Lecture 2 Readings PDF

12 Chapter 1 TABLE 1.1 Major Parts of this book and “At Work” chapters The “At Work” chapters exemplify how the material covered in each Part of the book can be used synthetically to understand a problem in animal physiology. Part “At Wor...

12 Chapter 1 TABLE 1.1 Major Parts of this book and “At Work” chapters The “At Work” chapters exemplify how the material covered in each Part of the book can be used synthetically to understand a problem in animal physiology. Part “At Work” chapter Part I: Fundamentals of Physiology Part II: Food, Energy, and Temperature The Lives of Mammals in Frigid Places (Chapter 11) Part III: Integrating Systems Animal Navigation (Chapter 18) Part IV: Movement and Muscle Plasticity in Response to Use and Disuse (Chapter 21) Part V: Oxygen, Carbon Dioxide, and Diving by Marine Mammals (Chapter 26) Internal Transport Part VI: Water, Salts, and Excretion Mammals of Deserts and Dry Savannas (Chapter 30) a limited set of animals. Comparative physiology is termed com- the book consists of six major subdivisions, Parts I through VI, parative because one of its major goals is to compare systematically each of which focuses on a particular set of functions. The chapters the ways that various sorts of animals carry out similar functions, within each part are listed in the Table of Contents and at the start such as vision, breathing, or circulation. Environmental physiol- of each part. Here we want to stress that each part (except Part ogy (also called physiological ecology) is the study of how animals I) ends with a unique type of chapter that we call an “At Work” respond physiologically to environmental conditions and challenges, chapter. Each “At Work” chapter takes a synthetic approach to a or—more briefly—“ecologically relevant physiology.” Integrative prominent, curiosity-provoking topic in the part. Table 1.1 lists physiology is a relatively new term referring to investigations with these topics. The principal goal of the “At Work” chapters is to show a deliberate emphasis on synthesis across levels of biological organiza- how the material in each of the parts can be used in an integrated way tion, such as research that probes the relations between molecular to understand animal function. and anatomical features of organs. Now, as they say in theater, “Let the play begin.” As we consider Our viewpoint in this book is mechanistic, evolutionary, com- the principal subject of this chapter—function on the ecological parative, environmental, and integrative. In other words, we stress: stage—the three major players are animals, environments, and „„The mechanisms by which animals perform their life- evolutionary processes (see Figure 1.3). We now address each. sustaining functions „„The evolution and adaptive significance of physiological Animals traits The features of animals that deserve mention in an initial overview „„The ways in which diverse phylogenetic groups of animals are the features that are of overriding importance. These include both resemble each other and differ that (1) animals are structurally dynamic, (2) animals are organized „„The ways in which physiology and ecology interact, in the systems that require energy to maintain their organization, and (3) present and during evolutionary time both time and body size are of fundamental significance in the lives „„The importance of all levels of organization for the full of all animals. understanding of physiological systems One of the most profoundly important properties of animals Overlapping with the classifications already discussed, physiol- is that the atoms of their bodies—their material building blocks— ogy is divided also into various branches or disciplines based on the undergo continuous, dynamic exchange with the atoms in their types of functions that are performed by animals. The organization environments throughout life. This structural dynamism—memo- of this book is based on the types of function. As Table 1.1 shows, rably termed “the dynamic state of body constituents” by Rudolf  Animals and Environments 13 Schoenheimer, who discovered it4—is a fundamental and crucially protein each day, and about 10% of the amino acids used to build important way in which animals differ from inanimate objects such the new protein molecules are acquired from food and therefore as telephones. After a telephone is manufactured, the particular new to the body. carbon and iron atoms that are built into its substance remain as Have you ever wondered why you need to worry every week long as the telephone exists. One might think by casual observation about whether you are eating enough calcium, iron, magnesium, that the composition of a person, lion, or crab is similarly static. and protein? The explanation is provided by the principles we are This illusion was abruptly dispelled, however, when Schoenheimer discussing. If you were an inanimate object, enough of each nec- and others began using chemical isotopes as research tools. essary element or compound could be built into your body at the Isotopes proved to be revealing because they permit atoms start, and you would then have enough forever. Instead, because to be labeled and therefore tracked. Consider iron (Fe) as an you are alive and dynamic—rather than inanimate and static—you example. Because most iron atoms in the natural world are of lose elements and compounds every day and must replace them. atomic weight 56 (56Fe), an investigator can distinctively label a As this discussion has illustrated, the material boundaries be- particular set of iron atoms by substituting the unusual (but stable tween an animal and its environment are blurred, not crisp. Atoms and nonradioactive) alternative isotope of iron having an atomic cross the boundaries throughout life, so that an atom that is part weight of 58 (58Fe). Suppose that we make a telephone in which of an animal’s tissues on one day may be lying on the forest floor all the iron atoms are of the unusual 58Fe isotope, so that we can or drifting in the atmosphere the next day, and vice versa. Possibly distinguish those iron atoms from the ones generally available. the most profound implication of these facts is that an animal is not Years later, all the iron atoms in the telephone will still be of the a discrete material object. unusual 58Fe type. Suppose, however, that we create a 58Fe-labeled person by feeding the person over the course of a year the unusual The structural property of an animal that 58 Fe isotope, so that isotopically distinctive iron atoms are built persists through time is its organization into hemoglobin molecules and other iron-containing molecules If the atomic building blocks of an animal are transient and con- throughout the person’s body. Suppose we then stop providing stantly changing, by what structural property is an animal defined? the unusual iron isotope in the person’s