Motor Mechanisms and Behavior 39

(1)

CAMPBELL BIOLOGY IN FOCUS

Urry • Cain • Wasserman • Minorsky • Jackson • Reece

Lecture Presentations by

39

Motor Mechanisms

and Behavior

(2)

Overview: The How and Why of Animal Activity

 Fiddler crabs feed with their small claw and wave their large claw

 Why do male fiddler crabs engage in claw-waving behavior?

 Claw waving is used to repel other males and to attract females

Video: Albatross Courtship

Video: Boobies Courtship

Video: Giraffe Courtship

(3)

Figure 39.1

(4)

 A behavior is an action carried out by muscles under control of the nervous system

 Behavior is subject to natural selection

(5)

Concept 39.1: The physical interaction of protein filaments is required for muscle function

 Muscle activity is a response to input from the nervous system

 Muscle contraction is an active process; muscle

relaxation is passive

(6)

 Muscle cell contraction relies on the interaction between protein structures

 Thin filaments consist of two strands of actin coiled around one another

 Thick filaments are staggered arrays of myosin

molecules

(7)

Vertebrate Skeletal Muscle

 Vertebrate skeletal muscle moves bones and the body and is characterized by a hierarchy of smaller and smaller units

 A skeletal muscle consists of a bundle of long fibers, each a single cell, running parallel to the length of

the muscle

 Each muscle fiber is itself a bundle of smaller

myofibrils, which contain thick and thin filaments

(8)

 Skeletal muscle is also called striated muscle because the regular arrangement of myofibrils creates a pattern of light and dark bands

 The functional unit of a muscle is called a sarcomere

and is bordered by Z lines

(9)

Figure 39.2

Muscle

Bundle of muscle fibers

Nuclei

Z lines Myofibril

Single muscle fiber (cell)

Plasma membrane

Sarcomere

Thick filaments

(myosin) M line

TEM

0.5 m

(10)

Figure 39.2a

Muscle

Bundle of

muscle fibers

Nuclei

Z lines Myofibril

Single muscle fiber (cell)

Sarcomere

Plasma

membrane

(11)

Figure 39.2b

Sarcomere

Thick filaments (myosin)

Z line

Sarcomere Thin filaments

(actin)

Z line M line

TEM

0.5 m

(12)

Figure 39.2c

Sarcomere

TEM 0.5 m

(13)

The Sliding-Filament Mechanism of Muscle Contraction

 According to the sliding-filament model, filaments slide past each other longitudinally, causing an

overlap between thin and thick filaments

(14)

Figure 39.3

Sarcomere Relaxed muscle

Contracting muscle

Fully contracted muscle

Z M Z Z M Z

Contracted sarcomere

0.5 m

(15)

Figure 39.3a

Sarcomere

Z

Z M

0.5 m

(16)

Figure 39.3b

Sarcomere

Z

Z M

0.5 m

(17)

Figure 39.3c

Contracted sarcomere

0.5 m

(18)

 The sliding of filaments relies on interaction between actin and myosin

 The “head” of a myosin molecule binds to an actin filament, forming a cross-bridge and pulling the thin filament toward the center of the sarcomere

 Muscle contraction requires repeated cycles of

binding and release

(19)

 Glycolysis and aerobic respiration generate the

ATP needed to sustain muscle contraction

(20)

Figure 39.4

Thin

filaments Thick filament

Thin filament moves

toward center of sarcomere.

Cross-bridge Myosin head (low

energy configuration)

Myosin head (low energy configuration)

Thin filament

Thick filament

Myosin head (high energy configuration)

ATP

Myosin-

binding sites

ATP

Actin

ADP P i ADP

P i

ADP P i

5

1

2

3

4

(21)

Figure 39.4a

Myosin head (low

energy configuration) Thin

filament

Thick filament

ATP

1 ATP

(22)

Figure 39.4b

2 Myosin head (high

energy configuration) Myosin-

binding sites Actin

ADP

P i

(23)

Figure 39.4c

3 Cross-bridge

ADP P _i

(24)

Figure 39.4d

4 Thin filament moves

toward center of sarcomere.

