Summary

This document is a lecture reading on the subject of animal biology. It includes concepts such as biological clocks, how body size relates to other traits within a species, and interactions with the environment. It explores these themes through examples and diagrams.

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Animals and Environments 19 80 they complete one timing cycle, they start 70 another, just as a wristwatch starts to time a...

Animals and Environments 19 80 they complete one timing cycle, they start 70 another, just as a wristwatch starts to time a Plains new day after it has completed timing of the 60 zebra Mountain Length of gestation (weeks) on log scale previous day. These sorts of biological clocks zebra African 50 buffalo emit signals that cause cells and organs to Greater undergo internally programmed, repeating kudu cycles in their physiological states, thereby 40 Mountain giving rise to periodic, clock-controlled reedbuck changes in an animal’s phenotype. An Wildebeest 30 enzyme under control of a biological clock, for instance, might increase in concentration Dikdik each morning and decrease each evening, not Bushbuck Warthog because the animal is responding to changes 20 in its outside environment, but because of the Gray duiker action of the clock. The changes in enzyme concentration might mean that an animal is 15 inherently better able to digest a certain type of food at one time of day than another, or is 5 10 20 50 100 200 500 1000 better able to destroy a certain type of toxin in Adult female body weight (kg) on log scale the morning than in the evening. Biological Figure 1.11 Length of gestation scales as a regular function clocks typically synchronize themselves with of body size in mammals The data points—each representing the external environment, but they go through their timing cycles a different species—are for African herbivorous mammals weighing inherently, and they can time physiological changes for days on 5–1000 kg as adults. The line (fitted by ordinary least squares regres- end without environmental input. We will discuss them in greater sion; see Appendix D) provides a statistical description of the overall detail in Chapter 15. trend and thus depicts the gestation length that is statistically expect- ed of an average or ordinary animal at each body size. Both axes Size in the lives of animals: Body size is one of use logarithmic scales, explaining why the numbers along the axes are not evenly spaced (see Appendix E). The red-colored data points an animal’s most important traits are for animals discussed in the text. (After Owen-Smith 1988.) How big is it? is one of the most consequential questions you can ask about any animal. This is true because within sets of related species, many traits vary in regular ways with their body sizes. With this information on expected gestation lengths, now we can The length of gestation, for example, is a regular function of body address the question asked earlier: Are the bushbuck and mountain size in mammals (Figure 1.11). Brain size, heart rate, the rate reedbuck specialized or ordinary? Notice that the length of gestation of energy use, the age of sexual maturity, and hundreds of other in the bushbuck is very close to what the line in Figure 1.11 predicts Hill Animal Physiology 4E physiological and morphological traits are also known to vary Sinauer Associates in for an animal of its size. The bushbuck, therefore, adheres to what regular, predictable ways with body size in mammals Morales and Studioother is expected for its size: It has an ordinary gestation length when its phylogenetically related sets of animal species. The study Figure of these 01.11 11-09-15 size is taken into account. The mountain reedbuck, however, is far regular relations is known as the study of scaling because related off the line. According to the line, as shown in Table 1.3, an animal species of large and small size can be viewed as scaled-up and scaled- of the reedbuck’s size is expected to have a gestation lasting 26.5 down versions of their type. weeks, but actually the reedbuck’s gestation lasts 32 weeks. Thus Knowledge of the statistical relationship between a trait and the reedbuck seems to have evolved a specialized, exceptionally body size is essential for identifying specializations and adapta- tions of particular species. To illustrate, let’s ask if two particular African antelopes, the bushbuck and mountain reedbuck, have TABLE 1.3 Predicted and actual gestation specialized or ordinary lengths of gestation. Answering this lengths for two African antelopes question is complicated precisely because there is no single norm of about the same body size of mammalian gestation length to use to decide. Instead, because Predicted Actual the length of gestation varies in a regular way with body size, a gestation length gestation biologist needs to consider the body sizes of the species to know Species (weeks)a length (weeks) what is average or ordinary. Bushbuck 27 26 Statistical methods can be used to derive a line that best fits a (Tragelaphus set of data. In the study of scaling, the statistical method that has scriptus) traditionally been considered most appropriate is ordinary least Mountain 26.5 32 squares regression (see Appendix D). The line in Figure 1.11 was reedbuck calculated by this procedure. This line shows the average trend in (Redunca the relationship between gestation length and body size. The line fulvorufula) is considered to show the length of gestation expected of an ordinary a Predicted lengths are from the statistically fitted line shown species at each body size. in Figure 1.11. 20 Chapter 1 long gestation. Similarly, the gray duiker seems to have evolved an exceptionally short length of gestation for its size (see Figure 1.11). In the last 20 years, physiologists have recognized that ordinary least squares regression may not always be the best procedure for fitting lines to scaling data because the ordinary least squares procedure does not take into account the family tree of the species studied; it simply treats each data point as being fully independent of all the other data points (see Appendix D). Increasingly, therefore, physiolo- gists have fitted lines not only by the ordinary least squares procedure but also by an alternative procedure based on phylogenetically independent contrasts, a method that takes the family tree into account (see Appendix G).8 Although these two approaches sometimes yield distinctly different results, they most often yield similar results, and in this book, the lines we present for scaling studies will be derived from the method of traditional, ordinary least squares regression. Figure 1.12 Krill and fish in the sea around Antarctica spend their Body-size relations are important for analyzing almost entire lives at body temperatures near –1.9°C Despite the low tem- all sorts of questions in the study of physiology, ecology, and peratures, these shrimplike krill (Euphausia superba) occur in huge, gregarious evolutionary biology. If all one knows about an animal spe- populations that blue whales greatly favor for food. The krill—which grow to cies is its body size, one can usually make useful predictions lengths of 3–6 cm—eat algal cells in the water and also graze on algae grow- about many of the species’ physiological and morphological ing on ice surfaces. They hatch, grow, feed, and mate at body temperatures traits by consulting known statistical relationships between near –1.9°C. the traits and size. Conversely, there is always the chance that a species is specialized in certain ways, and as soon as one has actual data on the species, one can identify potential ranges of variation of temperature, oxygen, and water across the specializations by the