diet. Thereafter—as time The answer comes from imagining that we can see the individual goes by—the isotopically distinctive 58Fe atoms in the body will molecules in an adult animal’s body. If we could, we would observe leave and will be replaced with atoms of the ordinary isotope, that the molecular structures and the spatial relations of molecules 56 Fe, from the environment. Years later, all the unusual iron atoms in tissues are relatively constant over time, even though the particu- will be gone. We see, therefore, that although the person may lar atoms constructing the molecules change from time to time. A outwardly appear to be structurally constant like a telephone, rough analogy would be a brick wall that retains a given size and the iron atoms in the substance of the person’s body at one time shape but in which the bricks are constantly being replaced, so that differ from those at another time. the particular bricks present during one month are different from The mechanistic reason for the turnover of iron atoms in an those present a month earlier. animal is that the molecular constituents of an individual’s body The structural property of an animal that persists through time break down and are rebuilt. A human red blood cell, for example, is the organization of its atomic building blocks, not the building typically lives for only 4 months. When a red blood cell is discarded blocks themselves. Thus an animal is defined by its organization. This and replaced, some of the iron atoms from the hemoglobin mol- characteristic of animals provides the most fundamental reason ecules of the old cell are excreted into the environment, and some why animals require inputs of energy throughout life. As we will of the iron atoms built into the new cell are acquired from food. discuss in detail in Chapter 7, the second law of thermodynamics In this way, even though the number of red blood cells remains says that for organization to be maintained in a dynamic system, relatively constant, the iron atoms of the cells are in dynamic use of energy is essential. exchange with iron atoms in the environment. Essentially all the atoms in the substance of an animal’s body Most cells of an animal are exposed undergo similar dynamic exchanges. Calcium atoms enter an to the internal environment, not the animal’s skeleton and later are withdrawn; some of the with- external environment drawn atoms are replaced with newly ingested calcium atoms Shifting our focus now to the cells of an animal’s body, it is im- from the environment. Proteins and fats throughout an animal’s portant first to stress that the conditions experienced by most body are continually broken down at substantial rates,5 and their of an animal’s cells are the conditions inside the body, not those resynthesis is carried out in part with molecules newly acquired outside. Most cells are bathed by the animal’s tissue fluids or from the environment, such as amino acids and fatty acids from blood. Thus the environment of most cells consists of the set of foods. Adult people typically resynthesize 2–3% of their body conditions prevailing in the tissue fluids or blood. Claude Bernard (1813–1878), a Frenchman who was one of the most influential 4 As chemists learned about and started to synthesize unusual isotopes in physiologists of the nineteenth century, was the first to codify this the 1930s, Rudolf Schoenheimer (1898–1941) was one of the first to apply the newfound isotopes to the study of animal metabolism. His classic book concept. He coined the term internal environment (milieu inté- on the subject, published posthumously as World War II raged, is titled The rieur) to refer to the set of conditions—temperature, pH, sodium Dynamic State of Body Constituents. (Na+) concentration, and so forth—experienced by cells within 5 See Chapter 2 (page 59) for a discussion of the ubiquitin–proteasome an animal’s body. The conditions outside the body represent the system that tags proteins for breakdown and disassembles them. external environment.  14 Chapter 1 The internal environment may be permitted (A) Temperature conformity (B) Chloride regulation to change when the external environment …but its blood Cl– concentration changes, or it may be kept constant When a salmon enters a river from remains almost constant, even the sea, its body temperature Animals have evolved various types of relations between their in- (including blood temperature) though river water is very dilute ternal environment and the external environment. If we think of changes if the river water is warmer in Cl– and seawater is very or cooler than the ocean water… concentrated in Cl–. the organization of the body as being hierarchically arranged, the relations between the internal and external environments repre- sent one of the potential hierarchical levels at which animals may exhibit organization. All animals consistently exhibit structural Blood Cl– concentration Blood temperature organization of their atoms and molecules. When we think about the relation between the internal and external environments, we are considering another level at which organization may exist. At this level, animals sometimes—but only sometimes—exhibit further organization by keeping their internal environment distinct from their external environment. Animals display two principal types of relation between their internal and external environments. On the one hand, when the Water temperature Water Cl– concentration conditions outside an animal’s body change, the animal may permit its internal environment to match the external conditions and thus Figure 1.9 Mixed conformity and regulation in a single spe- cies Salmon are temperature conformers but chloride regulators. change along with the outside changes. On the other hand, the The presentation of Cl– regulation is diagrammatic; the blood Cl– con- animal may maintain constancy in its internal environment. These centration is not in fact absolutely constant but is a little higher when alternatives are illustrated with temperature in Figure 1.8. If the the fish are in seawater than when they are in freshwater. temperature of an animal’s external environment changes, one option is for the animal to let its internal temperature change to match the external temperature (see Figure 1.8A). Another option is for the animal to maintain a constant internal temperature (see internal temperature match the surrounding water temperature (see Figure 1.8B). If an animal permits internal and external conditions Figure 1.9A). Simultaneously, salmon are excellent chloride regulators; to be equal, it is said to show conformity. If the animal maintains they maintain a nearly constant concentration of Cl– ions in their internal constancy in the face of external variability, it shows blood, regardless of how salty their environmental water is—that regulation. Conformity and regulation are