Myosin head (low

energy configuration)

(25)

The Role of Calcium and Regulatory Proteins

 The regulatory protein tropomyosin and the

troponin complex, a set of additional proteins, bind to actin strands on thin filaments when a muscle

fiber is at rest

 This prevents actin and myosin from interacting

(26)

Figure 39.5

Myosin- binding site

Tropomyosin

Actin Troponin complex

(a) Myosin-binding sites blocked

(b) Myosin-binding sites exposed

Ca

^2

-binding sites

Ca

^2

(27)

 For a muscle fiber to contract, myosin-binding sites must be uncovered

 This occurs when calcium ions (Ca ^2 ) bind to the troponin complex and expose the myosin-binding sites

 Contraction occurs when the concentration of Ca ^2

is high; muscle fiber contraction stops when the

concentration of Ca ^2 is low

(28)

 The stimulus leading to contraction of a muscle fiber is an action potential in a motor neuron that makes a synapse with the muscle fiber

 The synaptic terminal of the motor neuron releases the neurotransmitter acetylcholine

 Acetylcholine depolarizes the muscle, causing it to produce an action potential

Animation: Muscle Contraction

(29)

Figure 39.6

Synaptic terminal

Ca

^2

released from SR

Ca

^2

1 T tubule Sarcoplasmic

reticulum (SR)

Plasma membrane of muscle fiber

Myofibril

Axon of

motor neuron

Sarcomere

Mitochondrion

Synaptic terminal of motor neuron T tubule

Plasma membrane Sarcoplasmic

reticulum (SR) Synaptic cleft

ACh

Ca

^2

pump

Ca

^2

CYTOSOL

ATP

2

3 4 6

7

(30)

Figure 39.6a

Synaptic terminal

Ca

^2

released from SR

T tubule Sarcoplasmic

reticulum (SR)

Plasma membrane of muscle fiber

Myofibril

Axon of

motor neuron

Sarcomere

Mitochondrion

(31)

Figure 39.6b

Ca

^2

1 Synaptic terminal of motor neuron T tubule

Plasma membrane Sarcoplasmic

reticulum (SR) Synaptic cleft

ACh

Ca

^2

pump

Ca

^2

CYTOSOL

ATP

2

3 4 6

7

(32)

 Action potentials travel to the interior of the muscle fiber along transverse (T) tubules, infoldings of the plasma membrane

 The action potential along T tubules causes the sarcoplasmic reticulum (SR), a specialized endoplasmic reticulum, to release Ca ^2

 The Ca ^2 binds to the troponin complex on the thin

filaments, initiating muscle fiber contraction

(33)

 When motor neuron input stops, the muscle cell relaxes

 Transport proteins in the SR pump Ca ^2 out of the cytosol

 Regulatory proteins bound to thin filaments shift

back to the myosin-binding sites

(34)

 Amyotrophic lateral sclerosis (ALS; also known as Lou Gehrig’s disease) interferes with the excitation of skeletal muscle fibers; this disease is usually

fatal

 Myasthenia gravis is an autoimmune disease that attacks acetylcholine receptors on muscle fibers;

treatments exist for this disease

(35)

Nervous Control of Muscle Tension

 Contraction of a whole muscle is graded, which

means that the extent and strength of its contraction can be voluntarily altered

 There are two basic mechanisms by which the nervous system produces graded contractions

 Varying the number of fibers that contract

 Varying the rate at which fibers are stimulated

(36)

 In vertebrates, each motor neuron may synapse with multiple muscle fibers, although each fiber is

controlled by only one motor neuron

 A motor unit consists of a single motor neuron and

all the muscle fibers it controls

(37)

