Basic Principles of Animal Form and Function

Chapter 40  Basic Principles of Animal Form and Function
Lecture Outline
Overview: Diverse Forms, Common Challenges

• Animals inhabit almost every part of the biosphere.
• Despite their great diversity, all animals must solve a common set of problems.
• All animals must obtain oxygen, nourish themselves, excrete wastes, and move.
• Animals of diverse evolutionary histories and varying complexity must solve these general challenges of life.
• Consider the long, tongue-like proboscis of a hawk moth, a structural adaptation for feeding.
• Recoiled when not in use, the proboscis extends as a straw through which the moth can suck nectar from deep within tube-shaped flowers.
• Analyzing the hawk moth’s proboscis gives clues about what it does and how it functions.
• Anatomy is the study of the structure of an organism.
• Physiology is the study of the functions an organism performs.
• Natural selection can fit structure to function by selecting, over many generations, the best of the available variations in a population.
• Searching for food, generating body heat and regulating internal temperature, sensing and responding to environmental stimuli, and all other animal activities require fuel in the form of chemical energy.
• The concept of bioenergetics—how organisms obtain, process, and use energy resources—is a connecting theme in the comparative study of animals.

Concept 40.1 Physical laws and the environment constrain animal size and shape

• An animal’s size and shape, features often called “body plans” or “designs,” are fundamental aspects of form and function that significantly affect the way an animal interacts with its environment.
• The terms plan and design do not mean that animal body forms are products of conscious invention.
• The body plan or design of an animal results from a pattern of development programmed by the genome, itself the product of millions of years of evolution due to natural selection.
• Physical requirements constrain what natural selection can “invent.”
• An animal such as the mythical winged dragon cannot exist. No animal as large as a dragon could generate enough lift to take off and fly.
• Similarly, the laws of hydrodynamics constrain the shapes that are possible for aquatic organisms that swim very fast.
• Tunas, sharks, penguins, dolphins, seals, and whales are all fast swimmers.
• All have the same basic fusiform shape, tapered at both ends.
• This shape minimizes drag in water, which is about a thousand times denser than air.
• The similar forms of speedy fishes, birds, and marine mammals are a consequence of convergent evolution in the face of the universal laws of hydrodynamics.
• Convergence occurs because natural selection shapes similar adaptations when diverse organisms face the same environmental challenge, such as the resistance of water to fast travel.

 Body size and shape affect interactions with the environment.

• An animal’s size and shape have a direct effect on how the animal exchanges energy and materials with its surroundings.
• As a requirement for maintaining the fluid integrity of the plasma membrane of its cells, an animal’s body must be arranged so that all of its living cells are bathed in an aqueous medium.
• Exchange with the environment occurs as dissolved substances diffuse and are transported across the plasma membranes between the cells and their aqueous surroundings.
• For example, a single-celled protist living in water has a sufficient surface area of plasma membrane to service its entire volume of cytoplasm.
• Surface-to-volume ratio is one of the physical constraints on the size of single-celled protists.
• Multicellular animals are composed of microscopic cells, each with its own plasma membrane that acts as a loading and unloading platform for a modest volume of cytoplasm.
• This only works if all the cells of the animal have access to a suitable aqueous environment.
• For example, a hydra, built as a sac, has a body wall only two cell layers thick.
• Because its gastrovascular cavity opens to the exterior, both outer and inner layers of cells are bathed in water.
• Another way to maximize exposure to the surrounding medium is to have a flat body.
• For instance, a parasitic tapeworm may be several meters long, but because it is very thin, most of its cells are bathed in the intestinal fluid of the worm’s vertebrate host from which it obtains nutrients.
• While two-layered sacs and flat shapes are designs that put a large surface area in contact with the environment, these solutions do not permit much complexity in internal organization.
• Most animals are more complex and are made up of compact masses of cells, producing outer surfaces that are relatively small compared to the animal’s volume.
• Most organisms have extensively folded or branched internal surfaces specialized for exchange with the environment.
• The circulatory system shuttles material among all the exchange surfaces within the animal.
• Although exchange with the environment is a problem for animals whose cells are mostly internal, complex forms have distinct benefits.
• A specialized outer covering can protect against predators; large muscles can enable rapid movement; and internal digestive organs can break down food gradually, controlling the release of stored energy.
• Because the immediate environment for the cells is the internal body fluid, the animal’s organ systems can control the composition of the solution bathing its cells.
• A complex body form is especially well suited to life on land, where the external environment may be variable.