type of scaling analysis we have discussed. face of the globe. We also discuss highlights of how animals relate to these features. In later chapters, we will return to these topics in greater detail. Environments What is an environment? An important starting point in answering temperature  The temperature of the air, water, or any other this question is to recognize that an animal and its environment are material is a measure of the intensity of the random motions that interrelated, not independent, entities. They are in fact defined in the atoms and molecules in the material undergo. All atoms and terms of each other: The environment in any particular case cannot molecules ceaselessly move at random on an atomic-molecular be specified until the animal is specified. A dog, for instance, is an scale. A high temperature signifies that the intensity of this animal from our usual perspective, but if the animal of interest is atomic-molecular agitation is high. Most animals are temperature a tapeworm in the dog’s gut, then the dog is the environment. All conformers, and as we discuss temperature here, we will focus on animals, in fact, are parts of the environments of other animals. them. The conformers are our principal interest because the level The birds in the trees around your home are part of your environ- of atomic-molecular agitation in their tissues matches the level in ment, and you are part of theirs. The interdependence of animal the environments where they live. and environment is reflected in standard dictionary definitions. The lowest temperature inhabited by active communities of A dictionary defines an animal to be a living organism. An en- relatively large, temperature-conforming animals is –1.9°C, in the vironment is defined to be all the chemical, physical, and biotic polar seas.9 The open waters of the polar oceans remain perpetually components of an organism’s surroundings. at about –1.9°C, the lowest temperature at which seawater is liquid. Thus the shrimplike krill (Figure 1.12), the fish, the sea stars and Earth’s major physical and sea urchins, and the other invertebrates of these oceans have tissue chemical environments temperatures near –1.9°C from the moment they are conceived until The physical and chemical environments on our planet are remark- they die. They do not freeze. Whereas some, such as the krill, do ably diverse in their features, providing life with countless challenges not freeze because their ordinary freezing points are similar to the and opportunities for environmental specialization. Temperature, oxygen, and water are the “big three” in the set of physical and 9 The very lowest temperature at which any active communities of chemical conditions that set the stage for life. Here we discuss the temperature-conforming animals live occurs within the sea ice near the poles. Minute nematodes and crustaceans, as well as algae, live and 8 Appendix G explains the reasons why the family tree should ideally reproduce within the sea ice at temperatures that, in some places, are a be taken into account, as well as providing a conceptual introduction to few degrees colder than the temperature of –1.9°C that prevails in the phylogenetically independent contrasts. surrounding polar water. 184 Chapter 7 (A) Vesalius 1543: One of the first anatomically accurate (B) Mandelbrot 1983: A fractal model of a branching images of the human circulatory system system such as the circulatory system Fractal mathematics is being used to try to understand how the circulatory system changes in its inherent capabilities for transport as animals evolve to be bigger or smaller. Figure 7.13 As the circulatory system is scaled up and down in size and extent, con- straints predicated on fractal geometry may help give rise to allometric metabolic scaling A mammal’s metabolism is dependent on the distribution of required resources to tissues throughout the body. When Andreas Vesalius first described the circulatory system (A), its function was a mystery. Even oxygen transport could not be considered because oxygen had not yet been discovered! By now, with many old questions answered, new questions about circula- tory function have come to the fore. The invention of fractal mathematics by Benoit Mandelbrot in about 1980 may help biologists gain a better understanding of the evolution of circulatory structure and function. Fractal systems, as seen in (B), are “self-similar” at multiple scales, mean- ing that the patterns of branching of fine elements are miniatures of the patterns of branching of large elements. (A from De Humani Corporis Fabrica, produced by Andreas Vesalius in 1543, as reproduced in Saunders and O’Malley 1950; B after Mandelbrot 1983.) placental mammals, for example, several meticulous efforts have down over the course of evolution. Researchers are continuing to concluded that b is different in some mammalian orders than in explore hypotheses based on the properties of circulatory systems others. In addition, as already noted, b is greater when mammals and other transport systems. Several other types of fascinating are exercising than when they are at rest. Moreover, b is higher hypotheses are also being investigated at present. when only large-bodied species are analyzed than when only small-bodied species are.16 Summary As physiologists have searched for the mechanistic basis of Metabolic Scaling: The Relation between metabolism–size relations, a key question has been, what attributes Metabolic Rate and Body Size of animals are so common and so fundamental that they could explain the way in which metabolism varies with size? One attribute in „„BMR, SMR, and other measures of resting metabolic particular has attracted a great deal of attention: internal transport. rate are allometric functions of body weight within For metabolism to occur, internal transport of metabolic resources— phylogenetically related groups of animals (M = aW b, where b is usually in the vicinity of 0.7). Small-bodied notably O2 and metabolic fuels—is critical. In mammals and many species tend to have higher weight-specific metabolic other types of animals, this transport is carried out by the circulatory rates than do related large-bodied species, an effect so system. Physiologists therefore realized that they had to understand great that the weight-specific BMR is 20 times higher in how the circulatory system—first accurately described in 1543 by mice than in elephants. Andreas Vesalius (1514–1564) (Figure 7.13A)—changes in its „„Maximum aerobic metabolic rate also tends to be an inherent capabilities for transport as animals evolve to be bigger or allometric function of body weight in sets of related smaller. The new mathematics of fractal geometry—invented more species. In many cases studied thus far, the allometric than 400 years later to describe the properties of branching systems exponent for maximum metabolic rate differs from that (Figure 7.13B)—was marshaled to analyze this question. From for resting metabolic rate. this fractal research, a hypothesis was propounded, that allometric „„The allometric relation between metabolic rate and metabolism–size relations occur in part because of geometrically weight exerts important effects on the organization and imposed constraints. This hypothesis stresses that in fractally structure of both individual animals and ecosystems. Heart structured transport systems, rates of transport—and thus rates rates, breathing rates, mitochondrial densities, and dozens of other features of individual animals are allometric of supply of resources required for metabolism—are geometrically functions of body weight within sets of phylogenetically constrained in distinctive ways as body size is scaled up or scaled related species. In ecosystems, population biomasses 16 Accordingly, the log–log plot of metabolism–size data exhibits a bit of and other features of community organization may vary Hill Animal curvature Physiology and requires 4E complex equation than Equation 7.3 to be a more allometrically with individual body size. Sinauer in described Associates detail. Morales Studio Figure 07.13 11-09-15 11-10-15 Energy Metabolism 185 „„Physiologists are not agreed on the explanation for the Pacific sardines channel over 18% of their absorbed energy into growth allometric relations between metabolic rate and body during their first year of life. weight. Rubner’s surface “law,” based on heat loss from homeothermic animals, does not provide a satisfactory 20 explanation. Net growth efficiency (%) 15 Energetics of Food and Growth By their sixth year, Food and growth are important topics in animal energetics, aptly however, they channel 10 only 1% of their discussed together because one animal’s growth is another’s absorbed energy into food. A consequential attribute of foods as energy sources is that growth. 