40 Basic Principles of Animal CONCEPT 40.1 example of convergent evolution (see Concept 22.3).

Natural selection often results in similar adaptations when

Animal form and function diverse organisms face the same environmental challenge,

Form and Function are correlated at all levels

of organization
such as overcoming drag during swimming.
Physical laws also influence animal body plans with regard
to maximum size. As body dimensions increase, thicker skel-
etons are required to maintain adequate support. This limita-
Over the course of its life, an Emperor penguin faces the same
tion affects internal skeletons, such as those of vertebrates, as
KEY CONCEPTS fundamental challenges as any other animal, whether hydra,
well as external skeletons, such as those of insects and other
hawk, or human. All animals must obtain nutrients and oxygen,
40.1 Animal form and function
fight off infection, and survive to produce offspring. Given that
arthropods. In addition, as bodies increase in size, the muscles
are correlated at all levels of required for locomotion must represent an ever-larger fraction
organization p. 874
all animal species share these and other basic requirements, why
of the total body mass. At some point, mobility becomes lim-
does their form, including anatomy—biological structure—
40.2 Feedback control maintains the
vary so widely? The answer lies in natural selection and adapta-
ited. By considering the fraction of body mass in leg muscles
internal environment in many and the effective force such muscles generate, scientists can
tion. Natural selection favors those variations in a population
animals p. 881 estimate maximum speed for a wide range of body plans. In the
that increase relative fitness (see Concept 23.4). The evolution-
40.3 Homeostatic processes for case of the 6-meter-tall dinosaur Tyrannosaurus rex, there is con-
ary adaptations that enable survival vary among environments
thermoregulation involve form, troversy, with some scientists calculating a top running speed as
and species but frequently result in a close match of form to
function, and behavior p. 884 fast as that of an Olympic sprinter—30 km/hr (19 miles/hour),
function, as illustrated for the Emperor penguins in Figure 40.1.
but others inferring that T. rex was at best a fast walker.
40.4 Energy requirements are related Because structure and function are correlated, examining anat-
to animal size, activity, and
omy often provides clues to physiology—biological function.
environment p. 889
An animal’s size and shape are fundamental aspects of form
Exchange with the Environment
that significantly affect the way the animal interacts with its
Study Tip Figure 40.1 Emperor penguins (Aptenodytes forsteri) live in Antarctica, Earth’s
Animals must exchange nutrients, waste products, and gases
coldest and windiest continent. In summer, these birds catch fish by diving down environment. Although we may refer to size and shape as
Draw a diagram: When you encounter 500 meters in water only 2°C above freezing. In winter, the females forage and the with their environment, and this requirement imposes an addi-
elements of a “body plan” or “design,” this does not imply a
an example in the chapter of how an males incubate eggs while temperatures drop to –40°C and winds gust to 200 km/hr. tional limitation on body plans. Exchange occurs as substances
process of conscious invention. The body plan of an animal
animal maintains a steady internal dissolved in an aqueous solution move across the plasma mem-
is the result of a pattern of development programmed by the
state, draw a simple circuit diagram brane of each cell. A single-celled organism, such as the amoeba
genome, itself the product of millions of years of evolution.
(see example—illustrations are How do animals regulate their internal state even in in Figure 40.3a, has a sufficient membrane surface area in con-
optional!). Label the variable being
controlled, a perturbation that affects
changing or harsh environments? tact with its environment to carry out all necessary exchange.
Evolution of Animal Size and Shape In contrast, an animal is composed of many cells, each with
the variable, the response, and its effect Adaptations in form, function, and behavior help maintain an animal’s internal
EVOLUTION Many different body plans have arisen during its own plasma membrane across which exchange must occur.
in restoring the normal state. environment. Adaptations that limit variation in temperature and other internal
variables are widespread and diverse. Consider, for example, three adaptations the course of evolution, but these variations fall within certain The rate of exchange is proportional to the membrane surface
Variable: BODY TEMPERATURE that help an Emperor penguin stay warm: bounds. Physical laws that govern strength, diffusion, move- area involved in exchange, whereas the amount of material that
(example: penguins) ment, and heat exchange limit the range of animal forms. must be exchanged is proportional to the total body volume.
Normal body Form (anatomy): An insulating As an example of how physical laws A multicellular organization therefore works only if
temperature
layer of fat (blubber) reduces heat loss constrain evolution, let’s consider every cell has access to a suitable aqueous environ-
Perturbation: from most of the penguin’s body . Figure 40.2 Convergent
(blue body areas in this thermal how some properties of water limit the evolution in fast
ment, either inside or outside the animal’s body.
Effect: body cold weather Function (physiology): Rapid
temperature causes drop image). cycles of muscle contraction and possible shapes for animals that are swimmers. Many animals with a simple internal organiza-
warms toward in body relaxation during fast swimmers. Water is about 1,000 tion have body plans that enable direct exchange
normal temperature shivering produce times denser than air and also far more between almost all their cells and the external
heat at a cellular
viscous. Therefore, any bump on an environment. For example, a pond-dwelling
level.
Response: animal’s body surface that causes drag hydra has a saclike body plan and a body wall only
penguins huddle Seal
together, reducing impedes a swimmer more than it would two cell layers thick (Figure 40.3b). Because its gas-
exposed surface area a runner or flyer. Tuna and other fast trovascular cavity opens to the external environ-
ray-finned fishes can swim at speeds ment, both the outer and inner layers of cells are
up to 80 km/hr (50 miles/hour). Sharks, constantly bathed by pond water. Another com-
Go to Mastering Biology
penguins, dolphins, and seals are also mon body plan that maximizes exposure to the
For Students (in eText and Study Area) Penguin surrounding medium is a flat shape. Consider, for
relatively fast swimmers. As illustrated
• Get Ready for Chapter 40
Behavior: By packing together in by the three examples in Figure 40.2, instance, a parasitic tapeworm, which can reach
• Figure 40.17 Walkthrough:
Thermoregulation in Humans groups of up to several thousand, these animals all have a shape that is several meters in length (see Figure 33.11). A thin,
Emperor penguins greatly reduce fusiform, meaning tapered on both flat shape places most cells of the worm in direct
For Instructors to Assign (in Item Library) their exposure to wind and cold.
• Everyday Biology: How to Keep ends. The similar streamlined shape contact with its particular environment—the
Your Cool found in these speedy vertebrates is an Tuna nutrient-rich intestinal fluid of a vertebrate host.
• Tutorial: Thermoregulation

873 874 UNIT SEVEN Animal Form and Function

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 . Figure 40.3 Direct exchange with the environment. hundreds of thousands of times smaller than that for a water food gradually, controlling the release of stored energy. In but also regulates the level of sugar in the blood as a vital part
flea. Nevertheless, every cell in the whale has access to oxygen, addition, specialized filtration systems can adjust the compo- of the endocrine system.
nutrients, and other resources. How is this accomplished? sition of the internal fluid that bathes the animal’s body cells. Just as viewing the body’s organization from the “bottom
Mouth
In whales and most other animals, the evolutionary adap- In this way, an animal can maintain a relatively stable inter- up” (from cells to organ systems) reveals emergent properties, a
Gastrovascular tations that enable sufficient exchange with the environ- nal environment despite the fact that it is living in a change- “top-down” view of the hierarchy reveals the multilayered basis
cavity
ment are specialized surfaces that are extensively branched able external environment. A complex body plan is especially of specialization. Organ systems include specialized organs
Exchange or folded (Figure 40.4). In almost all cases, these exchange advantageous for animals living on land, where the external made up of specialized tissues and cells. Consider the human
Exchange
surfaces lie within the body, an arrangement that protects environment may be highly variable. digestive system: Each organ has specific roles. In the case of
their delicate tissues from abrasion or dehydration and allows the stomach, one role is to initiate protein breakdown. This
for streamlined body contours. The branching or folding process requires a churning motion powered by stomach mus-
greatly increases surface area (see Figure 33.8). In humans, for
Hierarchical Organization of Body Plans cles, as well as digestive juices secreted by the stomach lining.
Exchange
example, the exchange surfaces for digestion, respiration, and Cells form a working animal body through their emergent Producing digestive juices, in turn, requires highly specialized
circulation each have an area more than 25 times larger than properties, which arise from successive levels of structural and cell types: One cell type secretes a protein-digesting enzyme, a
that of the skin. functional organization (see Concept 1.1). Cells are organized second generates concentrated hydrochloric acid, and a third
0.1 mm 1 mm into tissues, groups of cells with a similar appearance and
Internal body fluids link exchange surfaces to body cells. produces mucus, which protects the stomach lining.
(a) An amoeba, a single-celled The spaces between cells are filled with fluid, known in many a common function. Different types of tissues are further The specialized and complex organ systems of animals are
(b) A hydra, an animal with
organism two layers of cells animals as interstitial fluid (from the Latin for “stand organized into functional units called organs. (The simplest built from a limited set of cell and tissue types. For example,
between”). Complex body plans also include a circulatory animals, such as sponges, lack organs or even true tissues.) lungs and blood vessels have different functions but are lined
fluid, such as blood. Exchange between the interstitial fluid Groups of organs that work together, providing an additional by tissues that are of the same basic type and therefore share
Our bodies and those of most other animals are composed and the circulatory fluid enables cells throughout the body to level of organization and coordination, make up an organ many properties.
of compact masses of cells, with an internal organization obtain nutrients and get rid of wastes (see Figure 40.4). system (Table 40.1). Thus, for example, the skin is an organ There are four main types of animal tissues: epithelial,
much more complex than that of a hydra or a tapeworm. For Complex body plans offer numerous benefits. For exam- of the integumentary system, which protects against infec- connective, muscle, and nervous. Figure 40.5 explores the
such a body plan, increasing the number of cells decreases the ple, an external skeleton can protect against predators, and tion and helps regulate body temperature. structure and function of each type. In later chapters, we’ll
ratio of outer surface area to total volume. As an extreme com- sensory organs can provide detailed information on the ani- Many organs have more than one physiological role. If the discuss how these tissue types contribute to the functions of
parison, the ratio of outer surface area to volume for a whale is mal’s surroundings. Internal digestive organs can break down roles are distinct enough, we consider the organ to belong to particular organ systems.
more than one organ system. The pancreas, for instance, pro-
duces enzymes critical to the function of the digestive system Mastering Biology Animation: Overview of Animal Tissues
c Figure 40.4 Internal exchange surfaces of EXTERNAL ENVIRONMENT
complex animals. Most animals have surfaces that CO2
are specialized for exchanging chemicals with the Food O2
surroundings. These exchange surfaces are Mouth
Table 40.1 Organ Systems in Mammals
usually internal but are connected to the ANIMAL
environment via openings on the body surface BODY Organ System Main Components Main Functions
(the mouth, for example). The exchange surfaces Digestive Mouth, pharynx, esophagus, stomach, intestines, liver, pancreas, Food processing (ingestion, digestion,

