncoupling protein) function exist, and a definitive model awaits further experimentation. The thermogenic capacity of BAT has been Venule known for decades, and the protein thermogenin Figure 18 Brown adipose tissue in hamsters was first characterized in the early 1980s. It ap- Hamsters possess thick pads of BAT behind the shoulders. pears only in mammals and is expressed only in

685 Thermal Physiology

BAT. However, in recent years it has become clear sensing neurons is received and interpreted by a that thermogenin is only one member of a large thermostat within the central nervous system. The gene family of uncoupling proteins (UCPs). In ad- central thermostat triggers the appropriate behav- dition to thermogenin, also called UCP-1, mam- ioral and physiological response. mals express at least two other UCPs (UCP-2 and UCP-3). Both can increase mitochondrial proton leak, but not enough to make a significant contri- A central thermostat integrates central bution to heat production. Instead of having a role and peripheral thermosensory information in thermogenesis, these UCPs appear to reduce Animals possess different types of neurons to sense oxidative stress by preventing production of su- and respond to temperature. Temperatures are peroxide anions by mitochondria. The UCP gene monitored peripherally and centrally by tempera- family is ancient, with members in ectothermic ture-sensitive neurons, both cold sensing and warm animals, such as fish, as well as plants, fungi, and sensing. Birds and mammals monitor temperature protists. It is likely that thermogenin arose in the using similar neurons, although the location of the mammalian lineage as a duplicated and then mu- central thermostat differs in the two taxa.

tated version of other UCPs. Mammals monitor TA by peripheral cold- sensitive neurons located in the skin and the vis-

cera. When TA decreases, peripheral neurons send Regulating Body Temperature signals to the hypothalamus (Figure 19). The pre- Control of body temperature in endothermic ani- optic area of the anterior hypothalamus has both mals requires coordination of multiple physiologi- cold-sensing and warm-sensing neurons that

cal systems. Animals must be able to monitor TB in monitor core body temperature. Information from critical anatomical regions. By monitoring internal the peripheral and the central thermal sensors is

core TB, animals can assess their overall thermal integrated in the posterior hypothalamus, which balance. Peripheral thermoreceptors allow ani- sends signals to the body to alter the rates of heat

mals to detect TA. The information from thermal

Vasoconstriction of skin blood vessels BAT

38 37 36

Shivering

38 37 36

38 37 36 Panting

Vasodilation of skin Sweating blood vessels

Figure 19 Hypothalamus and thermoregulation The hypothalamus is the thermal control center of mammals. It interprets signals from peripheral and central thermosensitive neurons and sends neuronal signals to other tissues, altering heat flux.

686 Thermal Physiology production and dissipation. The hypothalamus is much more responsive to information from the central thermoreceptors than from the peripheral Guard hair thermoreceptors. Changes of less than 1°C can excite central thermoreceptors, triggering a rapid Accessory hairs hypothalamic response. Conversely, peripheral Sebaceous thermoreceptors may record and respond to a glands change of several degrees without invoking a hypothalamic response. Surface temperatures can Epidermis change by several degrees without harming the animal, whereas the temperature of the central Erector nervous system must be more stable. muscle

Bird TB regulation is less understood but is clearly different from that of mammals. Heating or cooling the hypothalamus has little effect on the thermoregulatory response of birds. The central thermostat in birds appears to be the spinal cord, Nerves not the hypothalamus. However, the thermostat is still responsible for integrating information from central and peripheral thermosensors. When the central thermostat detects changes in temperature, it responds by firing neurons that lead to a Subcutaneous fat compensatory response. Both birds and mammals alter TB by changing rates of heat production and Figure 20 Hair follicles A hair is produced by cells heat dissipation. in the hair follicle. Erector muscles attached to the base of the hair contract in response to neural stimulation, causing the hair to become upright. Sebaceous glands secrete lipids Piloerection reduces heat losses into the follicle ducts. Earlier in this chapter we discussed how body coverings, such as hair and feathers, act as insulation pit of the hair follicle is composed of epidermal for endotherms. Since the efficiency of the insula- cells. Intimately associated with each hair follicle tory layer depends on its thickness, animals can is a sebaceous gland, which releases complex se- regulate heat loss by changing the orientation of cretions of lipid (squalene, wax esters, triglyc- the hair (in mammals) or feathers (in birds). Birds eride, fatty acids) that form a protective coating on (and mammals) get fluffier in the cold by forcing the hair and provide moisturization. their feathers (and hair) to orient perpendicular to Tiny smooth muscles, called erector muscles, the body surface. The mechanism by which this connect each hair follicle to the undersurface of the orientation is controlled is best understood with epidermis. When the erector muscle contracts, the mammalian hair, but the position of bird feathers hair is pulled perpendicular, a process termed pi- is controlled in a similar way. loerection, so that the fur offers better insulation. Hair itself is a collection of cells that possess The erector muscle contractility is regulated by abundant keratin, an intermediate filament of the numerous factors, both bloodborne and neural in cytoskeleton. The distal end of a hair is primarily origin. The situation is similar in birds, where dead tissue, but the proximal end is composed of erector muscles also control the orientation of the living cells embedded within the hair follicle. De- feathers. pending on the species, a hair follicle can produce either a single hair shaft or complex combinations of hairs of various lengths and structures. Changes in blood flow affect Whereas human hair follicles produce single thermal exchange hairs, dog hair follicles produce a primary guard All animals exchange heat at the external surfaces of hair and multiple secondary hairs—soft, fine hairs the body, but they are able to alter the effectiveness that form the undercoat of the fur (Figure 20). The of surface heat exchange by changing the pattern of