Figure 39.7

Synaptic terminals Spinal cord

Motor neuron cell body

Motor neuron axon

Motor unit 1

Motor unit 2

Nerve

Muscle fibers

Muscle

(38)

 Recruitment of multiple motor neurons results in stronger contractions

 A twitch results from a single action potential in a motor neuron

 More rapidly delivered action potentials produce a

graded contraction by summation

(39)

Figure 39.8

Tetanus

T en si o n Summation of two twitches Single

twitch

Action Time

potential Pair of action potentials

Series of action

potentials at

high frequency

(40)

 Tetanus is a state of smooth and sustained

contraction produced when motor neurons deliver

a volley of action potentials

(41)

Types of Skeletal Muscle Fibers

 There are several distinct types of skeletal muscles, each of which is adapted to a particular set of

functions

 They are classified by the source of ATP powering the muscle activity and the speed of muscle

contraction

(42)

Table 39.1

(43)

 Oxidative and glycolytic fibers are differentiated by their energy source

 Oxidative fibers rely mostly on aerobic respiration to generate ATP

 These fibers have many mitochondria, a rich blood supply, and a large amount of myoglobin

 Myoglobin is a protein that binds oxygen more

tightly than hemoglobin does

(44)

 Glycolytic fibers use glycolysis as their primary source of ATP

 Glycolytic fibers have less myoglobin than oxidative fibers and tire more easily

 In poultry and fish, light meat is composed of

glycolytic fibers, while dark meat is composed of

oxidative fibers

(45)

 Fast-twitch and slow-twitch fibers are differentiated by their speed of contraction

 Slow-twitch fibers contract more slowly but sustain longer contractions

 All slow-twitch fibers are oxidative

 Fast-twitch fibers contract more rapidly but sustain shorter contractions

 Fast-twitch fibers can be either glycolytic or oxidative

(46)

 Most human skeletal muscles contain both slow- twitch and fast-twitch muscles in varying ratios

 Some vertebrates have muscles that twitch at rates much faster than human muscles

 In producing its characteristic mating call, the male toadfish can contract and relax certain muscles

more than 200 times per second

(47)

Figure 39.9

(48)

Other Types of Muscle

 In addition to skeletal muscle, vertebrates have cardiac muscle and smooth muscle

 Cardiac muscle, found only in the heart, consists of striated cells electrically connected by intercalated disks

 Cardiac muscle can generate action potentials

without neural input

(49)

 In smooth muscle, found mainly in walls of hollow organs such as those of the digestive tract,

contractions are relatively slow and may be initiated by the muscles themselves

 Contractions may also be caused by stimulation

from neurons in the autonomic nervous system

(50)

Concept 39.2: Skeletal systems transform muscle contraction into locomotion

 Skeletal muscles are attached in antagonistic pairs, the actions of which are coordinated by the nervous system

 The skeleton provides a rigid structure to which muscles attach

 Skeletons function in support, protection, and

movement

(51)

Figure 39.10

Biceps Human forearm (internal skeleton)

E xt en si o n

Grasshopper tibia (external skeleton)

Biceps Triceps

Triceps

Extensor muscle

Flexor muscle

Extensor muscle

Flexor muscle

F le xi o n

(52)

Types of Skeletal Systems

 The three main types of skeletons are

 Hydrostatic skeletons (lack hard parts)

 Exoskeletons (external hard parts)

 Endoskeletons (internal hard parts)

(53)

Hydrostatic Skeletons

 A hydrostatic skeleton consists of fluid held under pressure in a closed body compartment

 This is the main type of skeleton in most cnidarians, flatworms, nematodes, and annelids

 Annelids use their hydrostatic skeleton for

peristalsis, a type of movement on land produced

by rhythmic waves of muscle contractions

(54)

Figure 39.11

Longitudinal muscle relaxed (extended)

Longitudinal muscle

contracted Circular muscle

contracted Circular muscle relaxed

Bristles Head end

Head end

Head end 1

2

3

(55)