250 om
are finely branched or folded, giving them a Respiratory anus (See Figure 41.8.) absorption, elimination)
very large area. The digestive, respiratory, and od
system
Blo

excretory systems all have such exchange Circulatory Heart, blood vessels, blood (See Figure 42.5.) Internal distribution of materials
A microscopic view of the lung
surfaces. Chemicals exchanged across reveals that it is much more Respiratory Lungs, trachea, other breathing tubes (See Figure 42.24.) Gas exchange (uptake of oxygen; disposal
these surfaces are transported throughout sponge-like than balloon-like. This of carbon dioxide)
the body via the circulatory system. Heart construction provides an expansive
Cells Immune and lymphatic Bone marrow, lymph nodes, thymus, spleen, lymph vessels Body defense (fighting infections and
VISUAL SKILLS Using this diagram, explain wet surface for gas exchange with
the environment (SEM). (See Figure 43.6.) virally induced cancers)
how exchange carried out by animals can be
described as both internal and external. Excretory Kidneys, ureters, urinary bladder, urethra (See Figure 44.12.) Disposal of metabolic wastes; regulation of
Nutrients Circulatory osmotic balance of blood
system
Endocrine Pituitary, thyroid, pancreas, adrenal, and other hormone- Coordination of body activities (such as
secreting glands (See Figure 45.8.) digestion and metabolism)
Interstitial
Reproductive Ovaries or testes and associated organs (See Figures 46.9 Gamete production; promotion of fertiliza-
fluid
and 46.10.) tion; support of developing embryo

Nervous Brain, spinal cord, nerves, sensory organs (See Figure 49.6.) Coordination of body activities; detection
of stimuli and formulation of responses
Digestive Excretory to them
system system
100 om

Integumentary Skin and its derivatives (such as hair, claws, sweat glands) Protection against mechanical injury,
50 om

(See Figure 50.5.) infection, dehydration; thermoregulation

The lining of the small intestine Anus Within the kidney, blood is filtered Skeletal Skeleton (bones, tendons, ligaments, cartilage) Body support, protection of internal
has finger-like projections that across the surface of long, narrow (See Figure 50.37.) organs, movement
expand the surface area for Unabsorbed Metabolic waste products blood vessels packed into Muscular Skeletal muscles (See Figure 50.26.) Locomotion and other movement
nutrient absorption (SEM). matter (feces) (nitrogenous waste) ball-shaped structures (SEM).

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 . Figure 40.5 Exploring Structure and Function in Animal Tissues . Figure 40.5 (continued) Exploring Structure and Function in Animal Tissues

Epithelial Tissue Connective Tissue

Occurring as sheets of cells, epithelial tissues, or epithelia (singu- Stratified squamous Connective tissue, consisting of a sparse population of cells scat- reticular fibers join connective tissue to adjacent tissues, and
lar, epithelium), cover the outside of the body and line organs and epithelium tered through an extracellular matrix, holds many tissues and elastic fibers make tissues elastic. If you pinch a fold of tissue
cavities within the body. Because epithelial cells are closely packed, organs together and in place. The matrix generally on the back of your hand, the collagenous and reticular
often with tight junctions, they function as a barrier against consists of a web of fibers embedded in a liquid, fibers prevent the skin from being pulled far from the bone,
Apical
mechanical injury, pathogens, and fluid loss. Epithelia also form jellylike, or solid foundation. Within the matrix whereas the elastic fibers restore the skin to its original shape
surface
active interfaces with the environment. For example, the epithe- are numerous cells called fibroblasts, which when you release your grip. Different mixtures of fibers
lium that lines the nasal passages is crucial for Basal secrete fiber proteins, and macrophages, which and foundation form the major types of connective tissue
olfaction, the sense of smell. Note surface engulf foreign particles and any cell debris by shown below.
how different cell shapes and phagocytosis. Mastering Biology Animation:
arrangements correlate with Connective tissue fibers are of three kinds: Connective Tissue
distinct functions. A stratified squamous epi- Collagenous fibers provide strength and flexibility,
thelium is multilayered and
Mastering Biology Blood
regenerates rapidly. New cells
Animation: Epithelial formed by division near the Blood has a liquid extracellular
Tissue basal surface push outward, matrix called plasma, which con-
replacing cells that are sloughed Loose connective tissue sists of water, salts, and dissolved
off. This epithelium is common- proteins. Suspended in plasma
ly found on surfaces subject to The most widespread connec- are erythrocytes (red blood cells),
Collagenous fiber
abrasion, such as the outer skin tive tissue in the vertebrate leukocytes (white blood cells),
and the linings of the mouth, body is loose connective tissue, and cell fragments called platelets.
anus, and vagina. which binds epithelia to Red cells carry oxygen, white cells
underlying tissues and holds function in defense, and platelets
organs in place. Loose con- aid in blood clotting.
nective tissue gets its name

120 om
from the loose weave of its Plasma
fibers, which include all three
Cuboidal epithelium Simple columnar Simple squamous Pseudostratified types. It is found in the skin Elastic fiber White
epithelium epithelium columnar epithelium and throughout the body. blood cells

50 om
Fibrous connective tissue
Fibrous connective tissue is
dense with collagenous fibers. Red blood cells
The single layer of platelike It is found in tendons, which
A cuboidal epithelium, with cells that form a simple squa- attach muscles to bones, and Adipose tissue Cartilage
dice-shaped cells specialized mous epithelium functions in in ligaments, which connect
The large, brick-shaped cells of A pseudostratified epithelium Adipose tissue is a specialized Cartilage contains collagenous
for secretion, makes up the the exchange of material by dif-

30 om
simple columnar epithelia are consists of a single layer of cells bones at joints. loose connective tissue that fibers embedded in a rubbery
epithelium of kidney tubules fusion. This type of epithelium,
often found where secretion or varying in height and the posi- stores fat in adipose cells dis- protein-carbohydrate complex
and many glands, including which is thin and leaky, lines
active absorption is important. tion of their nuclei. In many tributed throughout its matrix. called chondroitin sulfate.
the thyroid gland and salivary blood vessels and the air sacs Nuclei
For example, a simple columnar vertebrates, a pseudostratified Adipose tissue pads and insu- Cells called chondrocytes secrete
glands. of the lungs, where diffusion of
epithelium lines the intestines, epithelium of ciliated cells forms lates the body and stores fuel as the collagen and chondroitin
nutrients and gases is essential.
secreting digestive juices and a mucous membrane that lines fat molecules. Each adipose cell sulfate, which together make
absorbing nutrients. portions of the respiratory tract. Bone contains a large fat droplet that cartilage a strong yet flexible
The beating cilia sweep the film The skeleton of most vertebrates is made of bone, swells when fat is stored and support material. The skeletons
of mucus along the surface. a mineralized connective tissue. Bone-forming cells shrinks when the body uses that of many vertebrate embryos
called osteoblasts deposit a matrix of collagen. Calcium, fat as fuel. contain cartilage that is re-
Lumen Apical surface magnesium, and phosphate ions combine into a hard placed by bone as the embryo
Polarity of epithelia matures. Cartilage remains in
mineral within the matrix. The microscopic structure of
All epithelia are polarized, meaning that they have two differ- hard mammalian bone consists of repeating units called Lipid droplets some locations, such as the
ent sides. The apical surface faces the lumen (cavity) or outside osteons. Each osteon has concentric layers of the min- disks that act as cushions
of the organ and is therefore exposed to fluid or air. Specialized eralized matrix, which are deposited around a central between vertebrae.
projections often cover this surface. For example, the apical sur-

150 om
canal containing blood vessels and nerves.
face of the epithelium lining the small intestine is covered with
microvilli, projections that increase the surface area available Chondrocytes
Central canal
Basal surface for absorbing nutrients. Opposite the apical surface of each
epithelium is the basal surface.