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blood flow. Internal heat is equilibrated throughout ture of the skin, the greater the rate of heat loss. The the body by the blood. Where blood vessels ap- changes in vascular smooth muscle tone are con- proach the body surface, they will more readily lose trolled by the posterior hypothalamus. heat. Similarly, increasing the flow of blood through Changes in blood flow through these capillary the vessels increases the capacity for heat loss be- beds allow an endotherm to control heat ex- cause it warms the surface of the skin, the site of heat change. The effects are perhaps most obvious in loss by conduction, convection, and radiation. Caucasian humans, whose rapid changes in skin The regulation of the amount of blood flowing color reflect subdermal blood flow. Exercise in- into the vasculature is known as the vasomotor re- creases the core body temperature and triggers an sponse (Figure 21). Directly under the skin are cap- increase in blood flow to the skin, causing it to turn illary beds fed by subcutaneous arteries and red. Similarly, cold temperatures cause peripheral drained by veins that empty into a network called vasoconstriction, reducing blood flow to the hands the venous plexus. There is also direct exchange of and feet, causing them to turn white. Prolonged some blood between the veins and arteries through restriction of blood flow can cause the extremities connections called arteriovenous anastomoses, or to turn purple, as the blood pooled in the venous

metarterioles. At normal TB, the sympathetic ner- system is slowly deoxygenated. vous system constricts the arterioles to reduce blood flow. This tonic constriction is mediated by vascular Countercurrent exchangers smooth muscle in response to ␣ adrenergic signals. in the vasculature help retain heat When body temperature rises, there is a loss of tonic constriction and arterioles dilate to allow more In addition to restricting blood flow to the periph- blood into the skin vasculature. At the same time, ery, some animals are able to extract heat from the blood vessels of the anastomoses constrict, forc- warmed blood and transfer it to cooler blood. This ing more blood to move through the vessels near the is accomplished by arranging the vasculature into skin. The large volume and high compliance of the countercurrent heat exchangers (see Box 4, Math- venous system allows the blood to readily exchange ematical Underpinnings: Countercurrent Systems). heat to the skin surface. The greater the tempera- The exact arrangement depends upon the animal and tissue. Because fish breathe water, any metabolic heat Cold (low TA) Warm (high TA) is rapidly lost across the gills. Some regionally heterothermic fish, discussed earlier in this chapter, are active swimmers that produce abundant heat in their red muscle. In tuna, veins leaving the red mus- skin cle are juxtaposed to the arteries that supply the red Arteriole Arteriole muscle, allowing the transfer of myogenic heat from (constricted) (dilated) the veins back to the arteries (Figure 22). This al- AV shunt AV shunt lows red muscle to reach temperatures more than (dilated) (constricted) 10°C warmer than other tissues, including white muscle. Countercurrent heat exchangers are important in other regionally heterothermic fish. Billfish possess a modified eye muscle, called a heater or- gan, that warms the eye and optical nerves. Coun- tercurrent heat exchangers help retain heat in the Artery Vein Artery Vein optical system. Many large fish, such as bluefin tuna, use countercurrent heat exchangers in the Figure 21 Skin vasculature When blood travels close to the surface of the animal, heat is lost across the gastrointestinal tract to retain the heat of digestion. skin. When temperatures are cold (left), blood is diverted Countercurrent heat exchangers are used by from the skin through arteriovenous (AV) shunts, called endotherms to reduce heat loss at the periphery. arteriovenous anastomoses, reducing heat loss. When an Birds standing on cold surfaces, such as ice, can lose animal is in a hot environment, shunts are constricted and blood moves through the vessels closer to the skin surface, a great deal of heat through the feet (Figure 23). enhancing heat loss.