Exoskeletons

 An exoskeleton is a hard encasement deposited on the surface of an animal

 Exoskeletons are found in most molluscs and arthropods

 Arthropods have a jointed exoskeleton called a cuticle, which can be both strong and flexible

 About 30–50% of the arthropod cuticle consists of a polysaccharide called chitin

 An arthropod must shed and regrow its exoskeleton

(56)

Endoskeletons

 An endoskeleton consists of a hard internal skeleton, buried in soft tissue

 Endoskeletons are found in organisms ranging from sponges to mammals

 A mammalian skeleton has more than 200 bones

 Some bones are fused; others are connected at

joints by ligaments that allow freedom of movement

(57)

Figure 39.12

Skull Shoulder girdle Clavicle

Scapula Sternum

Carpals

Pelvic girdle Ulna

Radius Vertebra Humerus Rib

Tarsals Fibula

Tibia Patella Femur Metacarpals Phalanges

Types of joints

Ball-and-socket joint

Hinge joint

Pivot joint

(58)

Figure 39.13

Ball-and-socket joint

Hinge joint

Pivot joint Ulna

Radius Humerus

Ulna

Head of

humerus

Scapula

(59)

Size and Scale of Skeletons

 An animal’s body structure must support its size

 The weight of a body increases with the cube of its dimensions, while the strength of that body

increases with the square of its dimensions

(60)

 The skeletons of small and large animals have different proportions

 In mammals and birds, the position of legs relative

to the body is very important in determining how

much weight the legs can bear

(61)

Types of Locomotion

 Most animals are capable of locomotion, or active travel from place to place

 In locomotion, energy is expended to overcome

friction and gravity

(62)

Flying

 Active flight requires that wings develop enough lift to overcome the downward force of gravity

 Many flying animals have adaptations that reduce body mass

 For example, birds have no teeth or urinary bladder, and their relatively large bones have air-filled

regions

(63)

Locomotion on Land

 Walking, running, or hopping on land requires an animal to support itself and move against gravity

 Diverse adaptations for locomotion on land have evolved in vertebrates

 For example, kangaroos have large, powerful

muscles in their hind legs, suitable for hopping

(64)

Figure 39.14

(65)

Swimming

 In water, friction is a bigger problem than gravity

 Fast swimmers usually have a sleek, torpedo-like shape to minimize friction

 Animals swim in diverse ways

 Paddling with their legs as oars

 Jet propulsion

 Undulating their body and tail from side to side or

up and down

(66)

Concept 39.3: Discrete sensory inputs can

stimulate both simple and complex behaviors

 Niko Tinbergen identified four questions that should be asked about animal behavior

1. What stimulus elicits the behavior, and what

physiological mechanisms mediate the response?

2. How does the animal’s experience during growth

and development influence the response?

(67)

3. How does the behavior aid survival and reproduction?

4. What is the behavior’s evolutionary history?

 Behavioral ecology is the study of the ecological

and evolutionary basis for animal behavior

(68)

 Behavioral ecology integrates proximate and ultimate explanations for animal behavior

 Proximate causation addresses “how” a behavior occurs or is modified, including Tinbergen’s

questions 1 and 2

 Ultimate causation addresses “why” a behavior

occurs in the context of natural selection, including

Tinbergen’s questions 3 and 4

(69)

Fixed Action Patterns

 A fixed action pattern is a sequence of unlearned, innate behaviors that is unchangeable

 Once initiated, it is usually carried to completion

 A fixed action pattern is triggered by an external

cue known as a sign stimulus

(70)

 Tinbergen observed male stickleback fish responding to a passing red truck

 In male stickleback fish, the stimulus for attack behavior is the red underside of an intruder

 When presented with unrealistic models, the attack

behavior occurs as long as some red is present

(71)

Figure 39.15

(a)

(72)

Migration

 Environmental cues can trigger movement in a particular direction

 Migration is a regular, long-distance change in location

 Animals can orient themselves using

 The position of the sun and their circadian clock, an internal 24-hour activity rhythm or cycle