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700 om
10 om

Osteon
Chondroitin sulfate

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 Coordination and Control . Figure 40.6 Signaling in the endocrine and nervous
systems.
For an animal’s tissues and organ systems to function
effectively, they must act in concert with one another. (a) Signaling by hormones (b) Signaling by neurons
Muscle Tissue For example, when the wolf shown in Figure 40.5 hunts,
blood flow is regulated to bring adequate nutrients and STIMULUS STIMULUS
gases to its leg muscles, which in turn are activated by the
The tissue responsible for nearly all types of body movement is muscle
tissue. All muscle cells consist of filaments containing the proteins
brain in response to cues detected by the nose. What sig-
actin and myosin, which together enable muscles to contract. There nals coordinate activity? How do the signals move within Endocrine
are three types of muscle tissue in the vertebrate body: skeletal, cell Cell body
the body?
smooth, and cardiac. of neuron
Animals have two major systems for coordinating and
Mastering Biology Animation: Muscle Tissue controlling responses to stimuli: the endocrine and nervous Nerve Axon
systems (Figure 40.6). In the endocrine system, signal- impulse
Hormone
ing molecules released into the bloodstream by endocrine
cells are carried to all locations in the body. In the nervous Signal travels Signal travels
Skeletal muscle system, neurons transmit signals along dedicated routes everywhere along axon to
via the a specific
Attached to bones by tendons, skeletal connecting specific locations in the body. In each system, the bloodstream. location.
muscle, or striated muscle, is responsible
type of pathway used is the same regardless of whether the
for voluntary movements. Skeletal muscle
consists of bundles of long cells that are signal’s ultimate target is at the other end of the body or just a
Smooth muscle Cardiac muscle
called muscle fibers. During development, few cell diameters away.
skeletal muscle fibers form by the fusion of Smooth muscle, which lacks striations, Cardiac muscle forms the contractile Axon
The signaling molecules that are broadcast throughout Blood
many cells, resulting in multiple nuclei in is found in the walls of the digestive wall of the heart. It is striated like skeletal
each muscle fiber. The arrangement of con- tract, urinary bladder, arteries, and other muscle and has similar contractile proper- the body by the endocrine system are called hormones. vessel Nerve
tractile units, or sarcomeres, along the internal organs. The cells are spindle- ties. Unlike skeletal muscle, however, car- It takes seconds for hormones to be released into the blood- impulse
fibers gives the cells a striped (striated) shaped. Smooth muscles are responsible diac muscle has branched fibers that inter- stream and carried throughout the body. The effects are often
appearance. In adult mammals, build- for involuntary body activities, such as connect via intercalated disks, which relay
ing muscle increases the size but not the churning of the stomach and constric- signals from cell to cell and help synchronize long-lasting, however, because hormones can remain in the
bloodstream for minutes or even hours. Axon
number of muscle fibers. tion of arteries. heart contraction.
Different hormones cause distinct effects, and only
Nuclei cells that have receptors for a particular hormone respond
(Figure 40.6a). Depending on which cells have receptors for
that hormone, the hormone may have an effect in just a
Muscle single location or in sites throughout the body. For example,
fiber
thyroid-stimulating hormone (TSH) acts solely on cells in the
Sarcomere thyroid gland. They in turn release thyroid hormone, which Response: Limited to cells

100 om 25 om 25 om
Response: Limited to cells that connect by specialized
Nucleus Muscle fibers Nucleus Intercalated disk acts on nearly every body tissue to increase oxygen consump- that have a receptor ( ) for junctions to an axon that
tion and heat production. the signal transmits an impulse
In the nervous system, signals called nerve impulses travel
Nervous Tissue to specific target cells along communication lines consisting
mainly of axons (Figure 40.6b). Transmission in the nervous VISUAL SKILLS After comparing the two diagrams, explain why a
Nervous tissue functions in the receipt, processing, and transmission of information. Nervous tissue contains particular nerve impulse signal has only one physical pathway but a
neurons, or nerve cells, which transmit nerve impulses, as well as support cells called glial cells, or simply system is extremely fast; nerve impulses take only a frac-
particular hormone molecule can have multiple physical pathways.
glia. In many animals, a concentration of nervous tissue forms a brain, an information-processing center. tion of a second to reach the target and last only a fraction
of a second.
Mastering Biology Animation: Nervous Tissue
Nerve impulses can act on other neurons, on muscle cells, Because the two major communication systems of the
15 om
Neurons Glia and on cells and glands that produce secretions. Unlike the body differ in signal type, transmission, speed, and dura-
Glia
Neurons are the basic Neuron: The various types of glia endocrine system, the nervous system conveys information tion, it is not surprising that they are adapted to different
units of the nervous sys- Dendrites help nourish, insulate, and by the pathway the signal takes. For example, a person can functions. The endocrine system is especially well adapted
tem. A neuron receives replenish neurons, and
Cell body distinguish different musical notes because each note’s fre- for coordinating gradual changes that affect the entire body,
nerve impulses from in some cases, modulate
other neurons via its neuron function. quency activates neurons in the ear that connect to slightly such as growth, development, reproduction, metabolic pro-
cell body and multiple Axon different locations in the brain. cesses, and digestion. The nervous system is well suited for
extensions called den-
Communication in the nervous system usually involves directing immediate and rapid responses to the environment,
drites. Neurons transmit
Axons of more than one type of signal. Nerve impulses travel along such as reflexes and other rapid movements. Nevertheless,
impulses to neurons, neurons
45 om

muscles, or other cells axons, sometimes over long distances, as changes in voltage. the two systems often work in close coordination. Both
via extensions called In contrast, passing information from one neuron to another help maintain a stable internal environment, our next topic
axons, which are often Blood
bundled together into (Fluorescent LM) vessel often involves very short-range chemical signals. of discussion.
nerves. (Confocal LM)