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BOX 4 MATHEMATICAL UNDERPINNINGS Countercurrent Systems

Many physiological processes depend on are required to transfer the entities between proximal countercurrent systems—a marriage of struc- and distal regions. The loop of Henle of the kidney tural and functional features that improves the effi- tubule is part of a countercurrent multiplier. The gradi- ciency of exchange processes. Consider a scenario in ents set up within the medulla result from active trans- which a tube drains a tank of hot water. As the water port of ions, and the resulting transfer of water. Because flows through the tube, heat is dissipated to the sur- a countercurrent multiplier requires transporters, it re- rounding environment, which in this example is the air. quires metabolic energy to create and maintain the gra- At some point along the length of the tube (if it is long dient. If flow through the tube ceases, or ion pumping is enough), the water reaches ambient temperature. How- reduced, the gradient can collapse. The efficiency of the ever, the tube can be rearranged to reduce the magni- countercurrent multiplier also depends on the length of tude of heat loss. Imagine what would happen to heat the proximal and distal arms of the system that gener- exchange if you were to align the lower (distal) end of the ate the gradient. tube alongside the upper (proximal) end, creating a hairpin structure. Water flowing through one segment would run in the opposite direction of the water in the other segment. With this arrangement, some of the Hot Hot heat lost from the proximal segment is gained by the distal segment. Instead of a gradient from one end to the other, a thermal gradient forms along the length of Warm the hairpin loop, coolest at the turn and warmest near the top. This is the basis of a countercurrent system. The longer the hairpin loop, the greater the gradients that can be built. Some researchers distinguish between two types of countercurrent systems: exchangers and multipliers. Countercurrent exchangers transfer entities between inflow and outflow using only passive processes. The countercurrent heat exchanger, described above, is an example of such an exchanger; no specific transporter or pathway mediates the transfer of the entity (heat), and the gradient is due to the physical arrangement of the plumbing. The efficiency of the countercurrent exchanger depends on the volume of flow through the tubes and the overall gradients along the length of the Cold hairpin. Without counter- With counter- Countercurrent multipliers are like exchangers in current exchange current exchange most respects except that specific transport proteins

They can reduce heat loss by restricting blood flow Sweating reduces body temperature to the periphery, but over long periods this would by evaporative cooling cause the peripheral tissues to starve. Countercur- Small animals have a favorable ratio of surface area rent heat exchangers transfer heat from arteries to volume for heat loss, so evaporative cooling is emerging from the body core to veins returning used primarily by large animals. Many larger ani- from the cold periphery. Warming of the venous mals use specialized skin fluid secretions (sweat) to blood lessens the impact of the peripheral cooling. enhance evaporative cooling. Humans are probably Also, cooling the arterial blood decreases the ther- the smallest animals that effectively use sweating to mal gradient across the skin and therefore reduces cool their bodies. Sweat is a mixture of water, salts, heat loss. and some oils. The salt in sweat raises the boiling

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Spinal cord Venule Arteriole Spine Direction of Rete blood flow

Dorsal aorta

Postcardial vein Red muscle Cutaneous Red muscle veins Cutaneous arteries Figure 22 Countercurrent heat exchangers in tuna smaller lateral vessels run over the surface of the red muscle, muscle Each heterothermic scombrid species relies on with branches penetrating the muscle. (b) These lateral different combinations and numbers of retes to retain heat. (a) vessels are arranged in a countercurrent manner, with lateral Red muscle of bluefin tuna is served by cutaneous arteries and venules transferring myogenic heat to lateral arterioles. veins that run beneath the skin. From these main vessels, (Source: Part (a) modified from Carey, 1973)

point of water, making evaporative cooling more sympathetic nerves that control the activity of efficient. Loss of water and salts can affect ion and sweat glands. osmoregulation, but animals exposed to hot weather for long periods can change the chemical composition of their sweat to minimize ionic and Panting increases heat loss across the osmotic problems. They produce a larger volume respiratory surface of sweat with a lower NaCl content, preserving vi- Another way animals lose heat is through ventila- tal salts. Sweating is controlled by the anterior hy- tion. The properties that make a respiratory sur- pothalamus and triggered by activation of the face good at gas exchange—high vascularity, moist surfaces, and high airflow—also enhance heat loss. Whether respiratory heat loss is beneficial or detrimental Blood flow depends on the situation. In the cold, 32°C birds and mammals minimize heat

loss from respiration, but at high TA, animals may alter their breathing pattern to accentuate heat loss. Artery Cooling through ventilation is a strategy that must balance respira- Vein tory demands with thermoregulation. Cooling is enhanced when 1°C animals increase ventilation frequency while reducing tidal volume. Shallow, rapid breathing is a sign that an animal may be overheated. Gular fluttering is a cooling behavior seen in birds, characterized by rapid contraction and relaxation of Figure 23 Peripheral vasoconstriction in cold endotherms Birds standing on cold surfaces can alter the flow of blood into the feet, reducing heat loss. the throat muscles. Mammals pant. The blood vessels of the leg and foot are arranged in parallel, allowing the formation Each of these behaviors cools the of a countercurrent heat exchanger. animal multiple ways. First, rapid