extremes; intermediate is, regardless of how high or low the outside Cl– concentration is responses are common. (see Figure 1.9B). Animals frequently show conformity with respect to some char- Regulation demands more energy than conformity because acteristics of their internal environment while showing regulation regulation represents a form of organization. How does regulation with respect to others. Consider a salmon, for example (Figure represent organization? The answer is that during regulation, an 1.9). Like most fish, salmon are temperature conformers; they let their animal maintains constancy inside its body, and it maintains distinc- tions between inside and outside conditions. Both the constancy and Hill the distinctions Animal Physiologyare 4E types of organization. A familiar analogy Sinauer Associates for the energy costs of regulation in animals is provided by home (A) Temperature conformity (B) Temperature regulation Morales Studio heating. Considerable energy is required to keep the inside of a Figure 01.09 11-09-15 An animal’s internal environment …or the internal house at 22°C (72°F) during the cold of winter. This energy cost may be permitted to vary when its environment may external environment changes… be held constant. is entirely avoided if the inside temperature is simply allowed to match the outside temperature. 40 40 Homeostasis in the lives of animals: Internal Internal temperature (°C) Internal temperature (°C) constancy is often critical for proper function 30 30 Homeostasis is an important concept regarding the nature and signifi- cance of internal constancy. Soon we will define homeostasis using the words of Walter Cannon (1871–1945), who coined the term. To 20 20 fully appreciate the concept, however, we must first recognize its historical roots in medicine. The two men who contributed the most toward developing the concept of homeostasis, Claude Bernard and 10 10 20 30 40 10 10 20 30 40 Walter Cannon, were physicians and medical researchers, concerned External temperature (°C) External temperature (°C) primarily with human physiology. Healthy humans maintain re- markable constancy of conditions in their blood and tissue fluids. Figure 1.8 Conformity and regulation These examples from The concept of homeostasis was thus conceived during studies of the study of temperature illustrate the general principles of conformity a species that exhibits exceptional internal constancy, and later the (A) and regulation (B). concept was extrapolated to other animals.  Animals and Environments 15 BOX 1.1 Negative Feedback The type of control that Claude Ber- the control system adds glucose to the In positive feedback, a control system nard discovered in his studies of blood blood if the blood glucose concentra- reinforces deviations of a controlled vari- glucose is what today we call negative tion—the controlled variable—falls below able from its set point. Positive feedback feedback. In any control system, the con- its set-point concentration, thereby op- is much less common in physiological trolled variable is the property that is be- posing the deviation of the blood con- systems than negative feedback. It is ing kept constant or relatively constant centration from the set point. The control more common during normal function by the system’s activities. The set point is system removes glucose from the blood than is usually recognized, however. the level at which the controlled variable if the glucose concentration rises too For example, positive feedback occurs is to be kept. Feedback occurs if the sys- high, thereby again opposing the devia- when action potentials (nerve impulses) tem uses information on the controlled tion of the concentration from its set develop in nerve cells (see Figure 12.16), variable itself to govern its actions. In point. Biologists and engineers who study and it also occurs during the birth pro- negative feedback, the system responds control systems have established that no cess in mammals (see Figure 17.18). In to changes in the controlled variable by control system can maintain perfect con- the first case, a relatively small change bringing the variable back toward its set stancy in a controlled variable; putting in the voltage across the cell membrane point; that is, the system opposes devia- the case roughly, a controlled variable of a nerve cell modulates the properties tions of the controlled variable from the must be a moving target for a control of the membrane in ways that make the set point. There are many detailed mech- system to act on it. Thus the blood glu- voltage change become greater. In the anisms by which negative feedback cose concentration is not kept perfectly second, muscular contractions acting can be brought about in physiological constant by the glucose control system, to expel the fetus from the uterus induce systems. Negative feedback, however, is but during normal health it is kept from hormonal signals that stimulate ever- virtually synonymous with homeostasis varying outside a narrow range. You will more-intense contractions. and occurs in all homeostatic systems. find a more thorough discussion of con- In the case of the blood glucose trol systems based on negative feedback level that so intrigued Claude Bernard, in Box 10.3. Claude Bernard was the first to recognize the impressive stability A modern translation might go like this: Animals are able to lead of conditions that humans maintain in their blood and tissue fluids. lives of greater freedom and independence to the extent that they One of Bernard’s principal areas of study was blood glucose in mam- maintain a stable internal environment, sheltering their cells from mals. He observed that the liver takes up and releases glucose as the variability of the outside world. necessary to maintain a relatively constant glucose concentration in Walter Cannon, a prominent American physiologist who was the blood. If blood glucose rises, the liver removes glucose from the born in the same decade that Claude Bernard died, introduced the blood. If blood glucose falls, the liver releases glucose into the blood. word homeostasis to refer to internal constancy in animals. In certain Bernard stressed that, as a consequence, most cells in the body of a ways, Bernard’s and Cannon’s views were so similar that Bernard mammal experience a relatively constant environment with respect might have invented the homeostasis concept, but the implications of to glucose concentration (Box 1.1). Bernard’s research and that of internal constancy were clearer by Cannon’s time. Because animals later investigators also revealed that most cells in a mammal’s body dynamically interact with their environments, the temperature, pH, experience a relatively constant temperature—and a relatively constant ion concentrations, and other properties of their bodies are incessantly O2 level, osmotic pressure, pH, Na+ concentration, Cl– concentra- being drawn away from stability. Cannon emphasized that for an tion, and so on—because various organs and tissues regulate these animal to be internally stable, vigilant physiological mechanisms must properties at consistent levels in the body fluids bathing the cells. be present to correct deviations from stability. Thus when Cannon Claude Bernard devoted much thought to the significance of introduced and defined the term homeostasis, he intended it to internal constancy in humans and other mammals. He was greatly mean not just internal constancy, but also the existence of regulatory impressed with how freely mammals are able to conduct their systems that automatically make adjustments to maintain internal lives regardless of outside conditions. Mammals, for example, can constancy. In his own words, Cannon described homeostasis as wander about outdoors in the dead of winter, seeking food and “the coordinated physiological processes which maintain most of mates, whereas fish and insects—in sharp contrast—are often the [constant] states in the organism.” driven into a sort of paralysis by winter’s cold. Bernard reasoned An essential aspect of Cannon’s perspective was his conviction that mammals are able to function in a consistent way regardless that homeostasis is good. Cannon argued, in fact, that homeostasis of varying outside conditions because the cells inside their bodies is a signature of highly evolved life. He believed that animal spe- enjoy constant conditions. He thus stated a hypothesis that remains cies could be ranked according to their degree of homeostasis; in probably the most famous in the history of animal physiology: his view, for example, mammals were superior to frogs because “Constancy of the internal environment is the condition for free life.” mammals maintain a greater degree of homeostasis. 16 Chapter 1 homeostasis in the modern study of animal physiology change. Details of their internal environment may change. Moreover, The concept of ranking animals using degrees of homeostasis the regulatory processes that maintain homeostasis must change seems misguided to most biologists today. Bernard and Cannon, from time to time so that homeostasis can prevail, much as day- having focused on mammals, articulated ideas that are truly in- to-day adjustments in the fuel consumption of a home furnace are dispensable for understanding mammalian biology and medicine. required to maintain a constant air temperature inside the home However, the mere fact that mammals exhibit a high degree of during winter. homeostasis does not mean that other animals should be held to An important organizing principle for understanding the role mammalian standards. Animals that exhibit less-complete ho- of time in the lives of animals is to recognize five major time frames meostasis than mammals coexist in the biosphere with mammals. within which the physiology of an animal can change. The time Indeed, the vast majority of animals thriving today do not achieve frames fall into two categories: (1) responses of physiology to changes “mammalian standards” of homeostasis. Thus most biologists to- in the external environment and (2) internally programmed changes day would argue that a high degree of homeostasis is merely one of physiology. Table 1.2 lists the five time frames classified in this of several ways to achieve evolutionary and ecological success. way. We will recognize these five time frames throughout this book In this view, Bernard and Cannon did not articulate universal as we discuss various physiological systems. requirements for success, but instead they clarified the properties The concept of the five time frames overlies other ways of and significance of one particular road to success. organizing knowledge about animal function. For example, Recent research has clarified, in fact, that organisms sometimes the concept of time frames overlies the concepts of regulation, achieve success in the biosphere precisely by letting their internal conformity, and homeostasis that we have just discussed. When environment vary with the external environment: the antithesis of we speak of regulation, conformity, and homeostasis, we refer to homeostasis. Consider, for example, insects that overwinter within types of responses that animals show in relation to variations in plant stems in Alaska. They survive by ceasing to be active, allow- their external environments. When we speak of the time frames, ing their internal temperatures to fall below –40°C, and tolerating we address when those responses occur. such low tissue temperatures. Any attempt by such small animals to maintain an internally constant temperature from summer to physiology responds to changes in the external en- winter would be so energetically costly that it would surely end in vironment in three time frames  Individual animals sub- death; thus the tolerance of the insects to the change of their internal jected to a change in their external environment exhibit acute and temperature in winter is a key to their survival. Even some mam- chronic responses to the environmental change. Acute responses, mals—the hibernators—survive winter by abandoning constancy by definition, are responses exhibited during the first minutes or of internal temperature; hibernating mammals allow their body hours after an environmental change. Chronic responses are temperatures to decline and sometimes match air temperature. expressed following prolonged exposure to new environmental For lizards in deserts, tolerance of profound dehydration is often conditions. You might wonder why an individual’s immediate a key to success. responses to an environmental change differ from its long-term Both constancy and inconstancy of the internal environment— responses. The answer is that the passage of time permits bio- regulation and conformity—have disadvantages and advantages: chemical or anatomical restructuring of an animal’s body. When an animal suddenly experiences a change in its environment, its „„Regulation. The chief disadvantage of regulation is that it immediate responses must be based on the “old,” preexisting prop- costs energy. The great legacy of Bernard and Cannon is that erties of its body because the animal has no time to restructure. A they clarified the advantage that animals enjoy by paying morphological example is provided by a person who suddenly is the cost: Regulation permits cells to function in steady required to lift weights after months of totally sedentary existence. conditions, independent of variations in outside conditions. The sedentary person is likely to have small arm muscles, and her „„Conformity. The principal disadvantage of conformity is immediate, acute response to her new weight-lifting environment that cells within the body are subject to changes in their will likely be that she can lift only light