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 CONCEPT CHECK 40.1 fluid. In addition, conforming need not involve changes . Figure 40.8 A nonliving example of temperature that “damps” its stimulus (see Figure 1.10). This type of feed-
in an internal variable: Many marine invertebrates, such regulation: control of room temperature. Regulating room back regulation plays a major role in homeostasis in animals.
1. What properties do all types of epithelia share? temperature depends on a sensor/control center (a thermostat)
as spider crabs (genus Libinia), let their internal solute con- For example, when you exercise vigorously, you produce heat,
2. VISUAL SKILLS Consider the idealized animal in Figure 40.4. that detects temperature change and activates mechanisms that
At which sites must oxygen cross a plasma membrane in centration conform to the relatively stable salinity of their reverse that change. which increases your body temperature. Your nervous system
traveling from the external environment to the cytoplasm ocean environment. detects this increase and triggers sweating. The evaporation
of a body cell?
of sweat from your skin then cools your body, helping return
3. WHAT IF? Suppose you are standing at the edge of a cliff
body temperature to its set point and eliminating the stimulus.
and suddenly slip, barely managing to keep your balance and Homeostasis Homeostasis is a dynamic equilibrium, an interplay
avoid falling. As your heart races, you feel a burst of energy,
due in part to a surge of blood into dilated (widened) vessels The steady body temperature of a river otter and the Thermostat turns between external factors that tend to change the internal
in your muscles and an upward spike in the level of glucose in stable concentration of solutes in a freshwater bass are air conditioner on. environment and internal control mechanisms that oppose
your blood. Why might you expect that this “fight-or-flight” examples of homeostasis, which means the maintenance Room temperature Room temperature
response requires both the nervous and endocrine systems? increases. decreases.
such changes. Note that physiological responses to stimuli
of internal balance. In achieving homeostasis, animals are not instantaneous, just as switching on a furnace does
For suggested answers, see Appendix A.
maintain a “steady state”—a relatively constant inter- not immediately warm a room. As a result, homeostasis
nal environment—even when the external environment moderates but doesn’t eliminate changes in the internal
changes significantly. environment. Fluctuation is greater if a variable has a normal
CONCEPT 40.2 Many animals exhibit homeostasis for a range of physi- ROOM TEMPERATURE
range—an upper and lower limit—rather than a set point.
AT 20°C
cal and chemical properties. For example, humans maintain
Feedback control maintains a fairly constant body temperature of about 37°C (98.6°F), a
(set point) This is equivalent to a heating system that is programmed
to produce heat when the room temperature drops to 19°C
the internal environment in blood pH within 0.1 pH unit of 7.4, and a blood glucose con- (66°F) and to stop heating when the temperature reaches
centration that is predominantly in the range of 70–110 mg
many animals of glucose per 100 mL of blood.
21°C (70°F). Regardless of whether there is a set point or a
normal range, homeostasis is enhanced by adaptations that
Room temperature Room temperature
Many organ systems play a role in managing an animal’s increases. decreases. reduce fluctuations, such as insulation in the case of tempera-
Thermostat turns
internal environment, a task that can present a major chal- heater on. ture and physiological buffers in the case of pH.
lenge. Imagine if your body temperature soared every time
Mechanisms of Homeostasis
Unlike negative feedback, positive feedback is a control
you took a hot shower or slurped a steaming bowl of soup. Homeostasis requires a control system. Before exploring
mechanism that amplifies the stimulus. In animals, positive-
Faced with environmental fluctuations, animals manage their homeostasis in animals, let’s get a basic picture of how a
feedback loops do not play a major role in homeostasis, but
internal environment by either regulating or conforming. control system works by considering a nonliving example:
instead help drive processes to completion. During child-
the regulation of room temperature. Let’s assume you DRAW IT Label at least one stimulus, response, and sensor/control
birth, for instance, the pressure of the baby’s head against
want to keep a room at 20°C (68°F), a comfortable tem- center in the above figure.
Regulating and Conforming perature for normal activity. You set a control device—the
sensors near the opening of the mother’s uterus stimulates
below 20°C, the thermostat turns on a radiator, furnace, or the uterus to contract. These contractions result in greater
Compare the two sets of data in Figure 40.7. The river otter’s thermostat—to 20°C. A thermometer in the thermostat
other heater (Figure 40.8). When the room exceeds 20°C, pressure against the opening of the uterus, heightening the
body temperature is largely independent of that of the sur- monitors the room temperature. If the temperature falls
the thermostat switches off the heater. If the temperature contractions and thereby causing even greater pressure, ulti-
rounding water, whereas the largemouth
then drifts below 20°C, the thermostat activates another mately causing the baby to be born.
bass’s body warms or cools when the . Figure 40.7 The relationship between body and environmental temperatures in
an aquatic temperature regulator and an aquatic temperature conformer. The river
heating cycle. If the temperature instead rises above 20°C,
water temperature changes. We can
otter regulates its body temperature, keeping it stable across a wide range of environmental the thermostat activates a cooling mechanism, such as by Alterations in Homeostasis
convey these two trends by labeling
temperatures. The largemouth bass, meanwhile, allows its internal environment to conform turning on an air conditioner. The set points and normal ranges for homeostasis can change
the otter a regulator and the bass a con- to the water temperature. Like a home heating system, the homeostatic control sys- under various circumstances. In fact, regulated changes in the
former with regard to body temperature.
tem in animals maintains a variable, such as body tempera- internal environment are essential to normal body functions.
An animal is a regulator for an envi- 40
ture or solute concentration, at or near a particular value, or Some regulated changes occur during a particular stage in life,
ronmental variable if it uses internal River otter
(temperature regulator) set point. A fluctuation in the variable above or below the such as the radical shift in hormone balance that occurs dur-
mechanisms to control internal change
set point serves as the stimulus detected by a sensor. The ing puberty. Other regulated changes are cyclic, such as the
Body temperature (°C)

in the face of external fluctuation. In 30

sensor signals a control center, which triggers a response, a variation in hormone levels responsible for a woman’s men-
contrast, an animal is a conformer if it
physiological activity that helps return the variable to the set strual cycle (see Figure 46.14).
allows its internal condition to change Largemouth bass
20
point. In the home heating example, a drop in temperature In all animals (and plants), certain cyclic alterations in
in accordance with external changes in (temperature
conformer) below the set point acts as a stimulus, the thermostat serves metabolism reflect a circadian rhythm, a set of physiologi-
the particular variable.
as the sensor and control center, and the heater produces cal changes that occur roughly every 24 hours (Figure 40.9).
An animal may allow some inter-
10 the response. One way to observe this rhythm is to monitor body tem-
nal conditions to conform to the
perature, which in humans typically undergoes a cyclic rise
environment but regulate others. For
Feedback Control in Homeostasis and fall of more than 0.6°C (1°F) in every 24-hour period.
instance, the bass conforms to the
0 If you examine the circuit in Figure 40.8, you can see that Remarkably, a biological clock maintains this rhythm even
temperature of the water in which it
0 10 20 30 40 either response (heating or cooling) reduces the stimulus (the when variations in human activity, room temperature, and
lives, but regulates the solute concen-
Ambient (environmental) temperature (°C) change in temperature) that triggered that response. The cir- light levels are minimized (see Figure 40.9a). A circadian
tration in its blood and interstitial
cuit thus displays negative feedback, a control mechanism rhythm is thus intrinsic to the body, although the biological

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 . Figure 40.9 Human circadian rhythm. . Figure 40.10 Acclimatization by mountain climbers in the CONCEPT 40.3 Variation in Body Temperature
Himalayas. To lessen the risk of altitude sickness when ascending
Body Melatonin Animals also differ in whether their body temperature is vari-
temperature concentration
a high peak, climbers acclimatize by camping partway up the
mountain. Spending time at an intermediate altitude allows the Homeostatic processes for able or constant. An animal whose body temperature varies
Core body temperature (°C)

thermoregulation involve form,

Melatonin concentration
37.1 60 circulatory and respiratory systems to become more efficient in with its environment is called a poikilotherm (from the Greek
capturing and distributing oxygen at a lower concentration.

in blood (pg/mL)
poikilos, varied). In contrast, a homeotherm has a relatively
function, and behavior constant body temperature. For example, the largemouth
36.9 40
In this section, we’ll examine the regulation of body tempera- bass is a poikilotherm, and the river otter is a homeotherm
ture as an example of how form and function work together (see Figure 40.7).
in regulating an animal’s internal environment. Later chap- From the descriptions of ectotherms and endotherms, it
36.7 20
ters in this unit will discuss other physiological systems might seem that all ectotherms are poikilothermic and all
involved in maintaining homeostasis. endotherms are homeothermic. In fact, there is no fixed rela-
36.5 0 Thermoregulation is the process by which animals tionship between the source of heat and the stability of body
2 6 10 2 6 10
PM PM PM AM AM AM maintain their body temperature within a normal range. temperature. Many ectothermic marine fishes and inver-
Time of day Body temperatures outside the normal range can reduce the tebrates inhabit waters with such stable temperatures that
efficiency of enzymatic reactions, alter the fluidity of cellular their body temperature varies less than that of mammals and
(a) Variation in core body temperature and melatonin concentra- membranes, and affect other temperature-sensitive biochemi- other endotherms. Conversely, the body temperature of a few
tion in blood. Researchers studied resting but awake volunteers in
an isolation chamber with constant temperature and low light. cal processes, potentially with fatal results. endotherms varies considerably. For example, the body tem-
(Melatonin is a hormone secreted by the pineal gland.) In talking about thermoregulation, we will need to perature of some bats drops from 40°C to a few degrees above
talk about heat. Formally, heat is defined as thermal zero when they enter hibernation.
Midnight energy in transfer from one body of matter to another (see
Start of Lowest
melatonin secretion heart rate Concept 8.1). Here, however, we will use the term heat to
SLE refer simply to thermal energy. . Figure 40.11 Thermoregulation by internal or external
Greatest EP sources of heat. Endotherms obtain heat from their internal
Lowest body
muscle strength metabolism, whereas ectotherms rely on heat from their
temperature
Endothermy and Ectothermy external environment.
6 PM 6 AM
Homeostasis is sometimes altered by acclimatization, Heat for thermoregulation can come from either internal
Most rapid metabolism or the external environment. Humans and other
rise in blood an animal’s physiological adjustment to changes in its
pressure external environment. For instance, when an elk moves mammals, as well as birds, are endothermic, meaning that
Fastest they are warmed mostly by heat generated by metabolism.
reaction time up into the mountains from sea level, the lower oxygen
concentration in the high mountain air stimulates the Some fishes and insect species and a few nonavian reptiles
Highest risk
Noon of cardiac arrest animal to breathe more rapidly and deeply. As a result, are also mainly endothermic. In contrast, amphibians,
more CO2 is lost through exhalation, raising blood pH many nonavian reptiles and fishes, and most invertebrates
(b) The human circadian clock. Metabolic activities undergo daily above its normal range. As the animal acclimatizes over are ectothermic, meaning that they gain most of their heat
cycles in response to the circadian clock. As illustrated for a typical several days, changes in kidney function cause it to from external sources. Endothermy and ectothermy are not
individual who rises early in the morning, eats lunch around noon, mutually exclusive, however. For example, a bird is mainly
and sleeps at night, these cyclic changes occur throughout a excrete urine that is more alkaline, returning blood pH
24-hour day. to its normal range. Other mammals, including humans, endothermic but may warm itself in the sun on a cold morn-
are also capable of acclimatizing to dramatic altitude ing, much as an ectothermic lizard does.
changes (Figure 40.10), although health risks remain. Endotherms can maintain a stable body temperature even
(a) King penguins (Aptenodytes patagonicus), endotherms
in the face of large fluctuations in the environmental tem-
clock is normally coordinated with the cycle of light and perature. In a cold environment, an endotherm generates
darkness in the environment (see Figure 40.9b). For example, CONCEPT CHECK 40.2 enough heat to keep its body substantially warmer than its
the hormone melatonin is secreted at night, and more is 1. MAKE CONNECTIONS How does negative feedback in surroundings (Figure 40.11a). In a hot environment, endo-
released during the longer nights of winter. External stimuli thermoregulation differ from feedback inhibition in an thermic vertebrates have mechanisms for cooling their bod-
can reset the biological clock, but the effect is not immediate. enzyme-catalyzed biosynthetic process (see Figure 8.21)? ies, enabling them to withstand temperatures that are intoler-
That is why flying across several time zones results in jet lag, 2. If you were deciding where to put the thermostat in a able for most ectotherms.
house, what factors would govern your decision? How do
a mismatch between the circadian rhythm and local environ- these factors relate to the fact that many homeostatic con- Many ectotherms adjust their body temperature
ment that persists until the clock fully resets. trol sensors in humans are located in the brain? by behavioral means, such as seeking out shade or bask-
Noting the importance of biological clocks to human 3. MAKE CONNECTIONS Like animals, cyanobacteria have a ing in the sun (Figure 40.11b). Because their heat source
health and disease, the Nobel Prize Committee awarded the circadian rhythm. By analyzing the genes that maintain bio- is largely environmental, ectotherms generally need to
logical clocks, scientists concluded that the 24-hour rhythms
2017 Nobel Prize in Physiology or Medicine to Americans consume much less food than endotherms of equiva-
of humans and cyanobacteria reflect convergent evolution
Jeffrey Hall, Michael Rosbash, and Michael Young, who stud- (see Concept 26.2). What evidence would have supported lent size—an advantage if food supplies are limited.
ied the fruit fly Drosophila to map out the molecular mecha- this conclusion? Explain. Ectotherms also usually tolerate larger fluctuations in
nisms that underlie circadian rhythms. For suggested answers, see Appendix A. their internal temperature. (b) Florida red-bellied turtles (Pseudemys nelsoni ), ectotherms