Concept 40.2 Animal form and function are correlated at all levels of organization

• Life is characterized by hierarchical levels of organization, each with emergent properties.
• Animals are multicellular organisms with their specialized cells grouped into tissues.
• In most animals, combinations of various tissues make up functional units called organs, and groups of organs work together as organ systems.
• For example, the human digestive system consists of a stomach, small intestine, large intestine, and several other organs, each a composite of different tissues.
• Tissues are groups of cells with a common structure and function.
• Different types of tissues have different structures that are suited to their functions.
• A tissue may be held together by a sticky extracellular matrix that coats the cells or weaves them together in a fabric of fibers.
• The term tissue is from a Latin word meaning “weave.”
• Tissues are classified into four main categories: epithelial tissue, connective tissue, nervous tissue, and muscle tissue.
• Occurring in sheets of tightly packed cells, epithelial tissue covers the outside of the body and lines organs and cavities within the body.
• The cells of an epithelium are closely joined and in many epithelia, the cells are riveted together by tight junctions.
• The epithelium functions as a barrier protecting against mechanical injury, invasive microorganisms, and fluid loss.
• The cells at the base of an epithelial layer are attached to a basement membrane, a dense mat of extracellular matrix.
• The free surface of the epithelium is exposed to air or fluid.
• Some epithelia, called glandular epithelia, absorb or secrete chemical solutions.
• The glandular epithelia that line the lumen of the digestive and respiratory tracts form a mucous membrane that secretes a slimy solution called mucus that lubricates the surface and keeps it moist.
• Epithelia are classified by the number of cell layers and the shape of the cells on the free surface.
• A simple epithelium has a single layer of cells, and a stratified epithelium has multiple tiers of cells.
• A “pseudostratified” epithelium is single-layered but appears stratified because the cells vary in length.
• The shapes of cells on the exposed surface may be cuboidal (like dice), columnar (like bricks on end), or squamous (flat like floor tiles).
• Connective tissue functions mainly to bind and support other tissues.
• Connective tissues have a sparse population of cells scattered through an extracellular matrix.
• The matrix generally consists of a web of fibers embedding in a uniform foundation that may be liquid, jellylike, or solid.
• In most cases, the connective tissue cells secrete the matrix.
• There are three kinds of connective tissue fibers, which are all proteins: collagenous fibers, elastic fibers, and reticular fibers.
• Collagenous fibers are made of collagen, the most abundant protein in the animal kingdom.
• Collagenous fibers are nonelastic and do not tear easily when pulled lengthwise.
• Elastic fibers are long threads of elastin.
• Elastin fiber provides a rubbery quality that complements the nonelastic strength of collagenous fibers.
• Reticular fibers are very thin and branched.
• Composed of collagen and continuous with collagenous fibers, they form a tightly woven fabric that joins connective tissue to adjacent tissues.
• The major types of connective tissues in vertebrates are loose connective tissue, adipose tissue, fibrous connective tissue, cartilage, bone, and blood.
• Each has a structure correlated with its specialized function.
• Loose connective tissue binds epithelia to underlying tissues and functions as packing material, holding organs in place.
• Loose connective tissue has all three fiber types.
• Two cell types predominate in the fibrous mesh of loose connective tissue.
• Fibroblasts secrete the protein ingredients of the extracellular fibers.
• Macrophages are amoeboid cells that roam the maze of fibers, engulfing bacteria and the debris of dead cells by phagocytosis.
• Adipose tissue is a specialized form of loose connective tissue that stores fat in adipose cells distributed throughout the matrix.
• Adipose tissue pads and insulates the body and stores fuel as fat molecules.
• Each adipose cell contains a large fat droplet that swells when fat is stored and shrinks when the body uses fat as fuel.
• Fibrous connective tissue is dense, due to its large number of collagenous fibers.
• The fibers are organized into parallel bundles, an arrangement that maximizes nonelastic strength.
• This type of connective tissue forms tendons, attaching muscles to bones, and ligaments, joining bones to bones at joints.
• Cartilage has an abundance of collagenous fibers embedded in a rubbery matrix made of a substance called chondroitin sulfate, a protein-carbohydrate complex.
• Chondrocytes secrete collagen and chondroitin sulfate.
• The composite of collagenous fibers and chondroitin sulfate makes cartilage a strong yet somewhat flexible support material.
• The skeleton of a shark and the embryonic skeletons of many vertebrates are cartilaginous.
• We retain cartilage as flexible supports in certain locations, such as the nose, ears, and intervertebral disks.
• The skeleton supporting most vertebrates is made of bone, a mineralized connective tissue.
• Bone-forming cells called osteoblasts deposit a matrix of collagen.
• Calcium, magnesium, and phosphate ions combine and harden within the matrix into the mineral hydroxyapatite.
• The combination of hard mineral and flexible collagen makes bone harder than cartilage without being brittle.
• The microscopic structure of hard mammalian bones consists of repeating units called osteons.
• Each osteon has concentric layers of mineralized matrix deposited around a central canal containing blood vessels and nerves that service the bone.
• Blood functions differently from other connective tissues, but it does have an extensive extracellular matrix.
• The matrix is a liquid called plasma, consisting of water, salts, and a variety of dissolved proteins.
• The liquid matrix enables rapid transport of blood cells, nutrients, and wastes.
• Suspended in the plasma are erythrocytes (red blood cells), leukocytes (white blood cells), and cell fragments called platelets.
• Red cells carry oxygen.
• White cells function in defense against viruses, bacteria, and other invaders.
• Platelets aid in blood clotting.
• Muscle tissue is composed of long cells called muscle fibers that are capable of contracting when stimulated by nerve impulses.
• Arranged in parallel within the cytoplasm of muscle fibers are large numbers of myofibrils made of the contractile proteins actin and myosin.
• Muscle is the most abundant tissue in most animals, and muscle contraction accounts for most of the energy-consuming cellular work in active animals.
• There are three types of muscle tissue in the vertebrate body: skeletal muscle, cardiac muscle, and smooth muscle.
• Attached to bones by tendons, skeletal muscle is responsible for voluntary movements.
• Skeletal muscle consists of bundles of long cells called fibers.
• Each fiber is a bundle of strands called myofibrils.
• Skeletal muscle is also called striated muscle because the arrangement of contractile units, or sarcomeres, gives the cells a striped (striated) appearance under the microscope.
• Cardiac muscle forms the contractile wall of the heart.
• It is striated like skeletal muscle, and its contractile properties are similar to those of skeletal muscle.
• Unlike skeletal muscle, cardiac muscle carries out the unconscious task of contraction of the heart.
• Cardiac muscle fibers branch and interconnect via intercalated disks, which relay signals from cell to cell during a heartbeat.
• Smooth muscle, which lacks striations, is found in the walls of the digestive tract, urinary bladder, arteries, and other internal organs.
• The cells are spindle-shaped.
• They contract more slowly than skeletal muscles but can remain contracted longer.
• Controlled by different kinds of nerves than those controlling skeletal muscles, smooth muscles are responsible for involuntary body activities.
• These include churning of the stomach and constriction of arteries.
• Nervous tissue senses stimuli and transmits signals from one part of the animal to another.
• The functional unit of nervous tissue is the neuron, or nerve cell, which is uniquely specialized to transmit nerve impulses.
• A neuron consists of a cell body and two or more processes called dendrites and axons.
• Dendrites transmit impulses from their tips toward the rest of the neuron.
• Axons transmit impulses toward another neuron or toward an effector, such as a muscle cell that carries out a body response.
• In many animals, nervous tissue is concentrated in the brain.

