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. 2 661 Thermal Physiology 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 662 Thermal Physiology Heat Exchange and Thermal Strategies The most important physiological pa- rameter 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—tol- erance 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,

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