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ventilation increases the heat loss Nasal (40/min) across the respiratory surface by convection. Second, and perhaps In more important, the rapid ventila- Out tion causes water to evaporate from the surface of the airway, from the

pulmonary surface to the tongue. An- Airflow Mouth imals that rely on ventilatory cooling often possess well-vascularized res- In piratory surfaces that are kept wet Out through secretions. These ventilatory patterns could alter the nature Time of the blood gas profile, impinging on respiratory physiology. The increase (a) Low temperature in ventilation frequency is offset in part by a reduction in tidal volume. Nasal Reindeer provide a good example of the links between respiration In and thermoregulation. Although Out they live in the cold, reindeer are at risk of heat stress because of their large size and thick layer of fur insu- Airflow Mouth (~200/min) lation. At normal cold temperatures (10°C), a reindeer breathes through In its nose at low frequency. The upper Out part of the nasal cavity is rich in capillaries, and nasal respiration helps cool the nearby brain regions. When Time a reindeer becomes too warm, it (b) High temperature shifts its respiratory pattern. Breath- Figure 24 Heat loss during panting Like other mammals, reindeer alter ing frequency increases, and the an- breathing to increase heat loss. Reindeer breathe through the nose at low imal begins to pant through the temperatures. The flow of air cools the blood circulating through the vessels that line mouth (Figure 24). Although this the nasal cavity. When temperatures increase, reindeer breathe through the mouth and at a faster rate (200–300 breaths per minute). change in breathing pattern may re- (Source: Aas-Hansen et al., 2000) duce direct cooling of the brain, it reduces body core temperature more efficiently. called relaxed endothermy. The time course and

magnitude of reduction in TB differ among animals and types of dormancy (Figure 25). An arctic Relaxed endothermy results squirrel, for example, can allow TB to fall close to in hypometabolic states the freezing point. However, even minor reduc-

In your course, you have encountered various tions in TB can offer important energetic savings forms of hypometabolism used by endotherms to for a dormant animal. survive adverse conditions. Hummingbirds, for Under normal (euthermic) conditions, mam- example, undergo a nightly reduction in metabolic mals and birds maintain TB within a narrow rates. Hibernating mammals also undergo a meta- range. A euthermic animal induces a compensa- bolic suppression during the long, cold winter tory response when its central thermostat—the months when food is scarce. Whether a daily dor- hypothalamus in mammals—senses a decrease in mancy (torpor) or a more prolonged seasonal dor- TB. During periods of relaxed endothermy, the an- mancy (hibernation), the hypometabolic phase is imal recalibrates its central thermostat to recog- accompanied by a decrease in TB, a phenomenon nize and defend a different TB set point. The

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5 35 endothermic animal may allow TB to fall close to TA, well below the euthermic set point. 4 30 The links between metabolism and T regula- TB B tion make it difficult to establish which parameter 3 Metabolic rate 25 causes hypometabolic cooling. For most animals entering dormancy, T and metabolic rate decline 2 20 B

Metabolic rate in parallel, and it is not clear if the colder TB slows uptake in ml/g per h) 2 metabolism, or alternately if the slower metabolic 1 15 ( ° C) Body temperature (O heat production causes cooling. In some studies, 010animals show a reduction in metabolic rate be- 0 1 2 3 4 Time (months) fore TB declines, suggesting that hypometabolism initiates the reduction in T . However, in (a) Hibernation B larger animals a delay in cooling upon entering dormancy is due in part to thermal inertia; the 36 large mass and low ratio of surface area to volume delay the impact of reduced thermogenesis, 2 34 allowing the animal to remain much warmer

32 than TA even with a reduced metabolic rate.

1 30 Metabolic rate uptake in ml/g per h) 2

28 ( ° C) Body temperature 2 CONCEPT CHECK (O

0208. How do endotherms generate heat? 0 6 12 18 9. What regions of the body detect and respond to Time (hours) changes in temperature? (b) Torpor 10. What are the various types of hypometabolism? Figure 25 Hypometabolic states Many 11. How do animals control heat flux across the endotherms respond to cold temperatures by entering external body surface? some form of dormancy. Body temperature generally declines in parallel with metabolic rate. The dormancy is called (a) hibernation when the metabolic depression lasts for weeks to months or (b) torpor when the animal enters a hypometabolic state in daily cycles.