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 It is a common misconception that ectotherms are “cold- outer covering of the body, consisting of the skin, hair, and nails . Figure 40.13 Countercurrent heat exchangers. A countercurrent exchange system traps heat in
blooded” and endotherms are “warm-blooded.” Ectotherms (claws or hooves in some species). the body core, thus reducing heat loss from the extremities, particularly when they are immersed in cold
water or in contact with ice or snow. In essence, heat in the arterial blood emerging from the body core
do not necessarily have low body temperatures. On the con-
is transferred directly to the returning venous blood instead of being lost to the environment.
trary, when sitting in the sun, many ectothermic lizards have Insulation
higher body temperatures than mammals. Thus, the terms Insulation, which reduces the flow of heat between an animal’s
cold-blooded and warm-blooded are misleading and are avoided body and its environment, is a major adaptation for thermo- Canada goose 1 Arteries carrying warm blood to the animal’s Bottlenose dolphin
in scientific communication. regulation in both mammals and birds. Insulation is found extremities are in close contact with veins
conveying cool blood in the opposite
both at the body surface—hair and feathers—and beneath—
direction, back toward the trunk of
layers of fat formed by adipose tissue. In addition, some ani- the body. This arrangement facili-
Balancing Heat Loss and Gain mals secrete oily substances that repel water, protecting the tates heat transfer from arteries to
Thermoregulation depends on an animal’s ability to control veins along the entire length of
insulating capacity of feathers or fur. Birds, for example, secrete the blood vessels.
the exchange of heat with its environment. That exchange oils that they apply to their feathers during preening.
can occur by any of four processes: radiation, evaporation, 2 Near the end of the leg or flipper, where 1
Often, animals can adjust their insulating layers to further
1 3 arterial blood has been cooled to far below Vein
convection, and conduction (Figure 40.12). In each, heat is regulate body temperature. Most land mammals and birds, Artery Vein the animal‘s core temperature, the artery
transferred from an object of higher temperature to one of lower Artery
for example, react to cold by raising their fur or feathers. This can still transfer heat to the even colder 3
358C 338
temperature. blood in an adjacent vein. The blood in the 3
action traps a thicker layer of air, thereby increasing the effec- veins continues to absorb heat as it passes
The essence of thermoregulation is maintaining a rate of heat tiveness of the insulation. Lacking feathers or fur, humans 308 278 warmer and warmer blood traveling in the
gain that equals the rate of heat loss. Animals do this through must rely primarily on fat for insulation. We do, however, get Arteries carry opposite direction in the arteries.
mechanisms that either reduce heat exchange overall or favor “goose bumps,” a vestige of hair raising inherited from our blood into the 208 188 3 As the blood in the veins approaches the
heat exchange in a particular direction. In mammals, several of goose’s leg and center of the body, it is almost as warm as 2
furry ancestors. foot. Veins the body core, minimizing the heat loss that
these mechanisms involve the integumentary system, the Insulation is particularly important for marine mammals, return the blood 108 98 results from supplying blood to body parts
such as whales and walruses. These animals swim in water to the heart. immersed in cold water. In the flippers of a dolphin, each artery is
. Figure 40.12 Heat exchange between an organism and its surrounded by several veins in a counter-
colder than their body core, and many species spend at least Key current arrangement, allowing efficient
environment. 2
part of the year in nearly freezing polar seas. Furthermore, the Warm blood Blood flow heat exchange between blood in the
Radiation is the emission of Evaporation is the removal of transfer of heat to water occurs 50 to 100 times more rapidly arteries and veins.
To From Cool blood Heat transfer
electromagnetic waves by all heat from the surface of a than heat transfer to air. Survival under these conditions is foot foot
objects warmer than absolute liquid that is losing some of its
zero. Here, a lizard absorbs molecules as gas. Evaporation made possible by an evolutionary adaptation called blubber,
heat radiating from the distant of water from a lizard‘s moist a very thick layer of insulating fat just under the skin. The
sun and radiates a smaller surfaces that are exposed to insulation that blubber provides is so effective that marine
amount of energy to the the environment has a strong iguana of the Galápagos Islands swims in the cold ocean, endothermic insects (bumblebees, honeybees, and some
surrounding air. cooling effect. mammals can maintain body core temperatures of about
its superficial blood vessels undergo vasoconstriction. moths) have a countercurrent exchanger that helps
36938°C 1979100°F2 without requiring much more energy
This process routes more blood to the body core, conserving maintain a high temperature in their thorax, where flight
from food than land mammals of similar size.
body heat. muscles are located.
In many birds and mammals, reducing heat loss
Circulatory Adaptations
from the body relies on countercurrent exchange,
Circulatory systems provide a major route for heat flow Cooling by Evaporative Heat Loss
the transfer of heat (or solutes) between fluids that are
between the interior and exterior of the body. Adaptations
flowing in opposite directions. In a countercurrent heat Many mammals and birds live in places where regulat-
that regulate the extent of blood flow near the body surface
exchanger, arteries and veins are located adjacent to each ing body temperature requires cooling at some times
or that trap heat within the body core play a significant role
other (Figure 40.13). Because blood flows through the and warming at others. If environmental temperature
in thermoregulation.
arteries and veins in opposite directions, this arrangement is above body temperature, only evaporation can keep
In response to changes in the temperature of their sur-
allows heat exchange to be remarkably efficient. As warm body temperature from rising. Water absorbs consider-
roundings, many animals alter the amount of blood (and
blood in the arteries moves outward from the body core, able heat when it evaporates (see Concept 3.2); this heat
hence heat) flowing between their body core and their
it transfers heat to the colder blood in the veins returning is carried away from the skin and respiratory surfaces with
skin. Nerve signals that relax the muscles of the vessel walls
from the extremities. Most importantly, heat is transferred water vapor.
result in vasodilation, a widening of superficial blood ves-
along the entire length of the exchanger, maximizing Some animals exhibit adaptations that greatly facilitate
sels (those near the body surface). As a consequence of the
Convection is the transfer of Conduction is the transfer
the rate of heat exchange and minimizing heat loss to evaporative cooling. A few mammals, including horses and
increase in vessel diameter, blood flow in the skin increases.
heat by the movement of air or of heat between molecules the environment. humans, have sweat glands. In many other mammals, as well
liquid past a surface, as when a of objects in contact with In endotherms, vasodilation usually increases the transfer
Although most sharks and fishes are temperature as in birds, panting is important. Some birds have a pouch
breeze contributes to heat loss each other, as when a of body heat to the environment by radiation, conduc-
from a lizard‘s dry skin or when lizard sits on a hot rock. conformers, countercurrent heat exchangers are found richly supplied with blood vessels in the floor of the mouth;
tion, and convection (see Figure 40.12). The reverse process,
blood moves heat from the in some large, powerful swimmers, including great fluttering the pouch increases evaporation. Pigeons can
body core to the extremities. vasoconstriction, reduces blood flow and heat transfer by
white sharks, bluefin tuna, and swordfish. By keep- use this adaptation to keep their body temperature close to
decreasing the diameter of superficial vessels.
VISUAL SKILLS If this figure showed a penguin (an endotherm) on an ing the main swimming muscles warm, this adaptation 40°C (104°F) in air temperatures as high as 60°C (140°F), as
ice floe rather than an iguana (an ectotherm) on a rock, would any of the Like endotherms, some ectotherms control heat exchange
enables vigorous, sustained activity. Similarly, many long as they have sufficient water.
arrows point in a different direction? Explain. by regulating blood flow. For example, when the marine