 The organ systems of an animal are interdependent.

• In all but the simplest animals (sponges and some cnidarians) different tissues are organized into organs.
• In some organs the tissues are arranged in layers.
• For example, the vertebrate stomach has four major tissue layers.
• A thick epithelium lines the lumen and secretes mucus and digestive juices.
• Outside this layer is a zone of connective tissue, surrounded by a thick layer of smooth muscle.
• Another layer of connective tissue encases the entire stomach.
• Many vertebrate organs are suspended by sheets of connective tissues called mesenteries in body cavities moistened or filled with fluid.
• Mammals have a thoracic cavity housing the lungs and heart that is separated from the lower abdominal cavity by a sheet of muscle called the diaphragm.
• Organ systems carry out the major body functions of most animals.
• Each organ system consists of several organs and has specific functions.
• The efforts of all systems must be coordinated for the animal to survive.
• For instance, nutrients absorbed from the digestive tract are distributed throughout the body by the circulatory system.
• The heart that pumps blood through the circulatory system depends on nutrients absorbed by the digestive tract and also on oxygen obtained from the air or water by the respiratory system.
• Any organism, whether single-celled or an assembly of organ systems, is a coordinated living whole greater than the sum of its parts.

Concept 40.3 Animals use the chemical energy in food to sustain form and function

• All organisms require chemical energy for growth, physiological processes, maintenance and repair, regulation, and reproduction.
• Plants use light energy to build energy-rich organic molecules from water and CO2, and then they use those organic molecules for fuel.
• In contrast, animals are heterotrophs and must obtain their chemical energy in food, which contains organic molecules synthesized by other organisms.
• The flow of energy through an animal—its bioenergetics—ultimately limits the animal’s behavior, growth, and reproduction and determines how much food it needs.
• Studying an animal’s bioenergetics tells us a great deal about the animal’s adaptations.
• Food is digested by enzymatic hydrolysis, and energy-containing food molecules are absorbed by body cells.
• Most fuel molecules are used to generate ATP by the catabolic processes of cellular respiration and fermentation.
• The chemical energy of ATP powers cellular work, enabling cells, organs, and organ systems to perform the many functions that keep an animal alive.
• Since the production and use of ATP generates heat, an animal continuously loses heat to its surroundings.
• After energetic needs of staying alive are met, any remaining food molecules can be used in biosynthesis.
• This includes body growth and repair; synthesis of storage material such as fat; and production of reproductive structures, including gametes.
• Biosynthesis requires both carbon skeletons for new structures and ATP to power their assembly.

 Metabolic rate provides clues to an animal’s bioenergetic “strategy.”