Integrating Systems Immune System and Thermoregulation

Deviations from optimum TB usually impair physiological processes (2–5). For these processes, an increase of functions, but in some cases endotherms can induce hy- 2–3°C in regional or systemic temperature would have a perthermia as part of the defense against pathogens. The relatively minor beneficial effect. However, some aspects

cellular and noncellular defense pathways are collec- of immune function demonstrate Q10 values ranging from tively called the immune response (Figure 26). When a 100 to 1000 and would be profoundly enhanced by the de- pathogen mounts a localized attack, such as at the site of gree of hyperthermia seen in inflammation and fever. a cut, the immune response includes an elevation in tem- Recall that blood carries T lymphocytes as part of the perature in that region: inflammation. A broad systemic adaptive immune system. When pathogens are present, pathogen attack induces a more elaborate immune re- some T lymphocytes undergo a maturation process that

sponse, which includes an increase in body core temper- enables them to become cytotoxic T cells. With a Q10 in ature: a fever. Whether arising from inflammation or a excess of 100, this maturation step is acutely sensitive to fever, the hyperthermic response serves to improve the temperature. The hyperthermia arising from an immune ability of the animal to combat the pathogen. Most im- response reflects a remarkable coordination between munological processes, such as the rate of movement of cellular signaling pathways, cardiovascular changes, and

or ingestion by phagocytotic immune cells, have Q10 val- central control of body temperature. ues within the range for other cellular and biochemical

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Pathogen

Infection

Macrophage

Cytokine production Paracrine Endocrine Blood vessels Interleukin 1 BBB

Vasodilation Permeability

Arterioles Capillaries Immune cells Capillaries

Temperature Edema Prostaglandin E2

Thermal set point

Blood flow Immune cell migration Hypothalamus

Blood vessels BAT Muscle

Vasoconstriction NST Shivering

Core temperature

Figure 26 Fever

Inflammation is a localized response to the pres- through the capillary beds. Since many of the surface tis- ence of something that activates the immune system. sues are cooler than body core temperature, the in- Consider what happens when a virus invades your respi- crease in blood flow may elevate skin by as much as ratory epithelium. The virus must first pass through the 10°C. The cytokines also alter the permeability of the physical barriers that separate your respiratory tissues capillary beds, allowing immune cells in the blood to from the environment. The mucus secreted from exo- squeeze between endothelial cells and enter the inter- crine cells in the respiratory epithelium reduces the pas- stitial fluids. The main features of inflammation are at- sive water loss but also acts as a barrier to external tributable to the changes in the vasculature: redness and viruses, bacteria, and particulates. If the virus passes warmth due to increased blood flow, swelling (edema) through the mucus, it must be able to enter living epithe- due to fluids moving from the main circulation through lial cells to propagate. The epithelium of the respiratory more permeable capillaries into the interstitial fluid. tract is thinner than that of the skin, the stratum If a regional infection spreads, or the infection oc- corneum, making it more vulnerable to viral attack. If the curs systemically, the animal mounts a more elaborate virus enters an epithelial cell, it will take over the normal immune response that includes an increase in body cellular protein synthetic machinery and cause the cell temperature: a fever. The body detects the presence of to produce the viral proteins needed for viral replication. a pathogen when cells of the immune system bind spe- After a time, the cell may lyse, releasing additional viral cific pathogen macromolecules, called exogenous pyro- particles. During this stage, infected and damaged gens. The most common exogenous pyrogens from cells release a number of factors (cytokines and bacterial infections are lipopolysaccharides, proteogly- prostaglandins) that alter cellular processes within the cans, and proteins, such as endotoxin. Recall that region. Though it doesn’t affect the virus directly, mucus macrophages phagocytose pathogenic bacteria. When a cells enhance secretions to bolster the physical barriers macrophage consumes bacteria, the bacterial macro- to reduce the likelihood of further infection. Recall that molecules cause the macrophage to secrete cytokines, cytokines, such as interleukin 1, also signal the local vas- such as interleukin 1. In addition to the local pro-inflam- culature, altering blood flow regionally. Through va- matory effects, interleukin works as an endogenous sodilatory effects on arterioles, more blood flows pyrogen. It causes other cells to synthesize

693 Thermal Physiology

another factor—a mediator—that exerts its effects on conservation. Though the neurocircuitry remains un- the brain. For example, interleukin 1 induces many cell clear, a fever also stimulates shivering thermogenesis. types in the periphery and in the vasculature of the brain Sympathetic neuronal activity stimulates thermogene-

to synthesize prostaglandin E2. It is not yet clear how sis in brown adipose tissue and vasoconstriction in cu- this mediator crosses the blood-brain barrier (BBB)—it taneous vasculature. may be through synthesis and secretion by the endothe- Since this immune response works by increasing

lial cells, or transport across the capillary endothelium— TB, it would seem to be practical only in animals that can

but once across the BBB, prostaglandin E2 binds to retain body heat. Indeed, this aspect of immunity is best neurons of the hypothalamus, where it alters the neuro- developed in endothermic animals, primarily birds and circuits that integrate peripheral and central thermal mammals. However, ectothermic vertebrates and many information. The pyrogenic mediator reduces the firing invertebrates show a behavioral fever in response to frequency of warm-sensitive neurons and increases pathogens. If a lizard is infected with bacteria or in- the firing frequency of cold-sensitive neurons. Thus, jected with an exogenous pyrogen, it demonstrates an

the pyrogenic mediator causes the hypothalamus to increase in its preferred TB, moving into a warmer envi- misinterpret the central and afferent thermal informa- ronment. Thus, the benefit effect of hyperthermia in the tion, and as a result the brain perceives that the body is immune response is likely a very ancient trait, though too cool. This triggers a range of compensatory re- the mechanisms by which body temperature is elevated sponses that increase the rate of heat production and differ among lineages. 2

