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8
Behavioral Ecology
Chapter 8 Behavioral Ecology
CONCEPT 8.1 An evolutionary approach
to the study of behavior leads to testable
predictions.
CONCEPT 8.2 Animals make behavioral
choices that enhance their energy gain
and reduce their risk of becoming prey.
Chapter 8 Behavioral Ecology
CONCEPT 8.3 Mating behaviors reflect
the costs and benefits of parental
investment and mate defense.
CONCEPT 8.4 There are advantages and
disadvantages to living in groups.
Baby Killers: A Case Study
Lions are the only cats that live in social
groups called prides.
Adult females in a pride are closely
related.
A pride hunts cooperatively, and females
often feed and care for each other’s
cubs.
Baby Killers: A Case Study
Adult male lions often kill the cubs of
another male in the pride. Why would
this behavior be adaptive?
Baby Killers: A Case Study
Young adult male lions are driven from
the pride and may form “bachelor
prides” that hunt together.
At 4 or 5 years, a male can challenge
adult males in an established pride.
If successful, the new male may kill cubs
recently sired by the vanquished male.
Baby Killers: A Case Study
A female lion will become sexually
receptive soon after her cubs are killed,
as opposed to 2 years if she has cubs.
The new male is increasing the chances
that he will sire cubs before he is
replaced by another, younger male.
Baby Killers: A Case Study
Many seemingly odd behaviors exist in
the animal world.
In many species, females are more
“choosy” than males in selecting a mate;
but in some species males are choosy,
and females try to mate with as many
males as possible.
Figure 8.2 Females That Fight to Mate with Choosy Males
Introduction
An animal’s behavioral decisions play a
critical role in activities such as
obtaining food, finding mates, avoiding
predators.
These decisions have costs and benefits
that affect an individual’s ability to
survive and reproduce.
Introduction
Behavioral ecology is the study of the
ecological and evolutionary basis of
animal behavior.
CONCEPT 8.1
An evolutionary approach to the study of
behavior leads to testable predictions.
Concept 8.1
An Evolutionary Approach to Behavior
Animal behaviors can be explained at
different levels:
Proximate causes (immediate)—or how
the behavior occurs.
Ultimate causes—why the behavior
occurs; the evolutionary and historical
reasons.
Behavioral ecologists mostly focus on
ultimate causes.
Concept 8.1
An Evolutionary Approach to Behavior
Because an individual’s ability to survive
and reproduce depends in part on its
behavior, natural selection should favor
individuals whose behaviors make
them efficient at foraging, obtaining
mates, and avoiding predators.
Animal behaviors are often consistent
with this prediction.
Concept 8.1
An Evolutionary Approach to Behavior
If the traits that confer advantage are
heritable, natural selection can result in
adaptive evolution:
• Traits that confer survival or
reproductive advantages tend to
increase in frequency over time.
Concept 8.1
An Evolutionary Approach to Behavior
Many studies have documented
adaptive behavioral change.
Silverman and Bieman (1993) showed
that cockroaches exposed to traps with
a bait containing an insecticide plus
glucose evolved glucose aversion,
which is controlled by a single gene.
Figure 8.3 An Adaptive Behavioral Response
Concept 8.1
An Evolutionary Approach to Behavior
Most aspects of animal behavior are
controlled by both genes and
environmental conditions.
Weber et al. (2013) studied burrow
construction in two mice species.
Concept 8.1
An Evolutionary Approach to Behavior
• Oldfield mice build a long entrance
tunnel and an escape tunnel,
possibly an adaptation to living in
open habitats that provide little
protective cover.
• Deer mice construct a simpler
burrow, with a short entrance tunnel
and no escape tunnel.
Figure 8.4 Distinctive Mouse Burrows
Concept 8.1
An Evolutionary Approach to Behavior
The two mice species can interbreed
and produce fertile offspring.
All of the F1 hybrid offspring built
burrows with escape tunnels, as did
about 50% of backcross mice (F1
hybrids mated with deer mice).
This indicates that building escape
tunnels is controlled by one genetic
locus.
Figure 8.5 The Genetics of Escape Tunnel Construction
Concept 8.1
An Evolutionary Approach to Behavior
Genetic mapping (quantitative trait locus
analysis, or QTL) also showed that
entrance tunnel length was controlled
by three genetic loci.
Although few studies have identified the
genes, many behaviors are known to
be heritable, and most are influenced
by multiple genes.
Concept 8.1
An Evolutionary Approach to Behavior
Individuals with an allele for a certain
behavior may not always perform that
behavior, and may change behavior
when in different environments.
But by assuming that genes affect
behaviors, and natural selection has
molded them over time, we can make
specific predictions about how animals
will behave.
CONCEPT 8.2
Animals make behavioral choices that
enhance their energy gain and reduce their
risk of becoming prey.
Concept 8.2
Foraging Behavior
Food availability can vary greatly over
time and space.
If energy is in short supply, animals
should invest in obtaining the highestquality food that is the shortest distance
away.
Concept 8.2
Foraging Behavior
Optimal foraging theory: Animals will
maximize the amount of energy gained
per unit of feeding time, and minimize
the risks involved.
The theory assumes that natural
selection acts on the foraging behavior
of animals to maximize their energy
gain.
Concept 8.2
Foraging Behavior
Profitability of a food item (P) depends
on how much energy (E) the animal
gets from the food relative to amount of
time (t) it spends obtaining the food:
E
P
t
Figure 8.6 Conceptual Model of Optimal Foraging
Concept 8.2
Foraging Behavior
An animal’s success in acquiring food
increases with the effort it invests; but
at some point, more effort results in no
more benefit, and the net energy
obtained begins to decrease.