5 lipids are at least twice as high in energy density—energy value per unit of weight (see Table 6.3)—as proteins and carbohydrates are. We asked at the start of this chapter why polar explorers 0 carry lipid-rich foods, such as meat mixed with pure lard. If they 1 2 3 4 5 6 are going to pull, push, and lift their food for many miles before Age (years) they eat it, the explorers should choose food that provides a lot Figure 7.14 Net growth efficiency during each year of life in of energy per kilogram transported. Similarly, migrating animals Pacific sardines (Sardinops sagax) When their populations often capitalize on the high energy density of lipids by carrying are thriving, these fish are a major food source for seals, predatory fish, their fuel as body fat. birds, and humans. (After Lasker 1970.) A key question about any food in relation to an animal’s physiol- ogy is how efficiently the animal can digest (or ferment) the food and absorb the products of digestion. The energy absorption efficiency is defined to be the fraction of ingested energy that is chemical-bond energy of tissue absorbed for use:17 added in net fashion by growth Gross growth efficiency = (7.8) absorbed energy ingested energy  Energy absorption efficiency = (7.7) ingested energy  This efficiency matters because the absorbed energy is the  chemical-bond energy of tissue energy actually available to an animal for use in its metabolism. added in net fashion by growth (7.9) To illustrate the importance of absorption efficiency, consider Net growth efficiency = absorbed energy the processing of ingested cellulose by humans and ruminants. Because people cannot digest cellulose, they cannot absorb it, The growth efficiency of animals (gross and net) typically and their absorption efficiency for cellulose is essentially 0%; if declines with age (Figure 7.14). This pattern is important in the they eat only cellulose, they starve. Ruminants such as cows, in analysis of energy flow in ecological communities. It is also impor- contrast, commonly achieve about 50% absorption efficiency for tant in agriculture and aquaculture because as growth efficiency cellulose because their rumen microbes ferment cellulose into declines as animals age, a decline occurs in the amount of product compounds that the animals can absorb; thus ruminants are (e.g., meat) that is obtained in return for a farmer’s or aquacultural- able to use about half of the energy available from cellulose in ist’s investment in feed. In the production of broiler chickens, for their metabolism. This example illustrates how the physiology of example, the birds are slaughtered at just 2–3 months of age because digestion and absorption, discussed in Chapter 6, bears on the atHillthat they are4Elarge enough to be meaty but their growth pointPhysiology Animal physiology of energy. Sinauer Associates efficiency—their growth in return for feed provided—is declining.19 Morales Studio Growing animals accumulate chemical-bond energy in Figure 07.14 11-10-15 their bodies by adding tissue consisting of organic molecules. Conclusion: Energy as the An important question in many contexts is how efficiently they are able to use their available food energy to build tissue. Two Common Currency of Life types of growth efficiency, termed gross growth efficiency and Energy features in virtually every biological process and in many net growth efficiency, are defined on the basis of whether the inanimate processes as well. It is a factor in animal growth, body food energy is expressed as the ingested energy or the absorbed maintenance, migration, photosynthesis, automobile operation, energy:18 building construction, ecosystem degradation, and war. When scientists attempt to analyze complex systems—from indi- 17 Recall that the energetic efficiency of a process is the output of high- vidual animals to entire ecosystems or even the entire planet—they grade energy expressed as a ratio of input (see Equation 7.1). When digestion, fermentation, and absorption are the functions of interest, the inevitably come up with long lists of processes that they must take output of high-grade energy is the absorbed energy, whereas the input is into account. Although the isolated study of individual processes the ingested energy. may be straightforward, the integration of multiple processes is 18 When growth is analyzed in terms of Equation 7.1, the output of high- usually not. One of the greatest challenges for the integrated study grade energy is the chemical-bond energy of added tissue, whereas the 19 input is food energy. Feed accounts for 60%–75% of a farmer’s costs. 186 Chapter 7 of complex systems is to find a common set of units of measure—a energy used in each category. Explain rigorously why heat is always made, regardless of the way energy is used. “common currency”—in which all the operative processes can be expressed so that they can be compared, added, or multiplied. 4. Small animals tend to expire sooner than related large ones if Energy is probably the single most promising common currency. forced to live on stored supplies. For instance, suppose you have a mouse and a dog that both start with body stores of fat equal to In the study of an individual animal, for example, processes as 20% of body weight. Explain why the mouse would be likely to diverse as growth, running, nerve conduction, blood circulation, die sooner if these animals could not find any food and thus had tissue repair, and thermoregulation can all be expressed in units of to live on their fat reserves. Which one would die sooner if they energy. The costs of all these processes can therefore be summed were trapped underwater and had only their stores of O2 to live to estimate the individual’s total cost of life, and the cost of life on while trying to escape? of an entire population can be calculated by multiplying the cost 5. Suppose that over a period of 4 h a dog was observed to consume per individual by the number of individuals present. Few, if any, 20 L of O2 and produce 14 L of CO2. Using Tables 7.1 and 7.2, other properties come close to energy in their potential to serve as estimate the dog’s total heat production over the 4 h. Explain why Table 7.2 is essential for your calculation. common currencies in this way. 6. Poultry scientists are doing research on the design of diets that are nutritionally complete for chickens but minimize the SDA. Postscript: The Energy Cost of These scientists believe that such diets would be particularly Mental Effort helpful to the poultry industry in southern states during the heat of summer. Why might this be true? Not the least of the energy costs of analyzing complex systems is 7. Before Mayer and Joule came along (see Box 7.1), people