Summary

k Heat Exchange and Thermal Strategies Poikilotherm TB changes with environmental k All animals are subject to the physical laws that conditions, whereas homeotherm TB remains govern heat fluxes, although the effects on en- nearly constant. dotherms have a greater impact on their physi- k Ectotherm T is determined by environmental ology. Animals that live in water lose heat more B conditions, whereas endotherm T is determined readily than those that live in air, because wa- B by metabolic heat production and conservation. ter has a higher thermal conductivity. k Heterotherms exhibit combinations of en- k Although heat production is a function of body dothermy and ectothermy. Temporal het- mass, heat exchange depends on body surface erotherms are typically endothermic for most of area. Consequently, an animal’s ratio of surface their lives, but undergo periods of hypothermia, area to volume has an important impact on such as in hibernation or torpor. Regional het- thermoregulation. erotherms are able to warm regions of the body

k External insulation, such as feathers and hair, cre- above ambient temperature (TA). ates a dead space that reduces the thermal gradi- k Endotherms have a thermoneutral zone—a ent between the animal and the environment. range of temperatures at which physiological k Movement of fluids, such as air and water, in- functions are optimal. Outside this temperature creases the rate of convective heat loss. range, the metabolic costs are greater and animal function suffers. k Thermal radiation is an important source of heat for animals, derived directly from the sun k Thermal effects in ectotherms are influenced by and also from an animal’s surroundings. Many thermal history animals can absorb thermal energy from solar k Eurytherms tolerate a wide T range, whereas radiation by basking, often aided by dark col- A stenotherms survive in a narrow T range. oration. Many animals employ evaporative A cooling to reduce overheating.

694 Thermal Physiology

Coping with a Changing Body Temperature k Heat is a natural consequence of metabolism, k Changes in TA have greater consequences for but endotherms have higher metabolic rates ectotherms than for endotherms, altering many than ectotherms of similar size. Greater mem- aspects of macromolecular structure and me- brane leakiness is one reason why endotherms tabolism. Temperature alters membrane fluid- have high metabolic rates. ity, which animals can mitigate by altering the k Endotherms use neural systems to detect exter- fatty acid chain length, saturation, phospholipid nal and internal temperatures. The mammalian profile, and cholesterol content. hypothalamus integrates central and periph- k Temperature also changes the rates of chemical eral thermal sensory information to cause phys- reactions, and consequently metabolic rate. It also iological systems to alter heat production and affects protein structure and enzyme kinetics. retention. When proteins undergo thermal denaturation, k Peripheral cold-sensitive neurons trigger the genes for stress proteins are induced. These changes in the orientation of hair, or piloerec- proteins help refold damaged proteins and target tion, to reduce heat loss. irreversibly damaged proteins for degradation. k Nerves and hormones also control the blood k Ectotherms living in cold environments often flow to the skin surface to regulate the rate of remodel their physiological systems to compen- heat loss. sate for the effects of temperature. Animals that have lived for long periods in extreme cold of- k Vasculature may be arranged into countercur- ten possess cold-adapted proteins. rent exchangers to help retain heat within the body core. k Many animals are able to survive freezing temperatures. The greatest risk is the uncontrolled k Overheated large mammals use sweating to en- formation of ice crystals, which can induce os- hance evaporative cooling, whereas smaller motic stress and can physically damage cellular mammals shed heat by panting. membranes. k Some endotherms can undergo short periods of k Freeze-tolerant animals use ice nucleators to metabolic suppression, during which body tem- control ice crystal growth. Freeze-avoiding ani- peratures decrease, although rarely to the point mals produce antifreeze proteins that prevent where the temperature matches the ambient tem- intracellular ice formation. perature. During relaxed endothermy, an animal resets its central thermostat to a new set point. Maintaining a Constant Body Temperature k Endothermic animals produce metabolic heat and retain it to elevate body temperature above the ambient temperature.

Review Questions

1. Compare and contrast the following terms: 5. Discuss the different sources of energy an ec-

homeothermy and poikilothermy; endothermy totherm can use to raise TB. and ectothermy; regional heterothermy and 6. What behaviors reduce heat losses due to temporal heterothermy. (a) conduction; (b) convection? 2. Water at 10°C feels colder than air at 10°C. 7. How do we know that antifreeze proteins Why? arose several times in evolution? 3. Why are antifreeze proteins found in marine 8. How do countercurrent heat exchangers work? fish but not freshwater fish? 4. Compare and contrast the mechanisms of thermogenesis. Which biochemical steps are responsible for heat production?