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 c Figure 40.14 chickadees, birds with a body mass of only 20 g, remain to seasonal temperature changes often includes adjusting
▼ Figure 40.16 Inquiry
Thermoregulatory behavior active and hold their body temperature nearly constant insulation—growing a thicker coat of fur in the winter and
in a dragonfly. By orienting its How does a Burmese python generate heat
at 40°C (104°F) in environmental temperatures as low as shedding it in the summer, for example.
body so that the narrow tip of
its abdomen faces the sun, the - 40°C 1- 40°F2. while incubating eggs? Acclimatization in ectotherms often includes adjust-
dragonfly minimizes heating Flying insects such as bees and moths can also vary ments at the cellular level. Cells may produce variants of
by solar radiation. Experiment Herndon Dowling and colleagues at the Bronx
heat production. Many such endothermic insects warm Zoo in New York observed that when a female Burmese py- enzymes that have the same function but different opti-
up by shivering before taking off. As they contract their flight thon incubated eggs by wrapping her body around them, mal temperatures. Also, the proportions of saturated and
muscles in synchrony, only slight wing movements occur, she raised her body temperature and frequently contracted unsaturated lipids in membranes may change; unsaturated
the muscles in her coils. To learn if the contractions were
Behavioral Responses but considerable heat is produced. Chemical reactions, and
elevating her body temperature, they placed the python and
lipids help keep membranes fluid at lower temperatures
Ectotherms, and hence cellular respiration, accelerate in the warmed-up flight her eggs in a chamber. As they varied the chamber’s temper- (see Figure 7.5).
sometimes endotherms, “motors,” enabling flight even in cold air. ature, they monitored the python’s muscle contractions as Remarkably, some ectotherms can survive subzero
In some mammals, endocrine signals released in response well as her oxygen uptake, a measure of her rate of cellular temperatures, producing “antifreeze” proteins that
control body tempera- respiration.
ture through behavioral to cold cause mitochondria to increase their metabolic activity prevent ice formation in their cells. In the Arctic and
and produce heat instead of ATP. This process, called nonshiv- Results The python’s oxygen consumption increased when Southern (Antarctic) Oceans, these proteins enable cer-
responses to changes the temperature in the chamber decreased. As shown in the
in the environment. ering thermogenesis, takes place throughout the body. Some tain fishes to survive in water as cold as - 2°C 128°F2, a full
graph, this increase in oxygen consumption paralleled an
When cold, they seek mammals also have a tissue called brown fat in their neck and increase in the rate of muscle contraction. degree Celsius below the freezing point of body fluids in
warm places, orienting between their shoulders that is specialized for rapid heat pro- other species.
themselves toward heat duction. (The presence of extra mitochondria is what gives 120
brown fat its characteristic color.) Brown fat is found in the
sources and expanding Physiological Thermostats and Fever

O2 consumption (mL O2/hr • kg)

the portion of their body surface exposed to the heat source infants of many mammals, representing about 5% of total body 100
In humans and other mammals, the sensors responsible for
(see Figure 40.11b). When hot, they bathe, move to cool weight in human infants. Long known to be present in adult
80 thermoregulation are concentrated in the hypothalamus,
areas, or turn in another direction, minimizing their absorp- mammals that hibernate, brown fat has also recently been
the brain region that also controls the circadian clock. Within
tion of heat from the sun. For example, a dragonfly’s “obe- detected in human adults (Figure 40.15). There, the amount
60 the hypothalamus, a group of nerve cells functions as a ther-
lisk” posture is an adaptation that minimizes the amount has been found to vary, with individuals exposed to a cool envi-
mostat, responding to body temperatures outside the normal
of body surface exposed to the sun and thus to heating ronment for a month having increased amounts of brown fat. 40 range by activating mechanisms that promote heat loss or
(Figure 40.14). Although these behaviors are relatively sim- Among the nonavian reptiles, endothermy has been
gain (Figure 40.17).
ple, they enable many ectotherms to maintain a nearly con- observed in some large species in certain circumstances. For 20
At body temperatures above the normal range, the hypo-
stant body temperature. example, researchers found that a female Burmese python
0 thalamic thermostat promotes cooling of the body
Social behavior contributes to thermoregulation in both (Python molurus bivittatus) incubating eggs maintained a
0 5 10 15 20 25 30 35 by dilation of vessels in the skin, sweating, or panting. When
endotherms and ectotherms. Among endotherms, for body temperature roughly 6°C (11°F) above that of the sur-
Contractions per minute body temperatures instead drop below the normal range,
example, behavior contributes significantly to the winter rounding air. Where did the heat come from? Further studies
the thermostat inhibits heat loss mechanisms and activates
survival of Emperor penguins (see Figure 40.1). Among showed that such pythons, like birds, can raise their body
Conclusion Because oxygen consumption, which gen- mechanisms that either save heat, such as constricting
ectotherms, honeybees are notable for their use of behavior erates heat through cellular respiration, was correlated vessels in the skin, or generate heat, such as shivering.
in achieving homeostasis for temperature. In cold weather, with the rate of muscle contraction, the researchers con-
cluded that the muscle contractions, a form of shivering, In the course of certain bacterial and viral infections,
they increase heat production and huddle together, thereby . Figure 40.15 Brown fat activity during cold stress. This
PET scan shows metabolically active brown fat deposits (indicated were the source of the Burmese python’s elevated body mammals and birds develop fever, an elevated body tem-
retaining heat. Individuals move between the cooler by the arrows) surrounding the neck. temperature. perature. A variety of experiments have shown that fever
outer edges of the huddle and the warmer center, thus Data from V. H. Hutchison, H. G. Dowling, and A. Vinegar, Thermoregulation reflects an increase in the normal range for the biological
circulating and distributing the heat. In hot weather, in a brooding female Indian python, Python molurus bivittatus, Science
thermostat. For example, artificially raising the temperature
151:694–696 (1966).
honeybees cool the hive by transporting water to the of the hypothalamus in an infected animal reduces fever in
hive and fanning with their wings, promoting evapora- WHAT IF? Suppose you varied air temperature and measured
oxygen consumption for a female Burmese python without a clutch of the rest of the body.
tion and convection. Thus, a honeybee colony uses many eggs. Since she would not show shivering behavior, how would you Among certain ectotherms, an increase in body tem-
of the mechanisms of thermoregulation characteristic of expect the snake’s oxygen consumption to vary with environmental perature upon infection reflects what is called a behavioral
individual animals. temperature?
fever. For example, the desert iguana (Dipsosaurus dorsalis)
responds to infection with certain bacteria by seeking
a warmer environment and then maintaining a body
Adjusting Metabolic Heat Production temperature through shivering (Figure 40.16). Whether cer- temperature that is elevated by 2–4°C (4–7°F). Similar obser-
Because endotherms generally maintain a body temperature tain groups of Mesozoic dinosaurs were similarly endother- vations in fishes, amphibians, and even cockroaches indicate
considerably higher than that of the environment, they mic is a matter of active debate. that fever is common to both endotherms and ectotherms.
must counteract continual heat loss. Endotherms can vary Now that we have explored thermoregulation, we’ll
heat production—thermogenesis—to match changing rates of Acclimatization in Thermoregulation conclude our introduction to animal form and function by
heat loss. Thermogenesis is increased by such muscle activ- Acclimatization contributes to thermoregulation in many considering the different ways that animals allocate, use,
ity as moving or shivering. For example, shivering helps animal species. In birds and mammals, acclimatization and conserve energy.