were the cost of operating our brain. This cost has some fascinating and well aware that if a person cranked a drill, heat appeared. For unexpected properties. From studies of tissue metabolic rates, we instance, the drilling of the bores of cannons was legendary know that in adult humans, the brain accounts for about 20% of for the heat produced. However, heat per se was believed to be resting metabolic rate (although it accounts for a much higher per- neither created nor destroyed, and thus no one thought that the centage in young children; see page 91); loosely put, in adulthood motion associated with drilling turned into heat. Mayer and Joule go down in history in part because they demonstrated the real one-fifth of our food is for our brain when we are at rest. This cost relation between motion and heat. Imagine that you were alive resembles an “idling” cost; the energy is expended whether we in the early nineteenth century, and like Mayer and Joule, you subjectively feel we are doing hard mental labor or not. Decades hypothesized that animal motion could turn into heat. Design an ago, the prominent physiologist Francis Benedict (1870–1957) experiment that would provide a rigorous test of your hypothesis. wanted to estimate how much the brain’s energy needs increase 8. Suppose you are measuring the metabolic rate of a young, with mental “effort.” So, of course, he recruited a group of college growing cow by using the material-balance method. What students to find out. He told the students on one occasion to sit for procedures could you use to take account of the cow’s growth, so tens of minutes keeping their minds as blank as possible. Then he that you measure a correct metabolic rate? had them spend an equal amount of time working mental arithmetic 9. Suppose you have measured the average rate of O2 consumption problems at a fevered pace. Measures of their metabolic rates under of two groups of laboratory rats that are identical, except that the two conditions indicated that the increase in energy consump- one group was injected with a hormone that is being tested to see if it affects metabolic rate. If the hormone-treated group tion caused by an hour of hard mental effort is slight, equivalent has a rate of O2 consumption 5% higher than the other, there to the energy of half a peanut! Benedict’s methods were crude by are physiological reasons why you cannot conclude that the today’s standards. Nonetheless, recent calculations from modern hormone has changed the metabolic rate. Explain, referring to neuroimaging methods confirm his conclusions. Thus the brain’s Table 7.1. According to the table, what might the hormone have high costs are largely steady costs, and thinking hard is not a way done to change the rate of O2 consumption without changing the metabolic rate? to stay slim. 10. Only nine species of existing land mammals grow to adult body weights over 1000 kg (1 megagram). All are herbivores that employ fermentative digestion. These “megaherbivores” are the Study Questions two species of elephants, the five species of rhinos, the common 1. Assuming that ten people plan to trek 500 miles to the North hippo, and the giraffe. What are the metabolic pros and cons of Pole, outline the steps you would take to calculate the amount such large size? Can you suggest why no terrestrial carnivores of food they should pack, taking into account the number of sled achieve such large size? dogs needed and the food needed for the dogs. 11. If there are many species of herbivores in a grassland ecosystem, 2. Suppose you use a tire pump to inflate a tire on a bicycle. The and if the species as populations are equally competitive in elevated pressure created in the tire represents a form of potential acquiring food, predict b in the following allometric equation: energy because the release of the pressure can do mechanical population biomass per square kilometer = aW b, where W is work (such as making a pinwheel turn). The potential energy in individual body weight. Do the data in Table 7.5 follow your the tire is derived from chemical-bond energy in your food. Trace equation? What hypotheses are suggested by the comparison? the energy from the time it enters your mouth at a meal until it ends up in the tire, identifying losses of energy as heat along the way. Go to sites.sinauer.com/animalphys4e for box extensions, quizzes, flashcards, 3. Define absorbed energy (assimilated energy). Then list the major categories of use of absorbed energy, and specify the fate of and other resources. 07_HILL4E.indd 186 4/1/16 10:32 AM Energy Metabolism 187 References Atkins, P. 2007. Four Laws That Drive the Universe. Oxford University Levine, J. A., N. L. Eberhardt, and M. D. Jensen. 1999. Role of non- Press, Oxford. Another gem from Peter Atkins, providing succinct exercise activity thermogenesis in resistance to fat gain in hu- explanations of the principles of thermodynamics, accessible to mans. Science 283: 212–214. An application of the concepts of general readers. energy metabolism to a significant human problem. Atkins, P. W. 1984. The Second Law. Scientific American Library, Nagy, K. A. 2005. Field metabolic rate and body size. J. Exp. Biol. New York. A serious book on the second law of thermodynam- 208: 1621–1625. This paper discusses in five lucid pages almost ics that is accessible to general readers, containing many useful all the issues currently in the forefront of vertebrate metabolic diagrams. allometry. Glazier, D. S. 2005. Beyond the ‘3/4-power law’: variation in the in- Owen-Smith, R. N. 1988. Megaherbivores: The Influence of Very tra- and interspecific scaling of metabolic rate in animals. Biol. Large Body Size on Ecology. Cambridge University Press, New Rev. 80: 611–662. A review of metabolic scaling in most groups York. A searching discussion of extremely large body size in terres- of animals and in plants, including discussion of hypotheses of trial mammals, providing an intriguing way to see the application causation. of many principles of animal energetics. Kolokotrones, T., V. Savage, E. J. Deeds, and W. Fontana. 2010. Raichle, M. E., and M. A. Mintun. 2006. Brain work and brain imag- Curvature in metabolic scaling. Nature 464: 753–756. A break- ing. Annu. Rev. Neurosci. 29: 449–476. This fascinating paper through paper, limited to mammals, showing that the time-hon- discusses cutting-edge knowledge of brain energetics, with a ored allometric equation for metabolic scaling does not in fact fit focus on insights provided by—and questions raised by—modern available data in detail because the data exhibit curvilinearity on neuroimaging. a log–log plot, with implications. Sinervo, B., and R. B. Huey. 1990. Allometric engineering: An experi- Lankford, T. E., Jr., J. M. Billerbeck, and D. O. Conover. 