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Synthesis Questions

1. Compare the effects of high and low tempera- 7. What gene regulatory changes must have ac- ture on molecules, cells, tissues and organisms. companied the evolution of brown adipose 2. How could you convert a stenothermal animal tissue? to a eurythermal animal? 8. Animal color influences many aspects of phys- 3. Summarize the physiological changes that ac- iology and ecology. Identify some examples of company thermal acclimation. animals whose color patterns are consistent with a role in thermoregulation. 4. Why do endothermic animals need both peripheral and central temperature-sensitive 9. Many mammals grow coats that differ in win- neurons? ter and summer. What factors affect the costs and benefits of seasonal shedding? 5. Thermoregulation requires active control of blood flow through vessels. How do animals 10. Compare and contrast the structures of hair dilate some blood vessels while constricting and feathers. others? 6. What would you expect to happen to blood pressure when a mammal is exposed to cold temperatures?

Quantitative Questions

1. The metabolic rate of a fish heart is studied at ing ATP level was 5 ␮mol/g tissue. Calculate various temperatures. The metabolic rate is the change in ATP levels over time that would 20 ␮mol ATP per min per g tissue at 25°C, result if the animal were moved to an environ- ␮ 8 mol ATP per min per g tissue at 10°C, ment that caused a 10°C increase in TB. ␮ 4 mol ATP per min per g tissue at 5°C, and 3. Recall the Stefan-Boltzmann equation, P ϭ 1 ␮mol ATP per min per g tissue at 2°C. Cal- σ 4 Ϫ 4 Ae (TB TA ) , where P is the radiating power, culate the Q10 values over this range of tem- A is its surface area, e is the ability of the ob- peratures and offer an explanation for the ject to emit radiation, σ is the Stefan constant, patterns. and T is the temperature of the body (TB) or 2. The levels of ATP are maintained through a surroundings (TA) in kelvins. Consider an ani- balance between the rates of ATP synthesis mal that uses a strategy of changing posture to and ATP utilization. For a given tissue (e.g., alter the surface area as a way of controlling heart) at a given TB (e.g., 15°C), assume that heat loss. It assumes a particular posture (a) the rates of ATP synthesis and utilization when it is in an environment that is 5°C below are both 10 ␮mol/min/g, (b) the rate of ATP its body temperature. How does it need to ϭ synthesis exhibits a Q10 2, (c) the rate of ATP change its surface area when it moves to a ϭ utilization has a Q10 2.05, and (d) the start- new environment that is 20°C cooler?

For Further Reading

See the Additional References section at the end This seminal paper uses a quantitative of the chapter for more readings related to the biophysical approach to describe how energy topics in this chapter. exchange between animals and the environment governs the thermal biology of the animal. Heat Exchange and Thermal Strategies Porter, W. P., and D. M. Gates. 1969. An interesting book, written for a lay audience, Thermodynamic equilibria of animals with that discusses the role of energy in biology. environment. Ecological Monographs 39: Brown, G. 1999. The energy of life. London: 227–244. HarperCollins.

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Much of thermal biology focuses on vertebrates, Willmer, P., G. Stone, and I. A. Johnston. 2000. and the insects are often understudied. Environmental physiology of animals. Oxford: Heinrich’s book reminds us of the many thermal Blackwell Science. strategies employed by insects. Heinrich, B. 1992. The hot-blooded insects. Cambridge, MA: Harvard University Press. Maintaining a Constant Body Temperature This book discusses the impact of changes in This book considers the regulatory mechanisms temperature on macromolecular structure and animals use to modify metabolic rate. For most biochemical processes. animals, this involves a reduction in body Hochachka, P. W., and G. N. Somero. 2002. temperature. Biochemical adaptation. Oxford: Oxford Hochachka, P. W., and M. Guppy. 1987. University Press. Metabolic arrest and the control of biological time. Oxford: Oxford University Press. These papers discuss the peculiarities of animals that live in the extreme cold, and consider This paper discusses recent studies on how the whether the data on metabolic rate of these hypothalamus senses and responds to thermal animals support the notion of metabolic cold conditions. adaptation. DiMicco, J. A., and D. V. Zaretsky. 2007. The Steffensen, J. F. 2002. Metabolic cold adaptation dorsomedial hypothalamus: A new player in of polar fish based on measurements of thermoregulation. American Journal of aerobic oxygen consumption: Fact or artefact? Physiology: Regulatory, Integrative, and Artefact! Comparative Biochemistry and Comparative Physiology 292: R47–R63 Physiology, Part A: Molecular and Integrative Physiology 132: 789–795. These papers consider the evolutionary origins of Various authors. 2002. Coping with the cold: hyperthermia and how hyperthermia in fever Molecular and structural biology of cold stress and inflammation may accentuate the immune survivors, D. J. Bowles, P. J. Lillford, D. A. response. Rees, and I. A. Shanks, eds. Philosophical Kluger, M. J., W. Kozak, C. A. Conn, L. R. Leon, Transactions of the Royal Society of London, and D. Soszynski. 1998. Role of fever in Series B: Biological Sciences 357: 829–956. disease. Annals of the New York Academy of Sciences 856: 224–233. This textbook considers how the environment Hanson, D. F. 1997. Fever, temperature, and the influences physiological processes. The sections immune response. Annals of the New York on thermal biology are particularly useful. Academy of Sciences 813: 453–464.