weights. However, if the conditions when outside conditions change. The chief person lifts weights repeatedly as time goes by, restructuring will advantage of conformity is that it is energetically cheap. occur; her muscles will increase in size. Thus her chronic response Conformity avoids the energy costs of keeping the internal to the weight-lifting environment will likely be that she can lift environment different from the external environment. heavy weights as well as light ones. Neither regulation nor conformity is categorically a defect or an A familiar physiological example of acute and chronic responses asset. One cannot understand mammals or medical physiology is provided by human reactions to work in hot weather. We all without understanding homeostasis, but one cannot understand know that when we are first exposed to hot weather after a period the full sweep of animal life without recognizing that physiological of living in cool conditions, we often feel quickly exhausted; we say flexibility is sometimes advantageous. the heat is “draining.” We also know that this is not a permanent state: If we experience heat day after day, we feel more and more Time in the lives of animals: able to work in the heat. Physiology changes in five time frames Figure 1.10 shows that these impressions are not merely sub- Time is a critical dimension for understanding the physiology of jective illusions. Twenty-four physically fit young men who lacked all animals because the physiology of animals invariably changes recent experience with hot weather were asked to walk at a fixed from time to time. Even animals that exhibit homeostasis undergo pace in hot, relatively dry air. Their endurance was measured as a  The acute response, displayed The chronic response, displayed Animals and Environments 17 when the men were first after a week of experience with the exposed to the hot environment hot environment, was dramatically on day 1, was low endurance; increased endurance; 23 of the 24 From research on the physiology of human work under hot none could continue walking for men could continue walking for 100 100 minutes. minutes. conditions, physiologists know that endurance under hot conditions changes because as people gain increased experience with heat, 24 their rates of sweat secretion increase, their sweat glands are able to maintain high rates of sweat secretion for dramatically lengthened men who could walk for 100 minutes Endurance measured as number of 20 periods of time, their sweat becomes more dilute (so they lose less salt), the blood flow to their skin becomes more vigorous (improving 16 delivery of internal heat to the body surface), and their heart rates during exercise in the heat become lower. Thus human physiology 12 is restructured in many ways by repeated exposure to heat. For a person who has been living in cool conditions, the acute physi- 8 ological responses to heat exposure are low exercise endurance, a low rate of sweat production, and so forth. Heat training poises a 4 person to express chronic physiological responses to heat, such as high exercise endurance and a high capacity to sweat. 0 1 2 3 4 5 6 7 The acute and chronic responses are, by definition, phenotypic Days of heat exposure responses of individual animals to environmental change. Popula- Figure 1.10 Heat acclimation in humans as measured by tions may exhibit a third category of response to environmental exercise endurance Twenty-four fit young men without recent change: evolutionary responses involving changes of genotypes. heat experience were asked to walk at 3.5 miles per hour in hot, dry Collectively, therefore, animals display responses to environmental air (49°C, 20% relative humidity). Their endurance was used as a change in three time frames: measure of their physiological capability to engage in moderate work under hot conditions. The acclimation illustrated by the chronic re- „„Individuals exhibit immediate, acute responses. sponse is reversible; if heat-acclimated men return to a life of no heat „„Individuals exhibit long-term, chronic responses. The exposure, they gradually revert to the level of endurance evident on length of time that an individual must be exposed to a new day 1. (After Pandolf and Young 1992.) environment for chronic responses to be fully expressed is usually a few days to a few weeks. „„Populations exhibit evolutionary responses. way of quantifying their physiological ability to sustain moderate Chronic responses by individual animals to environmental exercise under Hill Animal the hot Physiology 4Econditions. None of the men had sufficient change are so common, diverse, and important that their study Sinauer Associates endurance to walk for 100 minutes (min) on the first day. However, forms a special discipline with its own terminology. For many Morales Studio as the days passed and the men had more and more experience with physiologists, the concepts of acclimation and acclimatization provide Figure 01.10 10-30-15 hot conditions, their endurance increased, as indicated by a steady an important way to classify the chronic responses of individuals to increase in the number of men who could keep walking for 100 min. environmental change. A chronic response to a changed environ- TABLE 1.2 The five time frames in which physiology changes Type of change Description Changes in physiology that are responses to changes in the external environment 1. Acute changes Short-term changes in the physiology of individual animals: changes that individuals exhibit soon after their environments have changed. Acute changes are reversible. 2. Chronic changes (termed acclimation and Long-term changes in the physiology of individual animals: changes acclimatization; also termed phenotypic that individuals display after they have been in new environments for plasticity or phenotypic flexibility) days, weeks, or months. Chronic changes are reversible. 3. Evolutionary changes Changes that occur by alteration of gene frequencies over the course of multiple generations in populations exposed to new environments. Changes in physiology that are internally programmed to occur whether or not the external environment changes 4. Developmental changes Changes in the physiology of individual animals that occur in a programmed way as the animals mature from conception to adulthood and then to senescence (see Chapter 4). 5. Changes controlled by periodic Changes in the physiology of individual animals that occur in repeating biological clocks patterns (e.g., each day) under control of the animals’ internal biological clocks (see Chapter 15). 