2001. mental test of the causes of interpopulational differences in Evolution of intrinsic growth and energy acquisition rates. II. performance. Science 248: 1106–1109. Trade-offs with vulnerability to predation in Menidia menidia. Speakman, J. R., and E. Król. 2010. Maximal heat dissipation ca- Evolution 55: 1873–1881. A thought-provoking study of why growth pacity and hyperthermia risk: neglected key factors in the rates are not always maximized, focusing on evolutionary trade- ecology of endotherms. J. Anim. Ecol. 79: 726–746. Exposition of offs in a fish of great ecological importance. an interesting and novel idea for explaining metabolic scaling in endotherms. See also Additional References and Figure and Table Citations. Aerobic and Anaerobic Forms of Metabolism 8 A startled crayfish swims backward at jetlike speed by flipping its tail in a series of powerful The biochemistry of survival contractions. In this way, a crayfish discovered under a rock in a stream can move to anoth- Crayfish can escape rapidly from danger by tail flipping, provided er rock and disappear almost before its presence is appreciated. The tail muscles, which that the cells in their tail muscles account for 25% of a crayfish’s weight in some species, are a powerful tool for escape and can generate adenosine triphos- survival. To contract, however, the crayfish’s muscles—like all muscles—require adenosine phate (ATP) at high enough rates. The contractile apparatus in each triphosphate (ATP) as their immediate source of energy for contraction. When a crayfish is muscle cell requires ATP at a high startled, its first tail flip occurs instantly and is followed in short order by multiple additional rate to have energy for rapid, pow- flips. Each of these massive contractions requires a substantial amount of ATP. How is the erful contractions. ATP made available so promptly? The same question can be asked about the ATP that a human sprinter requires to run 100 meters (m) in 10 seconds (s). Burst exercise is a general term that refers to sudden, intense exercise. Besides escaping crayfish and sprinting people, burst exercise is illustrated by salmon leaping waterfalls during their upstream journey, cheetahs racing toward antelopes, and scallops jetting away from danger by clapping their shells. In most animals, burst exercise cannot be continued for long periods. Crayfish escaping by flipping their tails become exhausted after 15–30 flips, just as human sprinters are exhausted or nearly exhausted at the ends of their brief races. 190 Chapter 8 A second general form of physical activity is sustained exercise, situations often depends on the mechanistic peculiarities of the defined to be exercise that can be continued at a steady rate for a long particular biochemical pathways by which energy is extracted for period. Jogging by humans, migratory flight by birds, and steady use from foodstuff molecules. cruising by fish or crayfish are examples. During sustained exercise, the locomotory muscles must be supplied with ATP minute after Aerobic catabolism consists of four major sets minute for long, uninterrupted periods. What are the mechanisms of reactions that supply ATP in this way? Each cell in most animals possesses aerobic catabolic pathways: Comparing sustained exercise and burst exercise, do the mecha- pathways that, by use of O2, completely oxidize foodstuff molecules nisms that supply ATP during sustained exercise differ from the to CO2 and H2O and capture in ATP bonds much of the chemi- higher-intensity, shorter-duration mechanisms used during burst cal energy thereby released. These aerobic pathways typically can exercise? Do different types of animals differ in their ATP-producing oxidize all the major classes of foodstuffs. Here, for simplicity, we mechanisms? Is exercise ever forced to stop because of limitations emphasize just the catabolism of carbohydrates. Moreover, our aim in ATP-generating mechanisms? These are some of the questions is to provide an overview, not to duplicate detailed treatments avail- we discuss in this chapter. able in texts of cellular physiology or biochemistry. The principal ATP is not transported from one cell to another. Each cell, therefore, aerobic catabolic pathway can be subdivided into four major sets must make its own ATP. This is one of the most critically important of reactions: (1) glycolysis, (2) the Krebs cycle (citric acid cycle), (3) the properties of ATP physiology. Another key property is that ATP electron-transport chain, and (4) oxidative phosphorylation. is not stored by cells to any substantial extent. Because of these two properties, the rate at which a cell can do muscular work (or carry glycolysis Glycolysis is the series of enzymatically catalyzed out other forms of ATP-requiring physiological work) at any given reactions shown in Figure 8.1, in which glucose (or glycogen) is moment depends strictly on the rate at which that very cell is able converted to pyruvic acid. The enzymes and reactions of glycolysis to produce ATP at the moment. occur in the cytosol. The first step in glycolysis is that glucose is The following two complementary reactions, taken together, phosphorylated at the cost of an ATP molecule to form glucose- serve as a crucial energy shuttle and energy transduction mechanism 6-phosphate. Glucose-6-phosphate is then converted to fructose- in cells: 6-phosphate, and the latter is phosphorylated—also at the cost of ADP + Pi + energy from foodstuff molecules → ATP (8.1) an ATP molecule—to form fructose-1,6-diphosphate. The latter is cleaved to form two three-carbon molecules: dihydroxyacetone ATP → ADP + Pi + energy usable by cell processes (8.2) phosphate and glyceraldehyde-3-phosphate. These compounds are Each cell uses energy from carbohydrates, lipids, or other food- interconvertible, and when glucose is being catabolized for release stuff molecules to drive Equation 8.1, causing ATP to be formed of energy, the former is converted to the latter, yielding two mol- from adenosine diphosphate (ADP) and inorganic phosphate ions ecules of glyceraldehyde-3-phosphate. The reactions subsequent (HPO42–), symbolized Pi. In this way, the energy from the bonds of to glyceraldehyde-3-phosphate in Figure 8.1 are all multiplied by the foodstuff molecules is moved into bonds of ATP in the cell and 2 to emphasize that two molecules follow these pathways for each becomes poised for use in physiological work. Energy-demanding glucose molecule catabolized. processes in the cell—such as muscle contraction or ion pumping— The reaction that uses glyceraldehyde-3-phosphate is the only then split the ATP, as shown in Equation 8.2, thereby releasing the oxidation reaction in glycolysis and is particularly significant for energy they need. Cellular energy-demanding processes are not able understanding not only aerobic but also anaerobic catabolism. to draw energy directly from the bonds of foodstuff molecules. The Each molecule of glyceraldehyde-3-phosphate is oxidized, with energy-demanding processes, therefore, (1) are utterly dependent the addition of inorganic phosphate (P i), to a three-carbon di- on the cellular mechanisms that drive Equation 8.1 and (2) can take phosphate: 1,3-diphosphoglyceric acid. Although this reaction place only as fast as those mechanisms supply ATP. is an oxidation reaction, it does not itself require O2. Instead, it occurs by the simultaneous reduction of one molecule of Mechanisms of ATP Production and nicotinamide adenine dinucleotide (NAD) per molecule of glyceraldehyde-3-phosphate. NAD—which a cell synthesizes Their Implications from the vitamin niacin—is a relatively small, nonprotein mol- There are two major categories of catabolic, biochemical pathways ecule that undergoes reversible oxidation and reduction. When by which animal cells release energy from foodstuff molecules to a molecule of glyceraldehyde-3-phosphate is oxidized, two synthesize ATP and thus make energy available for the perfor- hydrogen atoms are removed from it and transferred to NAD, mance of physiological work. Some pathways, termed aerobic, reducing the NAD to form NADH 2.1 The two hydrogens remain require O2; others, termed anaerobic, can function without O2. bound to NADH2 only temporarily because they are soon passed In this section we examine the mechanistic features of the aerobic to another compound, regenerating NAD. and anaerobic catabolic pathways. In later sections of this chapter An alternative way to think about an oxidation–reduction reac- we will look at the interplay between the aerobic and anaerobic tion—such as the oxidation of glyceraldehyde-3-phosphate—is that modes of energy release when animals engage in burst or sus- electrons are transferred. In the reaction under discussion, NAD tained exercise, or when they