 The position of the sun or stars

 Earth’s magnetic field

(73)

Behavioral Rhythms

 Some animal behavior is affected by the animal’s circadian rhythm, a daily cycle of rest and activity

 Behaviors such as migration and reproduction are linked to changing seasons, or a circannual rhythm

 Daylight and darkness are common seasonal cues

 Some behaviors are linked to lunar cycles, which

affect tidal movements

(74)

Animal Signals and Communication

 In behavioral ecology, a signal is a behavior that causes a change in another animal’s behavior

 Communication is the transmission and

reception of signals

(75)

Forms of Animal Communication

 Animals communicate using visual, chemical, tactile, and auditory signals

 Fruit fly courtship follows a three-step stimulus-

response chain

(76)

1. A male identifies a female of the same species and orients toward her

 Chemical communication: he smells a female’s chemicals in the air

 Visual communication: he sees the female and

orients his body toward hers

(77)

2. The male alerts the female to his presence

 Tactile communication: he taps the female with a

foreleg

(78)

3. The male produces a courtship song to inform the female of his species

 Auditory communication: he extends and vibrates his wing

 If all three steps are successful, the female will

allow the male to copulate

(79)

 Honeybees show complex communication with symbolic language

 A bee returning from the field performs a dance to

communicate information about the distance and

direction of a food source

(80)

Figure 39.16

(a) Worker bees (b) Round dance (food near) (c) Waggle dance (food distant)

Location A Location B Location C Beehive

A

B C

30

(81)

Figure 39.16a

(a) Worker bees

(82)

Figure 39.16b

(b) Round dance

(food near)

(83)

Figure 39.16c

(c) Waggle dance (food distant)

Beehive

A

B C

30

(84)

Pheromones

 Many animals that communicate through odors

emit chemical substances called pheromones

(85)

 For example,

 A female moth can attract a male moth several kilometers distant

 A honeybee queen produces a pheromone that affects the development and behavior of female workers and male drones

 When a minnow or catfish is injured, an alarm

substance in the fish’s skin disperses in the water,

causing nearby fish to seek safety

(86)

 Animals use diverse forms of communication

 Nocturnal animals, such as most terrestrial mammals, depend on olfactory and auditory communication

 Diurnal animals, such as humans and most birds,

use visual and auditory communication

(87)

Concept 39.4: Learning establishes specific links between experience and behavior

 Innate behavior is developmentally fixed and does

not vary among individuals

(88)

Experience and Behavior

 Cross-fostering studies help behavioral ecologists to identify the contribution of environment to an

animal’s behavior

 A cross-fostering study places the young from one

species in the care of adults from another species

(89)

 Studies of California mice and white-footed mice have uncovered an influence of social environment on aggressive and parental behaviors

 Cross-fostered mice developed some behaviors

that were consistent with their foster parents

(90)

Table 39.2

(91)

 In humans, twin studies allow researchers to

compare the relative influences of genetics and

environment on behavior

(92)

Learning

 Learning is the modification of behavior based

on specific experiences

(93)

Imprinting

 Imprinting is the establishment of a long-lasting behavioral response to a particular individual

 Imprinting can only take place during a specific time in development, called the sensitive period

 A sensitive period is a limited developmental phase

that is the only time when certain behaviors can be

learned

(94)

 An example of imprinting is young geese following their mother

 Konrad Lorenz showed that when baby geese

spent the first few hours of their life with him, they imprinted on him as their parent

 The imprint stimulus in greylag geese is a nearby object that is moving away from the young geese

Video: Ducklings

(95)

Figure 39.17

(96)

Figure 39.17a

(a) Konrad Lorenz and geese

(97)

Figure 39.17b

(b) Pilot and cranes

(98)

 Conservation biologists have taken advantage of imprinting in programs to save the whooping crane from extinction