• The amount of energy an animal uses in a unit of time is called its metabolic rate—the sum of all the energy-requiring biochemical reactions occurring over a given time interval.
• Energy is measured in calories (cal) or kilocalories (kcal).
• A kilocalorie is 1,000 calories.
• The term Calorie, with a capital C, as used by many nutritionists, is actually a kilocalorie.
• Metabolic rate can be determined several ways.
• Because nearly all the chemical energy used in cellular respiration eventually appears as heat, metabolic rate can be measured by monitoring an animal’s heat loss.
• A small animal can be placed in a calorimeter, which is a closed, insulated chamber equipped with a device that records the animal’s heat loss.
• A more indirect way to measure metabolic rate is to determine the amount of oxygen consumed or carbon dioxide produced by an animal’s cellular respiration.
• These devices may measure changes in oxygen consumed or carbon dioxide produced as activity changes.
• Over long periods, the rate of food consumption and the energy content of food can be used to estimate metabolic rate.
• A gram of protein or carbohydrate contains about 4.5–5 kcal, and a gram of fat contains 9 kcal.
• This method must account for the energy in food that cannot be used by the animal (the energy lost in feces and urine).
• There are two basic bioenergetic “strategies” used by animals.
• Birds and mammals are mainly endothermic, maintaining their body temperature within a narrow range by heat generated by metabolism.
• Endothermy is a high-energy strategy that permits intense, long-duration activity of a wide range of environmental temperatures.
• Most fishes, amphibians, reptiles, and invertebrates are ectothermic, meaning they gain their heat mostly from external sources.
• The ectothermic strategy requires much less energy than is needed by endotherms, because of the energy cost of heating (or cooling) an endothermic body.
• However, ectotherms are generally incapable of intense activity over long periods.
• In general, endotherms have higher metabolic rates than ectotherms.

 Body size influences metabolic rate.

• The metabolic rates of animals are affected by many factors besides whether the animal is an endotherm or an ectotherm.
• One of animal biology’s most intriguing, but largely unanswered, questions has to do with the relationship between body size and metabolic rate.
• Physiologists have shown that the amount of energy it takes to maintain each gram of body weight is inversely related to body size.
• For example, each gram of a mouse consumes about 20 times more calories than a gram of an elephant.
• The higher metabolic rate of a smaller animal demands a proportionately greater delivery rate of oxygen.
• A smaller animal also has a higher breathing rate, blood volume (relative to size), and heart rate (pulse) and must eat much more food per unit of body mass.
• One hypothesis for the inverse relationship between metabolic rate and size is that the smaller the size of an endotherm, the greater the energy cost of maintaining a stable body temperature.
• The smaller the animal, the greater its surface-to-volume ratio, and thus the greater loss of heat to (or gain from) the surroundings.
• However, this hypothesis fails to explain the inverse relationship between metabolism and size in ectotherms, which do not use metabolic heat to maintain body temperature.
• Researchers continue to search for causes underlying this inverse relationship.

 Animals adjust their metabolic rates as conditions change.

• Every animal has a range of metabolic rates.
• Minimal rates power the basic functions that support life, such as cell maintenance, breathing, and heartbeat.
• The metabolic rate of a nongrowing endotherm at rest, with an empty stomach and experiencing no stress, is called the basal metabolic rate (BMR).
• The BMR for humans averages about 1,600 to 1,800 kcal per day for adult males and about 1,300 to 1,500 kcal per day for adult females.
• In ectotherms, body temperature changes with temperature of the surroundings, and so does metabolic rate.
• Therefore, the minimal metabolic rate of an ectotherm must be determined at a specific temperature.
• The metabolic rate of a resting, fasting, nonstressed ectotherm is called its standard metabolic rate (SMR).
• For both ectotherms and endotherms, activity has a large effect on metabolic rate.
• Any behavior consumes energy beyond the BMR or SMR.
• Maximal metabolic rates (the highest rates of ATP utilization) occur during peak activity, such as lifting heavy weights, all-out running, or high-speed swimming.
• In general, an animal’s maximum metabolic rate is inversely related to the duration of activity.
• Both an alligator (ectotherm) and a human (endotherm) are capable of intense exercise in short spurts of a minute or less.
• These “sprints” are powered by the ATP present in muscle cells and ATP generated anaerobically by glycolysis.
• Neither organism can maintain its maximum metabolic rate and peak activity level over longer periods of exercise, although the endotherm has an advantage in endurance tests.
• The BMR of a human is much higher than the SMR of an alligator.
• Both can reach high levels of maximum potential metabolic rates for short periods, but metabolic rate drops as the duration of the activity increases and the source of energy shifts toward aerobic respiration.
• Sustained activity depends on the aerobic process of cellular respiration for ATP supply.
• An endotherm’s respiration rate is about 10 times greater than an ectotherm’s.
• Only endotherms are capable of long-duration activities such as distance running.
• Between the extremes of BMR or SMR and maximal metabolic rate, many factors influence energy requirements.
• These include age, sex, size, body and environmental temperatures, quality and quantity of food, activity level, oxygen availability, hormonal balance, and time of day.
• Diurnal organisms, such as birds, humans, and many insects, are usually active and have their highest metabolic rates during daylight hours.
• Nocturnal organisms, such as bats, mice, and many other mammals, are usually active at night or near dawn and dusk and have their highest metabolic rates then.
• Metabolic rates measured when animals are performing a variety of activities give a better idea of the energy costs of everyday life.
• For most terrestrial animals, the average daily rate of energy consumption is 2–4 times BMR or SMR.
• Humans in most developed countries have an unusually low average daily metabolic rate of about 1.5 times BMR—an indication of relatively sedentary lifestyles.

