Chapter 40 Basic Principles of Animal Form and Function
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-
• 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
• 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
• 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
• 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
• Convergence occurs because natural selection shapes similar
adaptations when diverse organisms face the same
environmental challenge, such as the resistance of water to fast
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
• 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
• Life is characterized by hierarchical levels of organization, each with
• 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
• 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
• 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
• 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
• 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
• 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
• 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
• 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
• 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
• Cartilage has an abundance of collagenous fibers embedded in a
rubbery matrix made of a substance called chondroitin sulfate, a
• Chondrocytes secrete collagen and chondroitin sulfate.
• The composite of collagenous fibers and chondroitin sulfate
makes cartilage a strong yet somewhat flexible support
• 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
• 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
• 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
• 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
• 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
• 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
• Many vertebrate organs are suspended by sheets of connective
tissues called mesenteries in body cavities moistened or filled with
• 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
• 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
• Any organism, whether single-celled or an assembly of organ
systems, is a coordinated living whole greater than the sum of its
Concept 40.3 Animals use the chemical energy in food to sustain form
• 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,
• 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
• Endothermy is a high-energy strategy that permits
intense, long-duration activity of a wide range of
• 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
• 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
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
• 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
• 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
• 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
• 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.