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 c Figure 40.17 The thermostatic Blood vessels in skin . Figure 40.18 Bioenergetics of an animal: an overview. kilocalories (kcal). A kilocalorie equals 1,000 calories, or
function of the hypothalamus in dilate; capillaries fill 4,184 joules. (The unit Calorie, with a capital C, as used by
human thermoregulation. with warm blood; heat
Thermostat in radiates from skin surface. Organic molecules many nutritionists, is actually a kilocalorie.)
WHAT IF? Suppose at the end of a hard run hypothalamus in food Metabolic rate can be determined in several ways.
External
on a hot day you find that there are no drinks activates cooling
environment Because nearly all of the chemical energy used in cellular respi-
left in the cooler. If, out of desperation, you mechanisms.
dunk your head into the cooler, how might Sweat glands Animal ration eventually appears as heat, metabolic rate can be mea-
the ice-cold water affect the rate at which secrete body Digestion and sured by monitoring an animal’s rate of heat loss. For this
absorption Heat
your body temperature returns to normal? sweat, which approach, researchers use a calorimeter, which is a closed,
evaporates, Energy
Mastering Biology Figure Walkthrough cooling the insulated chamber equipped with a device that records the
Body temperature increases lost in
body. Body temperature feces heat an animal gives off to its environment. Metabolic rate
(such as when exercising or
in hot surroundings). decreases. Nutrient molecules can also be determined from the amount of oxygen consumed
in body cells Energy
lost in or carbon dioxide produced by an animal’s cellular respira-
nitrogenous tion (Figure 40.19). To calculate metabolic rate over longer
waste
NORMAL BODY TEMPERATURE periods, researchers record the rate of food consumption,
(approximately 36–38°C) Cellular the energy content of the food (about 4.5–5 kcal per gram of
Carbon Heat
skeletons respiration protein or carbohydrate and about 9 kcal per gram of fat), and
the chemical energy lost in waste products (feces and urine or
Body temperature decreases other nitrogenous wastes).
Body temperature (such as when in cold
increases. ATP
surroundings).
Skeletal muscles rapidly Biosynthesis:
contract, causing shivering, growth, Minimum Metabolic Rate
storage, and
which generates heat.
reproduction and Thermoregulation
Cellular Heat Animals must maintain a minimum metabolic rate for basic
work
functions such as cell maintenance, breathing, and circula-
Blood vessels in skin Thermostat in tion. Researchers measure this minimum metabolic rate
constrict, diverting blood hypothalamus
Heat differently for endotherms and ectotherms. The minimum
from skin to deeper activates warming
tissues and reducing heat mechanisms. metabolic rate of a nongrowing endotherm that is at rest,
MAKE CONNECTIONS Use the idea of energy coupling to
loss from skin surface. explain why heat is produced in the absorption of nutrients, has an empty stomach, and is not experiencing stress is
in cellular respiration, and in the synthesis of biopolymers called the basal metabolic rate (BMR). BMR is measured
(see Concept 8.3).
under a “comfortable” temperature range—a range that
requires only the minimum generation or shedding of heat.
The minimum metabolic rate of ectotherms is determined
CONCEPT CHECK 40.3 reproduction. The overall flow and transformation of body growth and repair, synthesis of storage material such at a specific temperature because changes in the environ-
1. What mode of heat exchange is involved in “wind chill,” energy in an animal—its bioenergetics—determines as fat, and production of gametes. The production and use of mental temperature alter body temperature and therefore
when moving air feels colder than still air at the same nutritional needs and is related to the animal’s size, activ- ATP generate heat, which the animal eventually gives off to metabolic rate. The metabolic rate of a fasting, nonstressed
temperature? Explain. ity, and environment. its surroundings (Figure 40.18).
2. Flowers differ in how much sunlight they absorb. Why
might this matter to a hummingbird seeking nectar on a
. Figure 40.19 Measuring the rate of oxygen consumption by a swimming shark.
cool morning? Energy Allocation and Use Quantifying Energy Use A researcher monitors the decrease in oxygen level over time in the recirculating water of a
3. WHAT IF? Why is shivering likely during the onset of a fever? Organisms can be classified by how they obtain chemical juvenile hammerhead’s tank.
For suggested answers, see Appendix A.
How much of the total energy an ani-
energy. Most autotrophs, such as plants, harness light energy
mal obtains from food does it need just
to build energy-rich organic molecules and then use those
to stay alive? How much energy must
molecules for fuel. Most heterotrophs, such as animals, obtain
be expended to walk, run, swim, or fly
CONCEPT 40.4 their chemical energy from food, which contains organic
from one place to another? What fraction
molecules synthesized by other organisms.
Energy requirements are related Animals use chemical energy harvested from the food they
of the energy intake is used for reproduc-
tion? Physiologists answer such questions
to animal size, activity, and eat to fuel metabolism and activity. Food is digested by enzy-
by measuring the rate at which an animal
matic hydrolysis (see Figure 5.2b), and nutrients are absorbed
environment by body cells. The ATP (adenosine triphosphate) produced by
uses chemical energy and how this rate
changes in different circumstances.
One of the unifying themes of biology, introduced cellular respiration and fermentation powers cellular work,
The sum of all the energy an animal
in Concept 1.1, is that life requires energy transfer enabling cells, organs, and organ systems to perform the
uses in a given time interval is called its
and transformation. Like other organisms, animals functions that keep an animal alive. Other uses of energy in
metabolic rate. Energy is measured
use chemical energy for growth, repair, activity, and the form of ATP include biosynthesis, which is needed for
in joules (J) or in calories (cal) and

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 ectotherm at rest at a particular temperature is called its . Figure 40.20 The relationship of metabolic rate to body size. Scientific Skills Exercise
standard metabolic rate (SMR).
Comparisons of minimum metabolic rates reveal the 103 Interpreting Pie Charts
different energy costs of endothermy and ectothermy. Elephant
How Do Energy Budgets Differ for Three Terrestrial
The BMR for humans averages 1,600–1,800 kcal per day 102 Horse Vertebrates? To explore bioenergetics in animal bodies,

BMR (L O2/hr) (log scale)

for adult males and 1,300–1,500 kcal per day for adult let’s consider typical annual energy budgets for three terrestrial
Human vertebrates that vary in size and thermoregulatory strategy:
females. These BMRs are about equivalent to the rate of
Sheep a 4-kg male Adélie penguin, a 25-g (0.025-kg) female deer
energy use by a 75-watt lightbulb. In contrast, the SMR of an 10
mouse, and a 4-kg female ball python. The penguin is well-
American alligator is only about 60 kcal per day at Dog insulated against his Antarctic environment but must expend
Cat
20°C (68°F). As this represents less than 1⁄20 the energy energy in swimming to catch food, incubating eggs laid by his
1 partner, and bringing food to his chicks. The tiny deer mouse
used by a comparably sized adult human, it is clear that lives in a temperate environment where food may be read-
ectothermy has a markedly lower energetic requirement Rat ily available, but her small size causes rapid loss of body heat.
than endothermy. 10–1 Ground squirrel Unlike the penguin and mouse, the python is ectothermic and Adélie penguin Deer mouse Ball python
Shrew 4-kg male 0.025-kg female 4-kg female
Mouse keeps growing throughout her life. She produces eggs but does
not incubate them. In this exercise, we’ll compare the energy 340,000 kcal/yr 4,000 kcal/yr 8,000 kcal/yr
Harvest mouse
10–2
Influences on Metabolic Rate 10–3 10–2 10–1 1 10 102 103
expenditures of these animals for five important functions:
basal (standard) metabolism, reproduction, thermoregulation,
Key
Metabolic rate is affected by many factors other than an ani- Body mass (kg) (log scale) activity, and growth. Basal (standard) metabolism Activity
mal being an endotherm or an ectotherm. Some key factors Reproduction Growth
(a) Relationship of basal metabolic rate (BMR) to body size for various How the Data Were Obtained Energy budgets were calculated
are age, sex, size, activity, temperature, and nutrition. Here for each of the animals based on measurements from field and Thermoregulation
mammals. From shrew to elephant, size increases 1 millionfold.
we’ll examine the effects of size and activity. laboratory studies.
Data from M. A. Chappell et al., Energetics of foraging in breeding Adélie pen-
8 guins, Ecology 74:2450–2461 (1993); M. A. Chappell et al., Voluntary running
Data from the Experiments Pie charts are a good way to
Size and Metabolic Rate Shrew compare relative differences in a set of variables. In the pie
in deer mice: speed, distance, energy costs, and temperature effects, Journal
of Experimental Biology 207:3839–3854 (2004); T. M. Ellis and M. A. Chappell,
BMR per kg body mass (L O2/hr • kg)