Additional References

Aas-Hansen, O., L. P. Folkow, and A. S. Blix. 2000. Panting in Feder, M. E., and G. E. Hofmann. 1999. Heat-shock proteins, reindeer (Rangifer tarandus). American Journal of molecular chaperones, and the stress response: Physiology: Regulatory, Integrative, and Comparative Evolutionary and ecological physiology. Annual Review of Physiology 279: R1190–R1195. Physiology 61: 243–282. Boutiler, R. G., and J. St-Pierre. 2000. Surviving hypoxia Hand, S. C. 1991. Metabolic dormancy in aquatic without really dying. Comparative Biochemistry and invertebrates. Advances in Comparative and Physiology, Part A: Molecular and Integrative Physiology Environmental Physiology 8: 1–50. 126: 481–490. Harrison, J. E., J. H. Fewell, S. P. Roberts, and H. G. Hall. Carey, F. G. 1973. Fishes with warm bodies. Scientific 1996. Achievement of thermal stability by varying American 228: 36–44. metabolic heat production in flying honeybees. Science Davies, P. L., J. Baardsnes, M. J. Kuiper, and V. K. Walker. 274: 88–90. 2002. Structure and function of antifreeze proteins. Heinrich, B. 1987. Thermoregulation in winter moths. Philosophical Transactions of the Royal Society of London, Scientific American March 1987. Series B: Biological Sciences 357: 927–935. Helmuth, B., J. G. Kingsolver, and DiMicco, J. A., and D. V. Zaretsky. 2007. The dorsomedial E. Carrington. 2005. Biophysics, physiological ecology, hypothalamus: A new player in thermoregulation. and climate change: Does mechanism matter? Annual American Journal of Physiology: Regulatory, Integrative, Review of Physiology 67: 177–201. and Comparative Physiology. 292: R47–R63

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Hochachka, P. W. 1986. Defense strategies against hypoxia Rolfe, D. F. S., and G. C. Brown. 1997. Cellular energy and hypothermia. Science 231: 234–241. utilization and molecular origin of standard metabolic rate Hochachka, P. W., P. L. Lutz, T. J. Sick, and M. Rosenthal. in mammals. Physiological Review 77: 731–758. 1993. Surviving hypoxia: Mechanisms of control and Scholander, P. F., V. Walters, R. Hock, and L. Irving. 1950. adaptation. Boca Raton, FL: CRC Press. Body insulation of some arctic and tropical mammals and Klingenberg, M., and K. S. Echtay. 2001. Uncoupling proteins: birds. Biological Bulletin 99: 225–236. The issues from a biochemist point of view. Biochimica et Schulte, P. M., H. C. Glemet, A. A. Fiebig, and D. A. Powers. Biophysica Acta–Bioenergetics 1504: 128–143. 2000. Adaptive variation in lactate dehydrogenase-␤ gene Knight, M. R. 2002. Signal transduction leading to low- expression: Role of a stress-responsive regulatory element. temperature tolerance in Arabidopsis thaliana. Proceedings of the National Academy of Sciences, USA 97: Philosophical Transactions of the Royal Society of London, 6597–6602. Series B: Biological Sciences 357: 871–875. Storz, G. 1999. An RNA thermometer. Genes and Logue, J. A., A. L. de Vries, E. Fodor, Development 13: 633–636. A. R. Cossins. 2000. Lipid compositional correlates of Sullivan, J. P., S. E. Fahrbach, J. F. Harrison, E. A. Capaldi, J. temperature-adaptive interspecific differences in H. Fewell, and G. E. Robinson. 2003. Juvenile hormone membrane physical structure. Journal of Experimental and division of labor in honey bee colonies: Effects of Biology 203: 2105–2015. allatectomy on flight behavior and metabolism. Journal of Reinertse, R. E., and S. Haftorn, 1986. Different metabolic Experimental Biology 206: 2287–2296. strategies of northern birds for nocturnal survival. Journal of Comparative Physiology, Part B: Biochemical, Systemic, and Environmental Physiology 156: 655–663. Credits

Credits listed in order of appearance. 624 Bruce Coleman Inc., Scott Nielsen/Bruce Coleman Inc. 625 Dorling Kindersley. 625 Getty Images, O. Louis Mazzatenta/National Geographics/Getty Images. 629 Photo courtesy of Dr. Brian Helmuth, University of South Carolina. 630 Photo Researchers, Inc., Gregory G. Dimijian M.D./Photo Researchers, Inc.

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