 Young whooping cranes can imprint on humans in

“crane suits” who then lead crane migrations using

ultralight aircraft

(99)

Spatial Learning and Cognitive Maps

 Spatial learning is the establishment of a memory that reflects the spatial structure of the environment

 Niko Tinbergen showed how digger wasps use

landmarks to find nest entrances

(100)

Figure 39.18

Experiment

Pinecone

Results

Nest

Nest No nest

(101)

 A cognitive map is an internal representation of

spatial relationships between objects in an animal’s surroundings

 For example, Clark’s nutcrackers can find food

hidden in caches located halfway between particular

landmarks

(102)

Associative Learning

 In associative learning, animals associate one feature of their environment with another

 For example, a blue jay will avoid eating butterflies

with specific colors after a bad experience with a

distasteful monarch butterfly

(103)

Figure 39.19

(104)

Figure 39.19a

(105)

Figure 39.19b

(106)

Figure 39.19c

(107)

 Animals can learn to link many pairs of features of their environment, but not all

 For example, pigeons can learn to associate danger with a sound but not with a color

 For example, rats can learn to avoid illness-inducing

foods on the basis of smells, but not on the basis of

sights or sounds

(108)

Cognition and Problem Solving

 Cognition is a process of knowing that may

include awareness, reasoning, recollection, and judgment

 For example, honeybees can distinguish “same”

from “different”

(109)

 Problem solving is the process of devising a strategy to overcome an obstacle

 For example, chimpanzees can stack boxes in order to reach suspended food

 For example, ravens obtained food suspended from

a branch by a string by pulling up the string

(110)

 Social learning is learning through the observation of others and forms the roots of culture

 For example, young chimpanzees learn to crack palm

nuts with stones by copying older chimpanzees

(111)

 Culture is a system of information transfer through observation or teaching that influences behavior of individuals in a population

 Culture can alter behavior and influence the fitness

of individuals

(112)

Concept 39.5: Selection for individual survival and reproductive success can explain most

behaviors

 Behavior enhances survival and reproductive success in a population

 Foraging is a behavior essential for survival and

reproduction that includes recognizing, capturing,

and eating food items

(113)

Evolution of Foraging Behavior

 Natural selection refines behaviors that enhance

the efficiency of feeding

(114)

 In Drosophila melanogaster, variation in a gene dictates foraging behavior in the larvae

 Larvae with one allele travel farther while foraging than larvae with the other allele

 Larvae in high-density populations benefit from

foraging farther for food, while larvae in low-density

populations benefit from short-distance foraging

(115)

 Natural selection favors different alleles depending on the density of the population

 Under laboratory conditions, evolutionary changes in the frequency of these two alleles were observed

over several generations

(116)

Figure 39.20

Low population density High population density

D. melanogaster lineages

M e an p a th l en g th ( cm )

R1 R2 R3 K1 K2 K3 7

6

5

4

3

2

1

0

(117)

Mating Behavior and Mate Choice

 Mating behavior includes seeking or attracting

mates, choosing among potential mates, competing for mates, and caring for offspring

 Mating relationships define a number of distinct

mating systems

(118)

Mating Systems and Sexual Dimorphism

 The mating relationship between males and females varies greatly from species to species

 In some species there are no strong pair-bonds

 In monogamous relationships, one male mates with one female

 Males and females with monogamous mating

systems have similar external morphologies

(119)

Figure 39.21

(a) Monogamous species

(120)

Figure 39.21a

(a) Monogamous species

(121)

Figure 39.21b

(b) Polygynous species

(122)

Figure 39.21c

(c) Polyandrous species

(123)

 In polygamous relationships, an individual of one sex mates with several individuals of the other sex

 Species with polygamous mating systems are usually sexually dimorphic: males and females have different external morphologies

 Polygamous relationships can be either polygynous

or polyandrous

(124)

 In polygyny, one male mates with many females

 The males are usually more showy and larger than

the females

(125)

 In polyandry, one female mates with many males

 The females are often more showy than the males

(126)