18 Chapter 1 BOX 1.2 The Evolution of Phenotypic Plasticity When animals express different geneti- The norm of reaction is genetically During tanning, the skin changes in its cally controlled phenotypes in different coded in the genome. Because of this, content of a dark-colored pigment called environments—when they acclimate the norm of reaction itself can evolve and melanin. Suppose that there are two pos- and acclimatize—they require controls is subject to natural selection. To see this, sible cutaneous phenotypes: high mela- that determine which particular pheno- suppose that an individual other than the nin and low melanin. Suppose also that types are expressed in which particular one just discussed expresses phenotype there are two environments: high sun and environments. As an illustration, suppose P1 in environment E1, P2 in E2, P3 in E3, low sun. One possible norm of reaction that an individual animal has four possi- and P4 in E4. In this case, the two individu- would be to express high melanin in low ble phenotypes, P1 through P4, and that als would differ in their norms of reaction. sun and low melanin in high sun. Another there are four environments, E1 through Suppose, now, that there is a population— norm of reaction would be to express high E4. One option is that the individual living in a variable environment—that is melanin in high sun and low melanin in could express phenotype P1 in environ- composed half of individuals with the low sun. Over the course of evolution, if ment E3, P2 in E4, P3 in E1, and P4 in E2. first reaction norm and half of individuals both of these reaction norms once ex- This set of correspondences between with the second. If individuals of the first isted, it is easy to understand why individu- phenotypes and environments consti- sort were to survive and reproduce more als with the second reaction norm would tutes the individual’s norm of reaction; successfully as the environment varied, have left more progeny than those with that is, if we think of the phenotypes as natural selection for the first reaction norm the first. That is, it is easy to understand one list and the environments as a sec- would occur. In this way the reaction norm the evolution of the sort of reaction norm ond list in a matching game, the norm itself would evolve in ways that would we see today among people with light of reaction is like the set of lines that better adapt the animals to the variable complexions. we would draw between items on the environment in which they live. Phenotypic plasticity itself can evolve, two lists to indicate which item on one A simple example is provided by tan- and norms of reactions can themselves matches which on the other. ning in people with light complexions. be adaptations. ment is called acclimation if the new environment differs from physiology undergoes internally programmed the preceding environment in just a few highly defined ways.6 changes in two time frames  The physiological properties Acclimation is thus a laboratory phenomenon. Acclimatization is of individuals sometimes change even if their external environ- a chronic response of individuals to a changed environment when ment stays constant. For instance, the type of hemoglobin in your the new and old environments are different natural environments blood today is different from the type you produced as a newborn. that can differ in numerous ways, such as winter and summer, This change in hemoglobin is internally programmed: It occurs or low and high altitudes. Thus animals are said to acclimatize to even if your external environment stays constant. Sometimes winter, but they acclimate to different defined temperatures in a internally programmed changes interact with environmental laboratory experiment. changes. For instance, an internally programmed change might Acclimation and acclimatization are types of phenotypic occur sooner, or to a greater amplitude, in one environment than in plasticity: the ability of an individual animal (a single genotype) to another. However, the internally programmed changes do not re- express two or more genetically controlled phenotypes. Phenotypic quire any sort of environmental activation. There are two principal plasticity is possible because the genotype of an individual can code types of internally programmed change: developmental changes for multiple phenotypes (Box 1.2). Growth of the biceps muscle and changes controlled by periodic biological clocks. during weight training provides a simple example of a change in Development is the progression of life stages from concep- phenotype under control of genetically coded mechanisms. Another tion to senescence in an individual. Different genes are internally example is that the particular suite of enzymes active in an adult programmed to be expressed at different stages of development, person may change from one time to another because the genes for giving rise to developmental changes in an animal’s phenotype. one suite of enzymes are expressed under certain environmental Puberty is a particularly dramatic example of internally programmed conditions, whereas the genes for another suite are expressed under developmental change in humans. The environment may change different conditions.7 We will discuss phenotypic plasticity in more the timing of puberty—as when the advent of sexual maturity is detail—with several additional examples—in Chapter 4. delayed by malnutrition—but puberty always occurs, no matter 6 Some authors restrict use of the word acclimation to cases in which just what the environment, illustrating that internally programmed one property differs between environments. changes do not require environmental activation. We will discuss 7 physiological development in much greater depth in Chapter 4. Enzymes that vary in amount as a result of changes in environmental conditions are termed inducible enzymes. An excellent illustration is Biological clocks are mechanisms that give organisms an provided by the P450 enzymes discussed at length in Chapter 2 (see internal capability to keep track of the passage of time. Most bio- page 52). logical clocks resemble wristwatches in being periodic; that is, after  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.  Animals and Environments 21 Figure 1.13 Butterfly biogeography: Latitude Species diversity is relatively low at the 70° cold, high latitudes The diagram shows 3 60° The number of species of the number of species of swallowtail butter- 9 terrestrial temperature flies (family Papilionidae) at various latitudes. conformers usually de- 50° The reason there are relatively few species of clines toward the poles. 16 animals at high latitudes may not be simply 40° the low temperatures there, but may in part be 19 a relay effect from the effects of cold on plants. 30° Plants decrease in diversity and annual pro- 30 ductivity toward the poles, affecting the food 20° supplies of animals. (After Scriber 1973.) 65 10° 81 Equator 79 The Canadian tiger 10° swallowtail (Papilio 69 canadensis) is one of 20° the species of butterflies 46 that lives farthest from 30° the equator. 