Larger animals have more body mass and therefore require 7 charts here, the sizes of the wedges represent the relative Metabolism, temperature relations, maternal behavior, and reproductive en-
annual energy expenditures for the functions shown in the ergetics in the ball python (Python regius), Journal of Comparative Physiology B
more chemical energy. Remarkably, the relationship between 6 157:393–402 (1987).
key. The total annual expenditure for each animal is given
overall metabolic rate and body mass is constant across a below its pie chart.
5 4. Now look at the total annual energy expenditures for each ani-
wide range of sizes and forms, as illustrated for various mam- mal. How much more energy does the penguin expend each year
INTERPRET THE DATA
mals in Figure 40.20a. In fact, for even more varied organisms 4 compared to the similarly sized python?
1. You can estimate the contribution of each wedge in a pie
ranging in size from bacteria to blue whales, metabolic rate 5. Which animal expends the most kilocalories per year on
Harvest mouse chart by remembering that the entire circle represents 100%,
remains roughly proportional to body mass to the three- 3 thermoregulation?
half is 50%, and so on. What percent of the mouse’s energy
quarter power (m3/4). Scientists are still researching the basis budget goes to basal metabolism? What percent of the 6. If you monitored energy allocation in the penguin for just
2 Mouse Sheep a few months instead of an entire year, you might find the
of this relationship, which applies to ectotherms as well penguin’s budget is for activity?
Rat Human Elephant growth category to be a significant part of the pie chart. Given
1 Cat 2. Without considering the sizes of the wedges, how do the three
as endotherms. Dog that adult penguins don’t grow from year to year, how would
Ground squirrel Horse pie charts differ in which functions they include? Explain these
The relationship of metabolic rate to size profoundly you explain this finding?
0 differences.
affects energy consumption by body cells and tissues. 10–3 10–2 10–1 1 10 102 103 3. Does the penguin or the mouse expend a greater proportion Instructors: A version of this Scientific Skills Exercise can be
As shown in Figure 40.20b, the energy it takes to main- Body mass (kg) (log scale) of its energy budget on thermoregulation? Why? assigned in Mastering Biology.
tain each gram of body mass is inversely related to body
(b) Relationship of BMR per kilogram of body mass to body size for
size. Each gram of a mouse, for instance, requires about
the same mammals as in (a).
20 times as many calories as a gram of an elephant, even
though the whole elephant uses far more calories than the INTERPRET THE DATA Based on the graph in (a), one observer
of about 1.5 times BMR—an indication of a relatively seden- A major adaptation that enables animals to save energy in the
whole mouse. The smaller animal’s higher metabolic rate suggests that a group of 100 ground squirrels has the same basal
metabolic rate as 1 dog. A second observer looking at the graph tary lifestyle. face of such difficult conditions is torpor, a physiological
per gram demands a higher rate of oxygen delivery. To meet disagrees. Who is correct and why? The fraction of an animal’s energy “budget” that is state of decreased activity and metabolism.
this demand, the smaller animal must have a higher breath- devoted to activity depends on many factors, including its Many birds and small mammals exhibit a daily torpor that
ing rate, blood volume (relative to its size), and heart rate. environment, behavior, size, and thermoregulation. In the is well adapted to feeding patterns. For instance, some bats
Thinking about body size in bioenergetic terms reveals insect twitching its wings consumes energy beyond the BMR Scientific Skills Exercise, you’ll interpret data on the annual feed at night and go into torpor in daylight. Similarly, chicka-
how trade-offs shape the evolution of body plans. As body or SMR. Maximum metabolic rates (the highest rates of ATP energy budgets of three terrestrial vertebrates. dees and hummingbirds, which feed during the day, often go
size decreases, each gram of tissue increases in energy cost. use) occur during peak activity, such as lifting a heavy object, into torpor on cold nights.
As body size increases, energy costs per gram of tissue sprinting, or swimming at high speed. In general, the maxi- All endotherms that exhibit daily torpor are relatively
decrease, but an ever-larger fraction of body tissue is required mum metabolic rate an animal can sustain is inversely related Torpor and Energy Conservation small; when active, they have high metabolic rates and thus
for exchange, support, and locomotion. to the duration of activity. Despite their many adaptations for homeostasis, animals may very high rates of energy consumption. The changes in body
For most terrestrial animals, the average daily rate of encounter conditions that severely challenge their abilities to temperature, and thus the energy savings, are often consid-
Activity and Metabolic Rate energy consumption is two to four times BMR (for endo- balance their heat, energy, and materials budgets. For exam- erable: the body temperature of chickadees drops as much
For both ectotherms and endotherms, activity greatly affects therms) or SMR (for ectotherms). Humans in most developed ple, at certain times of the day or year, their surroundings as 10°C (18°F) at night, and the core body temperature of a
metabolic rate. Even a person reading quietly at a desk or an countries have an unusually low average daily metabolic rate may be extremely hot or cold, or food may be unavailable. hummingbird can fall 25°C (45°F) or more.

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 Hibernation is long- . Figure 40.21 A hazel ▼ Figure 40.22 Inquiry
term torpor that is an adap- dormouse (Muscardinus
avellanarius) hibernating. What happens to the circadian clock during
tation to winter cold and
food scarcity. When a mam- hibernation?
mal enters hibernation, its
Experiment To determine whether the 24-hour biological
body temperature declines clock continues to run during hibernation, Paul Pévet and
as its body’s thermostat is colleagues at the University of Louis Pasteur in Strasbourg,
turned down (Figure 40.21). France, studied molecular components of the circadian
clock in the European hamster (Cricetus cricetus). The
Some hibernating mam-
researchers measured RNA levels for two clock genes—
mals cool to as low as Per2 and Bmal1—during normal activity (euthermia) and
192°C 134936°F2, during hibernation in constant darkness. The RNA samples
and at least one, the were obtained from the suprachiasmatic nuclei (SCN), a
pair of structures in the mammalian brain that control
Arctic ground squirrel
circadian rhythms.
(Spermophilus parryii), can enter a supercooled (unfrozen)
state in which its body temperature dips below 0°C (32°F). Results
Periodically, perhaps every two weeks or so, hibernating Day Night
animals undergo arousal, raising their body temperature Per2 Bmal1
and becoming active briefly before resuming hibernation.
100
Relative RNA level (%)
Metabolic rates during hibernation can be 20 times lower
than if the animal attempted to maintain normal body tem- 80
peratures of 36938°C 1979100°F2. As a result, hibernators 60
such as the ground squirrel can survive through the winter 40
on limited supplies of energy stored in the body tissues or as
20
food cached in a burrow. Similarly, the slow metabolism and
inactivity of estivation, or summer torpor, enable animals to 0
Euthermia Hibernation Euthermia Hibernation
survive long periods of high temperatures and scarce water.
What happens to the circadian rhythm in hibernating ani- Conclusion Hibernation disrupted circadian variation
mals? In the past, researchers reported detecting daily biological in the hamster’s clock gene RNA levels. Further experi-
rhythms in hibernating animals. However, in some cases the ments demonstrated that this disruption was not simply
animals were probably in a state of torpor from which they could due to the dark environment during hibernation, since
for nonhibernating animals RNA levels during a darkened
readily arouse, rather than “deep” hibernation. More recently, a daytime were the same as in daylight. The researchers con-
group of researchers in France addressed this question in a differ- cluded that the biological clock stops running in hibernat-
ent way, examining the machinery of the biological clock rather ing European hamsters and, perhaps, in other hibernators
as well.
than the rhythms it controls (Figure 40.22). Working with the
European hamster, they found that molecular components of Data from F. G. Revel et al., The circadian clock stops ticking during deep hiberna-
tion in the European hamster, Proceedings of the National Academy of Sciences
the clock stopped oscillating during hibernation. These findings USA 104:13816–13820 (2007).
support the hypothesis that the circadian clock ceases operation
during hibernation, at least in this species. WHAT IF? Suppose you discovered a new hamster gene and found
that the levels of RNA for this gene were constant during hibernation.
From tissue types to homeostasis, this chapter has focused What could you conclude about the day and night RNA levels for this
on the whole animal. We also investigated how animals gene during euthermia?
exchange materials with the environment and how size and
activity affect metabolic rate. For much of the rest of this unit,
we’ll explore how specialized organs and organ systems
CONCEPT CHECK 40.4
enable animals to meet the basic challenges of life. In Unit 6,
1. If a mouse and a small lizard of the same mass (both at rest)
we investigated how plants meet the same challenges.
were placed in experimental chambers under identical envi-
Figure 40.23, on the next two pages, highlights some funda- ronmental conditions, which animal would consume oxygen
mental similarities and differences in the evolutionary adapta- at a higher rate? Explain.
tions of plants and animals. This figure is thus a review of Unit 2. Which animal must eat a larger proportion of its weight in food
6, an introduction to Unit 7, and, most importantly, an illus- each day: a house cat or an African lion caged in a zoo? Explain.
tration of the connections that unify the myriad forms of life. 3. WHAT IF? Suppose the animals at a zoo were resting com-
fortably and remained at rest while the nighttime air tem-
perature dropped. If the temperature change were sufficient
Mastering Biology Interview with George to cause a change in metabolic rate, what changes would you
Bartholomew: Exploring connections between expect for an alligator and a lion?
animal physiology and the environment
For suggested answers, see Appendix A.

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