 Energy budgets reveal how animals use energy and materials.

• Different species of animals use the energy and materials in food in different ways, depending on their environment, behavior, size, and basic energy strategy of endothermy or ectothermy.
• For most animals, the majority of food is devoted to the production of ATP, and relatively little goes to growth or reproduction.
• However, the amount of energy used for BMR (or SMR), activity, and temperature control varies considerably between species.
• For example, the typical annual energy “budgets” of four vertebrates reinforces two important concepts in bioenergetics.
• First, a small animal has a much greater energy demand per kg than does a large animal of the same class.
• Second, an ectotherm requires much less energy per kg than does an endotherm of equivalent size.
• Further, size and energy strategy has a great influence on how the total annual energy expenditure is distributed among energetic needs.
• A human female spends a large fraction of her energy budget for BMR and relatively little for activity and body temperature regulation.
• The cost of nine months of pregnancy and several months of breast feeding amounts to only 5–8% of the mother’s annual energy requirements.
• Growth amounts to about 1% of her annual energy budget.
• A male penguin spends a much larger fraction of his energy expenditures for activity because he must swim to catch his food.
• Because the penguin is well insulated and fairly large, he has relatively low costs of temperature regulation despite living in the cold Antarctic environment.
• His reproductive costs, about 6% of annual energy expenditures, mainly come from incubating eggs and bringing food to his chicks.
• Penguins, like most birds, do not grow once they are adults.
• A female deer mouse spends a large fraction of her energy budget on temperature regulation.
• Because of the high surface-to-volume ratio that goes with small size, mice lose body heat rapidly to the environment and must constantly generate metabolic heat to maintain body temperature.
• Female deer mice spend about 12% of their energy budget on reproduction.
• In contrast to endotherms, the ectothermic python has no temperature regulation costs.
• Like most reptiles, she grows continuously throughout life.
• In one year, she can add 750 g of new body tissue and produce about 650 g of eggs.
• Through the python’s economical ectothermic strategy, she expends only 1/40 of the energy expended by the same-sized endothermic penguin.

Concept 40.4 Many animals regulate their internal environment within relatively narrow limits

• More than a century ago, physiologist Claude Bernard made the distinction between external environments surrounding an animal and the internal environment in which the cells of the animal actually live.
• The internal environment of vertebrates is called the interstitial fluid.
• This fluid exchanges nutrients and wastes with blood contained in microscopic vessels called capillaries.
• Bernard also recognized that many animals tend to maintain relatively constant conditions in their internal environment, even when the external environment changes.
• While a pond-dwelling hydra is powerless to affect the temperature of the fluid that bathes its cells, the human body can maintain its “internal pond” at a more or less constant temperature of about 37°C.
• Similarly, our bodies control the pH of our blood and interstitial fluid to within a tenth of a pH unit of 7.4.
• The amount of sugar in our blood does not fluctuate for long from a concentration of about 90 mg of glucose per 100 mL of blood.
• There are times during the course of the development of an animal when major changes in the internal environment are programmed to occur.
• For example, the balance of hormones in human blood is altered radically during puberty and pregnancy.
• Still, the stability of the internal environment is remarkable.
• Today, Bernard’s “constant internal milieu” is incorporated into the concept of homeostasis, which means “steady state,” or internal balance.
• Actually the internal environment of an animal always fluctuates slightly.
• Homeostasis is a dynamic state, an interplay between outside forces that tend to change the internal environment and internal control mechanisms that oppose such changes.

 Animals may be regulators or conformers for a particular environmental variable.

• Regulating and conforming are two extremes in how animals deal with environmental fluctuations.
• An animal is a regulator for a particular environmental variable if it uses internal control mechanisms to moderate internal change while external conditions fluctuate.
• For example, a freshwater fish maintains a stable internal concentration of solutes in its blood that is higher than the water in which it lives.
• An animal is a conformer for a particular environmental variable if it allows its internal conditions to vary as external conditions fluctuate.
• For example, many marine invertebrates live in environments where solute concentration (salinity) is relatively stable.
• Unlike freshwater fishes, most marine invertebrates do not regulate their internal solute concentration, but rather conform to the external environment.
• No organism is a perfect regulator or conformer.
• An animal may maintain homeostasis while regulating some internal conditions and allowing others to conform to the environment.
• For example, most freshwater fishes regulate their internal solute concentration but allow their internal temperature to conform to external water temperature.

 Homeostasis depends on feedback circuits.