face environmental stresses such 1 as O2 deficiency. There is a compelling reason to start with study The reduction of NAD is symbolized “NAD + 2 H → NADH2” or “NAD → NADH2” in this book because these simplified expressions compactly of the mechanistic features of the pathways of ATP production: emphasize the features of relevance for us. The true reaction is NAD+ + 2H The overall performance of animals during exercise and in other → NADH + H+. Aerobic and Anaerobic Forms of Metabolism 191 This is the number of carbon atoms in each serves as the immediate electron acceptor because it combines with compound. electrons (hydrogens) removed from glyceraldehyde-3-phosphate. The 1,3-diphosphoglyceric acid formed from the oxidation of Glucose (6C) glyceraldehyde-3-phosphate is next converted to a monophosphate, ATP 3-phosphoglyceric acid, with the formation of one ATP per mol- ecule. The 3-phosphoglyceric acid is then converted in two steps ADP to phosphoenolpyruvic acid, and the latter reacts to form pyruvic acid, again with the formation of one ATP per molecule. Glucose-6-phosphate (6C) Hexokinase Three important consequences of glycolysis deserve note: „„Each molecule of glucose is converted into two molecules of Fructose-6-phosphate (6C) pyruvic acid. ATP „„Two molecules of NAD are reduced to NADH2 per molecule of glucose catabolized. ADP „„Two molecules of ATP are used and four are formed for each Fructose-1,6-diphosphate (6C) glucose processed, providing a net yield of two ATP molecules per glucose molecule. Phosphofructokinase the krebs cycle (citric acid cycle) During aerobic Dihydroxy- Glyceraldehyde-3-phosphate (3C) catabolism, the pyruvic acid formed by glycolysis enters the acetone phosphate (3C) mitochondria by facilitated diffusion (mediated by a carrier protein just recently defined). The pyruvic acid is then oxidized in the mi- 2 NAD 2 Pi tochondria by a cyclic series of enzymatically catalyzed reactions called the Krebs cycle (citric acid cycle). This set of reactions, 2 NADH2 named after Hans Krebs, who in 1937 was the first to envision its Glyceraldehyde-3- features, is diagrammed in Figure 8.2. For simplicity the reac- phosphate dehydrogenase tions involved in the oxidation of just one molecule of pyruvic acid 2 1,3-Diphosphoglyceric acid (3C) are shown, although two molecules are in fact processed for each molecule of glucose. 2 ADP 2 ATP Pyruvic acid (3C) NAD 2 3-Phosphoglyceric acid (3C) CO2 NADH2 Phosphoglycerate kinase Coenzyme A 2 2-Phosphoglyceric acid (3C) Acetyl coenzyme A (2C) Coenzyme A Oxaloacetate (4C) 2 H2O NADH2 * Citrate (6C) NAD Enolase Isocitrate (6C) Malate (4C) 2 Phosphoenolpyruvic acid (3C) NAD * CO2 NADH2 2 ADP Fumarate (4C) FADH2 2 ATP α-Ketoglutarate (5C) FAD Succinate (4C) NAD 2 Pyruvic acid (3C) Coenzyme A CO2 Pyruvate kinase NADH2 GTP Coenzyme A Figure 8.1 The major reactions of glycolysis Each reac- Succinyl tion requires catalysis by an enzyme protein. The three-dimensional GDP * coenzyme A (4C) structures of six of these enzyme proteins are shown. Subunits of an enzyme are shown in different colors. The expression for reduction Figure 8.2 The major reactions of the Krebs cycle (citric Hill Animal of NAD, NAD → NADH4E Physiology + acid cycle) A molecule of H2O enters the reactions at each aster- 2, is shorthand; the actual reaction is NAD + Sinauer Associates+ isk (*). The expression for reduction of NAD, NAD → NADH2, is short- 2 H → NADH + H. Pi = inorganic phosphate. (Protein structures from Morales Studio hand; the actual reaction is NAD+ + 2 H → NADH + H+. Erlandsen et al. 2000.) Figure 08.01 11-12-15 192 Chapter 8 Pyruvic acid enters the Krebs cycle by participating in a complex O2. The reason is that O2 is in fact not a participant in any of the set of reactions in which it is oxidatively decarboxylated, forming CO2 reactions of glycolysis or the Krebs cycle; in a very narrow sense, and a two-carbon acetyl group that is combined with coenzyme A in all those reactions can proceed without O2. Nonetheless, O2 is the form of acetyl coenzyme A.2 In the process, a molecule of NAD is essential. The reason it is essential lies in the disposition of the reduced. Acetyl coenzyme A then reacts with oxaloacetate, with the end reduced NADH2 and FADH2 molecules. result that coenzyme A is released and the acetyl group is condensed NADH2 and FADH2 cannot serve as final resting places for with oxaloacetate (four-carbon) to form citrate (six-carbon). In the electrons because NAD and FAD are present in only limited ensuing series of reactions, oxaloacetate is ultimately regenerated and quantities in a cell. Whenever one of the mainstream molecules then again can combine with acetyl coenzyme A. For our purposes, in glycolysis or the Krebs cycle is oxidized, the electrons (or hydro- there is no need to review the reactions stepwise. It is more important gens) removed are transferred to NAD or FAD. As stressed earlier, to emphasize the overall outcomes of the reactions: therefore, NAD and FAD are the immediate electron acceptors. However, if the NADH 2 and the FADH 2 thereby formed were „„The six carbons of each glucose molecule catabolized emerge in simply allowed to accumulate, a cell would soon run out of NAD the form of six molecules of CO2 as the pyruvic acid molecules and FAD. Running out of NAD and FAD would bring glycolysis produced by glycolysis are processed by the Krebs cycle. The and the Krebs cycle to a halt because NAD and FAD are required. CO2 is formed by decarboxylation reactions. Such reactions Thus NAD and FAD cannot serve as final electron acceptors. The occur at two points in the Krebs cycle: in the conversion electron-transport chain regenerates NAD and FAD by removing of isocitrate to α-ketoglutarate and in the conversion electrons (hydrogens) from NADH2 and FADH2. For the ordinary of α-ketoglutarate to succinyl coenzyme A. These two operation of the electron-transport chain, O2 is required. In that decarboxylations, plus the one in the reaction of pyruvic way O2 is necessary for glycolysis and the Krebs cycle to function acid to form acetyl coenzyme A, account for the formation as they do during aerobic catabolism. of three molecules of CO2 for every molecule of pyruvic acid Let’s now focus on the electron-transport chain (respiratory processed (thus six molecules of CO2 formed per glucose chain) itself. It consists of a series of four major protein complexes molecule). (I–IV), plus other compounds, located in the inner membranes of „„For each glucose molecule catabolized, the Krebs cycle produces mitochondria (Figure 8.3). A key property of the constituents of eight molecules of NADH2 and two molecules of FADH2. the electron-transport chain is that each is capable of undergoing Oxidation reactions occur at four points in the Krebs cycle. reversible reduction and oxidation. The compounds function in a At three of these, NAD is reduced, forming NADH2. At discrete order—a chain. The chain takes electrons from NADH2 one (the oxidation of succinate to fumarate), another small, and FADH2 and passes them in sequence from one compound to nonprotein molecule that undergoes reversible oxidation the next in a series of reductions and oxidations (see Figure 8.3). and reduction—flavin adenine dinucleotide (FAD)—is Finally, the last compound in the electron-transport chain, known reduced, forming FADH2.3 Recall also that one NADH2 as complex IV or cytochrome oxidase, passes the electrons—along with is formed in the reaction of pyruvic acid to form acetyl “accompanying” protons (H+ ions)—to oxygen, reducing the O2 to coenzyme A. Considering all the oxidation reactions, the water.4 In this way, O2 acts as the final electron acceptor. The net effect processing of each pyruvic acid molecule results in four of the operation of the electron-transport