13 40° 0 10 20 30 40 50 60 70 80 90 Number of species of swallowtail butterflies freezing point of seawater,10 others, such as many fish, metabolically to –90°C (–130°F); in the Arctic, it descends to –70°C (–90°F). synthesize antifreeze compounds that keep them from freezing. The extremes of animal adaptation to low tissue temperature are Because the tissues of these animals are very cold, one might imagine represented by certain extraordinary species of Arctic insects that that the animals live in a sort of suspended animation. Actually, spend winters inside exposed plant stems or on the surface of pack however, the communities of temperature-conforming animals in ice. These insects are in a state of suspended animation at these the polar seas are active and thriving. In the ocean around Antarctica, times; they are quiescent, not active. Nonetheless, it is impressive for example, a sure sign of the vigor of the populations of krill and that some endure tissue temperatures of –60°C to –70°C, either fish is that they reproduce and grow prolifically enough to meet the in a frozen state (which they have adaptations to tolerate) or in an food needs of the famously huge populations of Antarctic whales, unfrozen supercooled state. seals, and penguins. The diversity of terrestrial temperature-conforming animals Are the low tissue temperatures of polar fish and invertebrates typically becomes lower and lower as latitude increases from the actually challenging for them, or do these low tissue temperatures temperate zone toward the poles, as exemplified in Figure 1.13. only seem challenging? One way to obtain an answer is to compare This decline in diversity toward the poles indicates that the very polar species with related nonpolar ones. Tropical species of fish cold terrestrial environments are demanding places for animals to clearly find low temperatures to be challenging. Many tropical fish, occupy, despite evolutionary adaptability. Hill Animal Physiology 4E in fact, die if cooled to +6°C, even if they are cooled very gradually. Sinauer Associates At the high end of the temperature scale, the temperature of Such observations Morales Studio emphasize that success at –1.9°C is not “auto- the air or water on Earth usually does not go higher than +50°C matic,” Figure and that 01.13 the polar species of fish have had to evolve special 11-09-15 (+120°F). Animals on land may experience even higher heat loads, adaptations to thrive with their tissues perpetually at –1.9°C. The however, by being exposed simultaneously to hot air and the sun’s polar species themselves often die if they are warmed to +6°C, radiation. Some temperature-conforming animals from hot environ- indicating that the tropical species also have special adaptations— ments—such as certain desert insects and lizards—can function adaptations that poise them to live at tropical temperatures. The at tissue temperatures of 45°C–55°C (Figure 1.14).11 These are the evolutionary divergence of these fish is dramatized by the fact that highest tissue temperatures known for animal life, suggesting that a single temperature can be lethally cold for tropical species and yet the high levels of molecular agitation at such temperatures pose be lethally warm for polar species! the greatest challenge that can be met by evolutionary adaptation Far greater extremes of cold are found on land than in aquatic in animal systems. environments. In Antarctica, the air temperature sometimes drops The hottest places in the biosphere are the waters of geother- 10 Dissolved salts and other dissolved compounds lower the freezing mally heated hot springs and underwater hot vents. These waters points of solutions. Krill and most other marine invertebrates have are often far above the boiling point when they exit Earth’s crust. total concentrations of dissolved matter in their blood similar to the Although aquatic animals typically stay where the waters have concentration in seawater. Consequently, their blood freezing points are about the same as the freezing point of seawater, and they do not freeze, 11 provided that the seawater remains unfrozen. Normal human body temperature is 37°C. 22 Chapter 1 secluded places on land, O2 is often freely available because—as counterintuitive as it may sound—O2 diffuses fairly readily from the open atmosphere through soil to reach burrow cavities, provided the soil structure includes gas-filled spaces surrounding the soil particles. As altitude increases, the O2 concentration of the air declines because the air pressure decreases and the air therefore becomes more and more rarified. Air at the top of Mt. Everest—8848 m above sea level—is 21% O2, like that at sea level; but the total air pressure is only about one-third as high as at sea level, and gas molecules within the air are therefore so widely spaced that each liter of air contains only about one-third as much O2 as at sea level. The high altitudes are among Earth’s most challenging places, reflected in the fact that the numbers of animal species are sharply reduced. At such altitudes, the maximum rate at which animals can acquire O2 is often much lower than at sea level, and functions are consequently limited. At elevations above 6500 m (21,000 ft), for example, people breathing from the atmosphere find that simply walking uphill is a major challenge because their level of exertion is limited by the low availability of O2 (Figure 1.15). Some animal species have evolved adaptations to succeed in the dilute O2 of rar- efied air in ways that humans cannot. One of the most remarkable species is the bar-headed goose (Anser indicus), which—in ways that physiologists still do not fully comprehend—is able to fly (without an oxygen mask!) over the crests of the Himalayas at 9000 m. Water-breathing animals typically face a substantially greater Figure 1.14 A thermophilic (“heat-loving”) lizard common challenge to obtain O2 than air-breathing animals do because the in North American deserts The desert iguana (Dipsosaurus supply of O2 for water breathers is the O2 dissolved in water, and the dorsalis) can often be seen abroad as the sun beats down on hot solubility of O2 in water is not high. Because of the low solubility days. Although it does not usually expose itself to body temperatures of O2, water contains much less O2 per liter than air does, even higher than 42°C, it can survive 48.5°C, one of the highest body tem- when the water is fully aerated. For example, aerated stream or peratures tolerated by any vertebrate animal. river water at sea level contains only 3–5% as much O2 per liter as air at sea level does. A common problem for animals living in slow-moving bodies of cooled to 35°C–45°C or lower, many prokaryotic microbes— water such as lakes, ponds, or marshes is that the O2 concentration bacteria and archaea—thrive at much higher temperatures than in the water may be even lower than in aerated water because dis- animals can. Some prokaryotes even