• Any homeostatic control system has three functional components: a receptor, a control center, and an effector.
• The receptor detects a change in some variable in the animal’s internal environment, such as a change in temperature.
• The control center processes the information it receives from the receptor and directs an appropriate response by the effector.
• One type of control circuit, a negative-feedback system, can control the temperature in a room.
• In this case, the control center, called a thermostat, also contains the receptor, a thermometer.
• When room temperature falls, the thermostat switches on the heater, the effector.
• In such a negative-feedback system, a change in the variable being monitored triggers the control mechanism to counteract further change in the same direction.
• Owing to a time lag between receptor and response, the variable drifts slightly above and below the set point, but fluctuations are moderate.
• Negative-feedback mechanisms prevent small changes from becoming too large.
• Most homeostatic mechanisms in animals operate on this principle of negative feedback.
• Human body temperature is kept close to a set point of 37°C by the cooperation of several negative-feedback circuits.
• In contrast to negative feedback, positive feedback involves a change in some variable that triggers mechanisms that amplify rather than reverse the change.
• For example, during childbirth, the pressure of the baby’s head against receptors near the opening of the uterus stimulates uterine contractions.
• These cause greater pressure against the uterine opening, heightening the contractions, which cause still greater pressure.
• Positive feedback brings childbirth to completion, a very different sort of process from maintaining a steady state.
• While some aspects of the internal environment are maintained at a set point, regulated change is essential to normal body functions.
• In some cases, the changes are cyclical, such as the changes in hormone levels responsible for the menstrual cycle in women.
• In other cases, a regulated change is a reaction to a challenge to the body.
• For example, the human body reacts to certain infections by raising the set point for temperature to a slightly higher level, and the resulting fever helps fight infection.
• Over the short term, homeostatic mechanisms can keep a process, such as body temperature, close to a set point, whatever it is at that particular time.
• Over the longer term, homeostasis allows regulated change in the body’s internal environment.
• Internal regulation is expensive.
• Animals use a considerable portion of their energy from the food they eat to maintain favorable internal conditions.

Concept 40.5 Thermoregulation contributes to homeostasis and involves anatomy, physiology, and behavior

• Thermoregulation is the process by which animals maintain an internal temperature within a tolerable range.
• This ability is critical to survival, because most biochemical and physiological processes are very sensitive to changes in body temperature.
• The rates of most enzyme-mediated reactions increase by a factor of 2 or 3 for every 10°C temperature increase until temperature is high enough to begin to denature proteins.
• The properties of membranes also change with temperature.
• Although different species of animals are adapted to different environmental temperatures, each species has an optimal temperature range.
• Thermoregulation helps keep body temperature within the optimal range, enabling cells to function effectively as external temperature fluctuates.

 Ectotherms and endotherms manage their heat budgets very differently.

• One way to classify the thermal characteristics of animals is to emphasize the role of metabolic heat in determining body temperature.
• Ectotherms gain most of their heat from the environment.
• An ectotherm has such a low metabolic rate that the amount of heat it generates is too small to have much effect on body temperature.
• Endotherms can use metabolic heat to regulate their body temperature.
• In a cold environment, an endotherm’s high metabolic rate generates enough heat to keep its body substantially higher than its surroundings.
• Many ectotherms can thermoregulate by behavioral means, such as basking in the sun or seeking out shade.
• In general, ectotherms tolerate greater variation in internal temperature than do endotherms.
• Animals are not classified as ectotherms or endotherms based on whether they have variable or constant body temperatures.
• It is the source of heat used to maintain body temperature that distinguishes ectotherms from endotherms.
• A different—and largely outdated—set of terms can be used to imply variable or constant body temperature.
• A poikilotherm is an animal whose internal temperature varies widely.
• A homeotherm is an animal that maintains relatively stable internal temperatures.
• Another common misconception is the idea that ectotherms are “cold-blooded” and endotherms are “warm-blooded.”
• Ectotherms do not necessarily have low body temperatures.
• While sitting in the sun, many ectothermic lizards have higher body temperatures than mammals.
• Biologists avoid the terms cold-blooded and warm-blooded because they are so misleading.
• Endothermy and ectothermy are not mutually exclusive thermoregulatory strategies.
• A bird is an endotherm but may warm itself in the sun on a cold morning, just as a lizard does.
• Endothermy has several important advantages.
• Being able to generate a large amount of metabolic heat enables endotherms to perform vigorous activity for much longer than is possible for most ectotherms.
• Sustained intense exercise, such as long-distance running or powered flight, is usually only possible for endotherms.
• Terrestrial animals can maintain stable body temperatures despite temperature fluctuations, which are more severe on land than in water.
• For example, no ectotherm can be active in below-freezing weather, but many endotherms function well in such conditions.
• Endothermic vertebrates also have mechanisms for cooling their bodies in hot environments, allowing them to withstand heat loads that would be intolerable for most ectotherms.
• However, ectotherms can tolerate larger fluctuations in their internal temperatures.
• Being endothermic is energetically expensive.
• For example, at 20°C, a human at rest has a BMR or 1,300 to 1,800 kcal per day.
• An American alligator of similar weight has an SMR of only 60 kcal per day.
• As a result, ectotherms need to consume far more food than ectotherms of equivalent size.
• This is a serious disadvantage if food supplies are limited.
• Ectothermy is an extremely effective and successful strategy in most of Earth’s environments, as is shown by the abundance and diversity of ectothermic animals.