chain is to take electrons NADH2 and one FADH2 (thus eight NADH2 and two from NADH2 and FADH2 (thereby regenerating NAD and FAD) FADH2 per glucose molecule). and pass the electrons to O2. „„Two molecules of ATP are produced in the Krebs cycle for each The special role played by O2 is important. The constituents molecule of glucose catabolized. Guanosine triphosphate of the electron-transport chain (e.g., cytochrome b-c 1 and (GTP) is formed from guanosine diphosphate (GDP) when cytochrome oxidase), just like NAD and FAD, are present in succinyl coenzyme A reacts to form succinate in the Krebs limited quantities in a cell and therefore cannot act as terminal cycle. GTP donates its terminal phosphate group to ADP, electron acceptors. In contrast, O2 is continuously supplied to a resulting in GDP and ATP. Thus one molecule of ATP is cell, and the product of its reduction, water, can be voided into generated for each molecule of pyruvic acid processed (two the environment, thereby carrying electrons out of the cell. An adult ATPs per glucose). person produces about 0.8 L of water per day in the process of voiding electrons from cells! electron transport, oxidative phosphorylation, and The electron-transport chain, in addition to reoxidizing NADH2 the role of o2 The final two of the four sets of reactions in and FADH2, is also pivotally involved in the transfer of energy from the aerobic catabolic pathway are the electron-transport chain and the bonds of foodstuff molecules to ATP. Molecular O2 has a much oxidative phosphorylation. We discuss them together because they higher affinity for electrons than NAD or FAD. Accordingly, a are often tightly linked. large decline in free energy takes place as the electrons originally A paradox you may have noticed is that thus far, in discussing taken from foodstuff molecules are passed through the electron- aerobic catabolism, we have not mentioned the involvement of transport chain. Much of this energy is captured in bonds of ATP. The process of forming ATP from ADP by use of energy released 2 Coenzyme A, an essential compound for aerobic metabolism, is 4 synthesized from pantothenic acid, a B vitamin. Two electrons and two protons combine with one oxygen atom. A molecule of oxygen (O2), therefore, can react with four electrons and four 3 Flavin adenine dinucleotide is synthesized from riboflavin (vitamin B2). protons. 230 Chapter 9 2. Temperature regulation may also be costly. To fly, bumblebees require the temperature of their flight muscles to be about 30°C or higher (see page 281). When the bees are flying, temperatures that high are maintained by the heat produced by the wing-flapping contractions of their flight muscles. When bees land on flowers (and stop flying) in cool weather, however, they are at risk of quickly cooling to below the necessary flight temperature—which would make them unable to take off again. To keep their flight muscles warm while they are alighted on flowers, bees produce heat by a process analogous to human shivering (see Chapter 10). The intensity and energetic cost of this form of shivering become greater as the air temperature decreases. Although shivering may be unnecessary at an air temperature of 25°C, shivering at 5°C may raise a stationary bee’s metabolic rate to as high a level as prevails during flight. Considering the costs of both flying and shivering, the average Figure 9.13 A case study in ecological energetics A breeding metabolic expenditure per unit of time for a bee to forage tends to colony of seabirds, such as this colony of terns, is ecologically depen- increase as the air becomes cooler. If the air is warm enough that dent on the populations of fish in its vicinity to obtain energy for life and no shivering is needed, a bee has a high metabolic rate when it is reproduction. With modern methods, physiologists can quantify the rate flying but a low rate when it is not. If the air is cold, the bee has a at which a colony harvests energy relative to the rates of production of high metabolic rate all the time, whether flying or stationary. prey populations. Now let’s turn to the energy rewards of foraging. The energy reward that can be obtained per unit of time from any particular species of flowering plant depends on (1) the volume of nectar Ecological Energetics obtained from each flower, (2) the sugar concentration of the nectar, Ecological energetics is the study of energy needs, acquisition, and (3) the number of flowers from which a bee can extract nectar and use in ecologically realistic settings. An example is provided per unit of time. The third property depends on the spacing of the by research on the energy needs and acquisition of breeding colo- flowers and the difficulty of penetrating flowers to obtain their nectar. nies of seabirds (Figure 9.13). Using the doubly labeled water Some species of plants yield sufficient sugar per flower that method or time–energy budgets, investigators estimate the daily bumblebees can realize a net energy profit when foraging from them energy demand of a colony, including costs of growth in the young regardless of the air temperature. For example, the rhododendron birds and costs of foraging flight in the adults, as discussed at the Rhododendron canadense, a plant with large flowers, typically yields start of this chapter. The energy demand of the colony can then sugar equivalent to about 1.7 J/flower. At 0°C, a large bee expends be compared with the energy available from the fish populations energy at a time-averaged rate of about 12.5 J/min while foraging. on which the birds feed. Studies of this sort have revealed that Accordingly, the bee could break even energetically by taking the seabird colonies sometimes consume one-fourth to one-third of nectar from about 7–8 flowers per minute. In fact, bees can tap all the productivity of prey fish in their foraging areas. This sort of almost 20 rhododendron flowers per minute. Thus, even at 0°C, ecological energetic analysis has helped biologists better understand bees foraging on the rhododendron are able to meet their costs of seabird population dynamics. For example, the high energy needs foraging plus accumulate a surplus of nectar to contribute to the hive. of some colonies help explain why some have been devastated by In contrast, some plants yield so little sugar per flower that they competition from human fishing. are profitable sources of nectar only when air temperatures are A more elaborate illustration of the power of ecological energetic relatively high (and the bees’ costs of foraging are thereby reduced). research is provided by Bernd Heinrich’s analysis of costs and For example, bees typically visit flowers of wild cherry (Prunus) only rewards in bumblebee foraging—an example of what Heinrich when the air is warm. The flowers yield sugar equivalent to only terms bumblebee economics. The starting point of his analysis is the about 0.21 J/flower. At 0°C, a bee would therefore have to tap about recognition that in ecologically realistic settings, the acquisition of 60 flowers per minute just to meet its costs of foraging. Tapping so food has energy costs as well as energy rewards. many flowers is impossible, meaning the bees cannot profitably When bumblebees (Bombus) forage, they fly from one flower (or forage on cherry flowers when the air is cold. flower cluster) to another, landing on each long enough to collect The study of bumblebee foraging exemplifies how an ecologi- available nectar. Two major costs of bumblebee foraging must be cally realistic accounting of energy costs and gains can help ecolo- considered: gists better understand the foraging choices that animals make. Bumblebees are observed to forage on rhododendron in all weather, 1. Flight is itself very costly. It can easily elevate the metabolic but they forage on wild cherry only in warm weather. From the rate of a bumblebee to 20–100 times its resting rate. The study of ecological energetics we now understand that energetic cost of flight per unit of time is essentially independent of considerations place constraints on foraging choices by helping to air temperature. dictate which flowers the bees can profitably exploit. The Energetics of Aerobic Activity 231 Study Questions References 1. How does the doubly labeled water method depend on the Alexander, R. M. 2003. Principles of Animal Locomotion. Princeton existence of isotopic equilibrium between the oxygen in H 2O and University Press, Princeton, NJ. that in CO2? Åstrand, P.