 Needs of the young are an important factor constraining evolution of mating systems

Mating Systems and Parental Care

(127)

 Consider bird species where chicks need a continuous supply of food

 A male maximizes his reproductive success by staying with his mate and caring for his chicks (monogamy)

 Consider bird species where chicks are soon able to feed and care for themselves

 A male maximizes his reproductive success by seeking

additional mates (polygyny)

(128)

 Certainty of paternity influences parental care and mating behavior

 Females can be certain that eggs laid or young born contain her genes; however, paternal certainty

depends on mating behavior

 Paternal certainty is relatively low in species with

internal fertilization because mating and birth are

separated over time

(129)

 Certainty of paternity is much higher when egg laying and mating occur together, as in external fertilization

 In species with external fertilization, parental care is

at least as likely to be by males as by females

(130)

Figure 39.22

(131)

 Sexual dimorphism results from sexual selection, a form of natural selection

 In intersexual selection, members of one sex choose mates on the basis of certain traits

 Intrasexual selection involves competition between members of the same sex for mates

Sexual Selection and Mate Choice

(132)

 Female choice is a type of intersexual competition

 Females can drive sexual selection by choosing

males with specific behaviors or features of anatomy

 For example, female stalk-eyed flies choose males with relatively long eyestalks

 Ornaments, such as long eyestalks, often correlate with health and vitality

Video: Chimp Agonistic

Video: Snakes Wrestling

Video: Wolves Agonistic

(133)

Figure 39.23

(134)

 Male competition for mates is a source of

intrasexual selection that can reduce variation among males

 Such competition may involve agonistic behavior, an often ritualized contest that determines which

competitor gains access to a resource

(135)

Concept 39.6: Inclusive fitness can account for the evolution of behavior, including altruism

 The definition of fitness can be expanded beyond

individual survival to help explain “selfless” behavior

(136)

Genetic Basis of Behavior

 Differences at a single locus can sometimes have a large effect on behavior

 For example, male prairie voles pair-bond with their mates, while male meadow voles do not

 The level of a specific receptor for a neurotransmitter

determines which behavioral pattern develops

(137)

Figure 39.24

(138)

Genetic Variation and the Evolution of Behavior

 When behavioral variation within a species

corresponds to environmental variation, it may

be evidence of past evolution

(139)

 The natural diet of western garter snakes varies by population

 Coastal populations feed mostly on banana slugs, while inland populations rarely eat banana slugs

 Studies have shown that the differences in diet are genetic

 The two populations differ in their ability to detect and respond to specific odor molecules produced by the banana slugs

Case Study: Variation in Prey Selection

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Figure 39.25

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 Natural selection favors behavior that maximizes an individual’s survival and reproduction

 These behaviors are often selfish

 On occasion, some animals behave in ways that

reduce their individual fitness but increase the fitness of others

 This kind of behavior is called altruism, or selflessness

Altruism

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 For example, under threat from a predator, an individual Belding’s ground squirrel will make an alarm call to warn others, even though calling increases the chances that the caller is killed

 For example, in naked mole rat populations,

nonreproductive individuals may sacrifice their lives

protecting their reproductive queen and kings from

predators

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Figure 39.26

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Inclusive Fitness

 Altruism can be explained by inclusive fitness

 Inclusive fitness is the total effect an individual has

on proliferating its genes by producing offspring and

helping close relatives produce offspring

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Hamilton’s Rule and Kin Selection

 William Hamilton proposed a quantitative measure for predicting when natural selection would favor altruistic acts among related individuals

 Three key variables in an altruistic act

 Benefit to the recipient (B)

 Cost to the altruistic (C)

 Coefficient of relatedness (the fraction of genes

that, on average, are shared; r)

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 Natural selection favors altruism when rB  C

 This inequality is called Hamilton’s rule

 Hamilton’s rule is illustrated with the following

example of a girl who risks her life to save her

brother

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