 Animals regulate the exchange of heat with their environment.

• Animals exchange heat with their external environment by four physical processes: conduction, convection, radiation, and evaporation.
• Heat is always transferred from a hotter object to a cooler object.
• Endotherms and thermoregulating ectotherms must manage their heat budgets so that rates of heat gain are equal to rates of heat loss.
• Five general categories of adaptations help animals thermoregulate.
• A major thermoregulatory adaptation in mammals and birds is insulation: hair, feathers, or fat layers.
• Insulation reduces the flow of heat between an animal and its environment and lowers the energy cost of keeping warm.
• In mammals, the insulating material is associated with the integumentary system, the outer covering of the body.
• Skin is a key organ of the integumentary system.
• Skin functions as a thermoregulatory organ by housing nerves, sweat glands, blood vessels, and hair follicles.
• It also protects internal body parts from mechanical injury, infection, and desiccation.
• Skin consists of two layers, the epidermis and the dermis, underlain by a tissue layer called the hypodermis.
• The epidermis is the outer layer of skin, composed largely of dead epithelial cells.
• The dermis supports the epidermis and contains hair follicles, oil and sweat glands, muscles, nerves, and blood vessels.
• Adipose tissue provides varying degrees of insulation, depending on the species.
• The insulating power of a layer of fur or feathers depends mostly on how much air the layer traps.
• Hair loses most of its insulating power when wet.
• Land mammals and birds react to cold by raising their fur or feathers to trap a thicker layer of air.
• Human goose bumps are a vestige of our hair-raising ancestors.
• Marine mammals have a very thick layer of insulating blubber just under their skin.
• The skin temperature of a marine mammal is close to water temperature.
• However, blubber insulation is so effective that marine mammals can maintain body core temperatures of 36–38°C.
• Many endotherms and ectotherms can alter the amount of blood flow between the body core and the skin.
• Elevated blood flow in the skin results from vasodilation, an increase in the diameter of superficial blood vessels near the body surface.
• Vasodilation is triggered by nerve signals that relax the muscles of the vessel walls.
• In endotherms, vasodilation usually warms the skin, increasing the transfer of body heat to a cool environment.
• The reverse process, vasoconstriction, reduces blood flow and heat transfer by decreasing the diameter of superficial vessels.
• Another circulatory adaptation is an arrangement of blood vessels called a countercurrent heat exchanger, which reduces heat loss.
• In some species, blood can either go through the heat exchanger or bypass it.
• The relative amount of blood that flows through the two paths varies, adjusting the rate of heat loss.
• Unlike most fishes, which are thermoconformers, some specialized endothermic bony fishes and sharks have circulatory adaptations to retain metabolic heat.
• Endothermic fishes include bluefin tuna, swordfish, and great white sharks.
• Large arteries convey most of the cold blood from the gills to tissues just under the skin.
• Branches deliver blood to the deep muscles, where small vessels are arranged into a countercurrent heat exchanger.
• Endothermy enables vigorous, sustained activity that is characteristic of these animals.
• Some reptiles also have physiological adaptations to regulate heat loss.
• In the marine iguanas of the Galápagos Islands, body heat is conserved by vasoconstriction of superficial blood vessels.
• Many endothermic insects (bumblebees, honeybees, some moths) have a countercurrent heat exchanger that helps maintain a high temperature in the thorax, where the flight muscles are located.
• In some insects, the countercurrent mechanism can be “shut down” to allow heat to be shed during hot weather.
• A bumblebee queen uses this mechanism to incubate her eggs.
• She generates heat by shivering her flight muscles and then transfers the heat to her abdomen, which she presses against her eggs.
• Many mammals and birds live in places where thermoregulation requires cooling as well as warming.
• If environmental temperature is above body temperature, evaporation is the only way to keep body temperature from rising.
• Terrestrial animals lose water by evaporation across the skin and when they breathe.
• Water absorbs considerable heat when it evaporates; it is 50 to 100 times more effective than air in transferring heat.
• Some animals have adaptations to augment evaporative cooling.
• Panting is important in birds and many mammals.
• Some birds have a pouch richly supplied with blood vessels in the floor of the mouth.
• Birds flutter the pouch to increase evaporation.
• Sweating or bathing moistens the skin and enhances evaporative cooling.
• Many terrestrial mammals have sweat glands controlled by the nervous system.
• Other mechanisms to promote evaporative cooling include spreading saliva on skin or regulating the amount of mucus secretion.
• Many endotherms and ectotherms use behavioral responses to control body temperature.
• Many ectotherms can maintain a constant body temperature by simple behaviors.
• Some animals hibernate or migrate to a more suitable climate.
• Amphibians regulate body temperature mainly by behavior, by moving to a location where solar heat is available or by seeking shade.
• Reptiles also thermoregulate behaviorally.
• When cool, they seek warm places, orient themselves toward a heat source, and expand the body surface exposed to the heat source.
• When hot, they move to cool places or turn away from the heat source.
• Many terrestrial invertebrates use similar behavioral mechanisms.
• Honeybees use a thermoregulatory mechanism that depends on social behavior.
• In cold weather, they increase heat production and huddle together to retain heat.
• They maintain a relatively constant temperature by changing the density of the huddling, and moving individuals between the cooler outer edges of the cluster and the warmer center.
• Honeybees expend considerable energy to keep warm during long periods of cold weather.
• This is the main function of the honey stored in the hive.
• Honeybees also cool the hive in hot weather by transporting water to it and fanning it with their wings to promote evaporation and convection.
• Endotherms vary heat production to counteract constant heat loss.
• For example, heat production is increased by such muscle activity as moving or shivering.
• Certain mammalian hormones can cause mitochondria to increase their metabolic activity and produce heat instead of ATP.
• This nonshivering thermogenesis (NST) takes place throughout the body.
• Some mammals have brown fat that is specialized for rapid heat production.
• Through shivering and NST, mammals and birds may increase their metabolic heat production to 5 or 10 times the minimal levels characteristic of warm weather.
• A few large reptiles can become endothermic in particular circumstances.
• For example, female pythons that are incubating eggs increase their metabolic rate by shivering, generating enough heat to elevate egg temperatures by 5–7°C during incubation.
• The smallest endotherms are flying insects such as bees and moths.
• These insects elevate body temperature by shivering before taking off.
• They contract their flight muscles in synchrony to produce only slight wing movements but considerable heat.
• The regulation of body temperature in humans is a complex system facilitated by feedback mechanisms.
• Nerve cells that control thermoregulation are concentrated in a brain region called the hypothalamus.
• The hypothalamus contains a group of nerve cells that functions as a thermostat.
• Nerve cells that sense temperature are in the skin, in the hypothalamus itself, and in other body regions.
• If the thermostat in the brain detects a decrease in the temperature of the blood below the set point, it inhibits heat loss mechanisms and activates heat-saving ones such as vasoconstriction of superficial vessels and erection of fur, while stimulating heat-generating mechanisms such as shivering.
• If the thermostat in the brain detects a rise in the temperature of the blood above the set point, it shuts down heat retention mechanisms and promotes body cooling by vasodilation, sweating, or panting.