-O., K. Rodahl, H. A. Dahl, and S. B. Strømme. 2003. Textbook of Work Physiology: Physiological Bases of Exercise, 2. From a list of your friends, select one (theoretically) for study to 4th ed. Human Kinetics, Champaign, IL. A definitive textbook determine his or her average daily metabolic rate. How would treatment of work in the full range of human endeavor, from daily you carry out research to create a time–energy budget for your life to sport. friend? Biro, P. A., and J. A. Stamps. 2010. Do consistent individual differ- 3. In your own words, explain why foraging on wild cherry flowers ences in metabolic rate promote consistent individual differ- is beneficial for bumblebees in warm weather but not in cold ences in behavior? Trends Ecol. Evol. 25: 653–659. weather. Dickinson, M. H., C. T. Farley, R. J. Full, and three additional au- thors. 2000. How animals move: An integrative view. Science 4. As noted in this chapter, the VO2max of people tends to decline 288: 100–106. A short but broadly conceived introduction to the after age 30 by about 9% per decade for sedentary individuals, biomechanics and functional morphology of animal locomotion. but it declines less than 5% per decade for people who stay active. Although the present chapter does not include these topics, they The average VO2max in healthy 30-year-olds is about 3.1 L/min. are intimately related to the energetic themes that the chapter Using the information given here, what would the average VO2max stresses. be in 60-year-olds who have been sedentary throughout their Gill, R. E., Jr., T. L. Tibbits, D. C. Douglas, and seven additional au- lives and in 60-year-olds who have stayed active (keep in mind thors. 2009. Extreme endurance flights by landbirds crossing that the decline is exponential)? Consider the activities in Table the Pacific Ocean: ecological corridor rather than barrier? 9.1, and recall from Chapter 7 that 1 kJ is equivalent to about 0.05 Proc. R. Soc. London, Ser. B 276: 447–457. L of O2 in aerobic catabolism. How would you expect sedentary Hammond, K. A., and J. Diamond. 1997. Maximal sustained energy and active people to differ in their capacities for each of those budgets in humans and animals. Nature 386: 457–462. activities in old age? Explain. Hannah, J. B., D. Schmitt, and T. M. Griffin. 2008. The energetic cost 5. For an animal engaging in sustained exercise, why is there not of climbing in primates. Science 320: 898. one single ideal speed? Heinrich, B. 1979. Bumblebee Economics. Harvard University Press, Cambridge, MA. One of the great essays on ecological energet- 6. List the possible reasons why two individuals of a certain species ics of the twentieth century. A rewarding and thoroughly enjoy- might differ in VO2max. able book. 7. Suppose that a bird’s metabolic rate while flying at 30 km/h is 8 Nagy, K. A. 2005. Field metabolic rate and body size. J. Exp. Biol. kJ/h. What is the bird’s cost of transport when flying at 30 km/h? 208: 1621–1625. A brief but thought-provoking paper on the costs of existence in free-living vertebrates. 8. Looking at Figure 9.8, how would you say animals and machines Piersma, T. 2011. Why marathon migrants get away with high meta- compare in their efficiencies in covering distance? bolic ceilings: towards an ecology of physiological restraint. J. Exp. Biol. 214: 295–302. An enticing paper on maximum 9. African hunting dogs depend on sustained chases by groups sustained rates of energy expenditure in people and other endo- of cooperating individuals to capture antelopes for food. If the therms—and why the rates of energy expenditure are what they members of two groups differ in their average VO2max, how might are. Provides lots to think about. the two groups differ in the strategies they use during hunting? Suarez, R. K., L. G. Herrera M., and K. C. Welch, Jr. 2011. The sugar 10. In mammals of all species, the peak rate of O2 consumption of oxidation cascade: aerial refueling in hummingbirds and nec- each mitochondrion is roughly the same. On the basis of patterns tar bats. J. Exp. Biol. 214: 172–178. Hummingbirds and nectar of how VO2max varies with body size in species of mammals, how bats have converged in that they hover at flowers to collect nec- would you expect the muscle cells of mammals of various body tar. This research shows that they reach a steady state in which they use the sugars they are collecting immediately for the mus- sizes to vary in how densely they are packed with mitochondria? cular work of hovering. Explain your answer. Weibel, E. R. 2000. Symmorphosis. On Form and Function in 11. What is the hypothesis of symmorphosis? How might you Shaping Life. Harvard University Press, Cambridge, MA. Even evaluate or test the hypothesis? if one is unconvinced by the theory of symmorphosis, this book provides a compact and lucid introduction to the suite of systems 12. Explain the concept that in high-performance muscle cells, responsible for O2 delivery in sustained exercise. mitochondria and contractile elements compete for space over scales of evolutionary time. See also Additional References and Figure and Table Citations. Go to sites.sinauer.com/animalphys4e for box extensions, quizzes, flashcards, and other resources. 09_HILL4E.indd 231 4/1/16 2:07 PM Thermal Relations 10 As this bumblebee flies from one flower cluster to another to collect nectar and pollen, tem- For a foraging bumblebee, perature matters for the bee in two crucial ways. First, the temperature of the bumblebee’s warming the thorax to a high temperature is a critical re- flight muscles determines how much power they can generate. The flight muscles must be at quirement The process adds to a tissue temperature of about 30–35°C to produce enough power to keep the bee airborne; the bee’s energy costs and food if the muscles are cooler, the bee cannot fly. The second principal way in which temperature needs on cool days. However, the flight muscles in the thorax require matters is that for a bumblebee to maintain its flight muscles at a high enough temperature high temperatures to produce suf- to fly, the bee must expend food energy to generate heat to warm the muscles. In a warm ficient power for flight. environment, all the heat required may be produced simply as a by-product of flight. In a cool environment, however, as a bumblebee moves from flower cluster to flower cluster—stopping at each to feed—it must expend energy at an elevated rate even during the intervals when it is not flying, either to keep its flight muscles continually warm enough to fly or to rewarm the flight muscles to flight temperature if they cool while feeding. Assuming that the flight muscles must be at 35°C for flight, they must be warmed to 10°C above air temperature if the air is at 25°C, but to 30°C above air temperature if the air is at 5°C. Thus, as the air becomes cooler, a bee must expend food energy at a higher and higher rate to generate heat to warm its flight muscles to flight temperature, meaning it must collect food at a higher and higher rate.

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