reproduce at temperatures solved O2 may become locally depleted by the metabolic activities above 100°C. of animals or microbes. Density layering of water—which prevents the water from circulating freely—is a common contributing factor oxygen  The need of most animals for oxygen (O2) is a con- to O2 depletion in the deep waters of lakes and ponds. Density sequence of their need for metabolic energy. The chemical reac- layering occurs when low-density water floats on top of high- tions that animals use to release energy from organic compounds density water, causing distinct water layers to form. When this remove some of the hydrogen atoms from the compounds. Each happens, there is often almost no mixing of oxygenated water from adult person, for example, liberates about one-fifth of a pound of the low-density surface layer (where photosynthesis and aeration hydrogen every day in the process of breaking down food mol- occur) into the high-density bottom layer. Thus O2 in the bottom ecules to obtain energy. Hydrogen liberated in this way cannot be layer is not readily replaced when it is used, and as microbes and allowed to accumulate in an animal’s cells. Thus an animal must animals in the bottom layer consume O2, the O2 concentration possess biochemical mechanisms for combining the hydrogen may fall to very low levels. with something, and O2 is the usual recipient. O2 obtained from In lakes during summer, density layering occurs because of the environment is delivered to each cell, where it reacts with the temperature effects: Sun-heated warm water tends to float on top free hydrogen produced in the cell, yielding water (see Figure 8.2). of colder and denser bottom water.12 The lake studied by a group The suitability of an environment for animals often depends of university students in Figure 1.16 provides an example of this on the availability of O2. In terrestrial environments at low and sort of density layering. The bottom waters of this lake contained moderate altitudes, the open air is a rich source of O2. Air consists 12 of 21% O2, and at low or moderate altitudes it is relatively dense In estuarine bodies of water along seacoasts—where freshwater and seawater mix—layering can occur because of salinity effects as well as because it is at relatively high pressure. Thus animals living in the temperature effects. Low-salinity water is less dense than—and tends to open air have a plentiful O2 resource. Even within burrows or other float on top of—high-salinity water.  Animals and Environments 23 O2 enters a lake only near the surface (through photo- synthesis or aeration). O2 concentration Temperature 2 When a sun-heated surface layer forms 4 in the summer, it Warm, low-density tends to float on top of layer of water; the deep, cold layer. 6 Depth (meters below surface) O2-rich 8 Little mixing between the two layers occurs 10 across the thermocline, the transition layer where temperature 12 changes rapidly with depth. 14 The failure of the 16 surface and deep Cold, high-density layers to mix cuts layer of water; off the O2 supply to 18 O2-depleted the deep waters. 20 0 2 4 6 8 10 Oxygen concentration (mL O2/L) 0 4 8 12 16 20 Figure 1.15 Performance in an O2-poor environment Temperature (°C) Because of the difficulty of acquiring O2 from rarefied air, the rate at which energy can be released from food molecules for use in Figure 1.16 Density layering can cut off the O2 supply to work by humans is reduced at high altitudes, and the simple act the deep waters of a lake Different densities of water do not mix of walking uphill becomes extremely arduous. Well-conditioned readily. The O2 concentration in the deep waters of a lake may fall to mountaineers are slowed to a walking rate of 100–200 m per near zero because the animals and microbes living there consume hour near the tops of the world’s highest mountains if they are O2 that is not replaced. (From data gathered by a group of animal breathing from the air rather than from oxygen tanks. Shown physiology students on a lake in northern Michigan in July.) here is Chantal Mauduit (1964–1998) during an unsuccessful attempt to reach the summit of Mt. Everest while breathing only atmospheric air. On an earlier expedition she had been the fourth woman to climb to the peak of K2 (8611 m), second high- photosynthesis that otherwise could replenish O2. Among the est mountain on Earth, without supplemental oxygen. fish living in such waters, the evolution of air breathing is one of the most remarkable features. Hundreds of species of fish in these waters are air breathers. Some take up inhaled O2 across essentially no dissolved O2 on the July day when the data were well-vascularized mouth linings or lunglike structures. Others collected. Deep-water O2 depletion has become more common in swallow air and absorb O2 in their stomachs or intestines, as recent decades in lakes, ponds, and estuaries as human popula- mentioned previously. For animals confronted with short-term Hill Animal Physiology 4E tions have enriched waters with organic matter. The organic matter or Associates Sinauer long-term O2 deficiency, whether in O2-depleted freshwater supports the growth of microbes that deplete dissolved O2. For environments Morales Studio or elsewhere, a potential solution over evolution- animals in deep waters to survive, they must be able to tolerate low Figureary time11-09-15 01.16 is to adopt a biochemistry that can attach hydrogen to O2 levels, or they must temporarily move to other places where O2 molecules other than O2. Many species—both air breathers and is more available. water breathers—have temporary options of this sort. Certain In certain sorts of water bodies, animals have faced the tissues in our own bodies, for example, can live without O2 for challenge of low O2 concentrations for millennia. Unlike ani- 10 min at a time by attaching hydrogen to pyruvic acid (making mals confronted with new, human-induced O2 depletion, the lactic acid). Suppose, however, that an animal’s entire body must animals living in primordially O2-poor waters have been able live with little or no O2 for many hours, days, weeks, or months. to undergo long-term evolutionary adaptation to low-O2 condi- Doing so is possible for some animals, but as the period without tions. Tropical rivers that are naturally very rich in organic matter, O2 lengthens, ever-fewer species have evolved biochemical as in the Amazon basin, provide examples of waters that have specializations that enable them to survive. Some exceptional been O2-poor for millennia. The warmth of these rivers not only animals are able to meet the most extreme challenge of living lowers the solubility of O2 in the water but also promotes rapid indefinitely in O2-free environments. Most that are currently multiplication of microbes that use O2. In addition, thick forest known to science are parasites (e.g., nematodes and tape