 Animals can acclimatize to a new range of environmental temperatures.

• Many animals can adjust to a new range of environmental temperatures by a physiological response called acclimatization.
• Ectotherms and endotherms acclimatize differently.
• In birds and mammals, acclimatization includes adjusting the amount of insulation and varying the capacity for metabolic heat production.
• Acclimatization in ectotherms involves compensating for temperature changes.
• Acclimatization responses in ectotherms often include adjustments at the cellular level.
• Cells may increase the production of certain enzymes or produce enzyme variants with different temperature optima.
• Membranes also change the proportions of saturated and unsaturated lipids to keep membranes fluid at different temperatures.
• Some ectotherms produce “antifreeze” compounds, or cryoprotectants, to prevent ice formation in body cells.
• These compounds allow overwintering ectotherms such as frogs and arthropods to withstand body temperatures well below zero.
• Arctic and antarctic fishes also have cryoprotectants to protect body tissues.
• Cells can make rapid adjustments to temperature changes.
• For example, mammalian cells grown in culture respond to increased temperature by producing and accumulating stress-induced proteins, including heat-shock proteins.
• These molecules, found in bacteria, yeast, plants, and animals, help to maintain the integrity of other proteins that would otherwise be denatured by severe heat.
• Stress-induced proteins help prevent cell death when an organism is challenged by severe changes in cellular environment.

 Animals may conserve energy through torpor.

• Some animals deal with severe conditions by an adaptation called torpor.
• Torpor is a physiological state in which activity is low and metabolism decreases.
• Hibernation is long-term torpor that is an adaptation to winter cold and food scarcity.
• When vertebrate endotherms enter torpor or hibernation, their body temperatures decline.
• Some hibernating mammals cool to 1–2°C, and a few drop slightly below 0°C in a supercooled, unfrozen state.
• Metabolic rates during hibernation may be several hundred times lower than if animals tried to maintain normal body temperatures.
• Hibernators can survive for very long periods on limited supplies of energy stored in body tissues or as food cached in a burrow.
• Estivation, or summer torpor, is also characterized by slow metabolism or inactivity.
• Estivation allows animals to survive long periods of high temperatures and scarce water supplies.
• Hibernation and estivation are often triggered by seasonal changes in the length of daylight.
• Some hibernators prepare for winter by storing food in their burrows or by eating huge quantities of food.
• Ground squirrels double their weight prior to hibernation.
• Many small mammals and birds exhibit a daily torpor that is adapted to their feeding patterns.
• For example, most bats and shrews feed at night and go into torpor during daylight hours.
• Chickadees and hummingbirds feed during the day and go into torpor on cold nights.
• The body temperature of a hummingbird may drop by 25–30°C at night.
• An animal’s daily cycle of activity and torpor appears to be a built-in rhythm controlled by its biological clock.
• Even if food is made available to a shrew, it will go through daily torpor.