Concept 8.2
Foraging Behavior
Tests of the model:
• Benefits may incorporate net energy
gained, time spent feeding, or risk of
predation.
• If optimal foraging is an adaptation to
limited food supplies, then we must
be able to relate the benefit to
survival and reproduction of the
animal.
Concept 8.2
Foraging Behavior
In a study of great tits, proportions of
prey types and encounter rates were
varied.
The time it took birds to subdue and
consume the prey (handling time) was
measured.
The model correctly predicted
consumption rates of large mealworms
as profitability of prey items varied.
Figure 8.7 Effect of Profitability on Food Selection
Concept 8.2
Foraging Behavior
A field study of Eurasian oystercatchers
(Meire and Ervynck 1986) showed that
the birds select prey items in a specific
size range.
• Small bivalves do not provide
enough energy to offset the energy
needed to find and open them.
• Largest bivalves are too difficult to
open.
Concept 8.2
Foraging Behavior
Marginal value theorem (Charnov
1976):
An animal should stay in a patch until
the rate of energy gain has declined to
match the average rate for the whole
habitat (giving up time).
Giving up time is also influenced by
distance between patches.
Figure 8.8 The Marginal Value Theorem
Concept 8.2
Foraging Behavior
The longer the travel time between food
patches, the longer an animal should
spend in a patch.
Cowie (1977) tested this in lab
experiments with great tits.
A “forest” of wooden dowels contained
food “patches” of plastic cups
containing mealworms.
Concept 8.2
Foraging Behavior
“Travel time” was manipulated by
covering food cups, and adjusting ease
of mealworm removal.
Results matched predictions made by
the theorem very well.
Figure 8.9 Effect of Travel Time between Patches
Concept 8.2
Foraging Behavior
Optimal foraging theory does not apply
as well to animals that feed on mobile
prey.
The assumption that energy is in short
supply, and that this dictates foraging
behavior, may not always hold.
Resources other than energy can be
important, such as nitrogen or sodium
content of food.
Concept 8.2
Foraging Behavior
For foragers, risk of exposure to their
own predators is also important.
Trade-offs that affect foraging decisions
may be related to predators,
environmental conditions, or
physiological conditions.
Concept 8.2
Foraging Behavior
Presence of wolves affected foraging
behavior of elk in the Yellowstone
ecosystem (Creel et al. 2005).
Radio collars were used to track elk
movements.
When wolves were present, elk moved
into forests that had more protection
but less food.
Figure 8.10 Elk Change Where They Feed in Response to Wolves
Figure 8.11 Movement Responses of Male and Female Elk
Concept 8.2
Foraging Behavior
Small bluegill sunfish were found to
spend more time foraging in vegetation
if a predator was present, which
provided only one-third the food of
more open habitats.
Larger sunfish (too large to be eaten by
the bass) foraged in ways predicted by
optimal foraging theory (Werner et al.
1983).
Concept 8.2
Foraging Behavior
Even a perceived risk of predation can
alter foraging patterns.
Song sparrows exposed to recordings of
predators fed their young fewer times
per hour than did sparrows that heard
recordings of nonpredators (Zanette et
al. 2011).
Figure 8.12 Young Receive Less Food When Parents Fear Predators
Concept 8.2
Foraging Behavior
Prey species have evolved a broad
range of defenses against their
predators.
Antipredator behaviors include those
that help prey avoid being seen, detect
predators, prevent attack, or escape
once attacked.
Figure 8.13 Examples of Antipredator Behaviors
CONCEPT 8.3
Mating behaviors reflect the costs and
benefits of parental investment and mate
defense.
Concept 8.3
Mating Behavior
Males and females often differ in
physical appearance; males often
posses weapons such as horns or
gaudy ornaments.
The sexes may also differ in behavior.
Many males fight, sing loudly, or
perform strange antics to gain access
to females.
Figure 8.14 A Male Shows Off
Figure 8.15 A Male Courtship Dance
Concept 8.3
Mating Behavior
Darwin proposed that the extravagant
features of some males resulted from
sexual selection:
• Individuals with certain
characteristics gain an advantage
over others of the same sex solely
with respect to mating success.
Concept 8.3
Mating Behavior
Example: Bighorn sheep with large
horns defeat other males to win the
right to mate with females; their genes
are passed to the offspring, and large
horn size becomes common.
Concept 8.3
Mating Behavior
A test of sexual selection hypothesis:
• Male long-tailed widowbirds have
extremely long tail feathers. They
establish territories—areas that
they defend against intruders.
• Andersson (1982) captured males
and altered the length of their tail
feathers.
Concept 8.3
Mating Behavior
• Males with lengthened tails had
higher mating success than control
males or males with shortened tails.
This supported the hypothesis that
female mating preferences affect male
mating success.
Many other studies since have found
similar results.
Figure 8.16 Males with Long Tails Get the Most Mates
Concept 8.3
Mating Behavior
In some species, males provide females
with a direct benefit for mating—gifts of
food, help in rearing young, access to a
territory with good nesting sites, food.
etc.
In other species, males provide
nothing—instead, females may receive
indirect genetic benefits.
Concept 8.3
Mating Behavior
The handicap hypothesis: a male that
can support a costly and unwieldy
ornament is likely to be a vigorous
individual whose overall genetic quality
is high.
Concept 8.3
Mating Behavior
The sexy son hypothesis: the female
receives indirect genetic benefits
through her sons, who will themselves
be attractive to females and produce
many grandchildren.
Concept 8.3
Mating Behavior
These hypotheses were tested in a
study of the stalk-eyed fly.
Eyestalk length is heritable, and females
prefer to mate with males with the
longest eyestalks.
Concept 8.3
Mating Behavior
By selecting for long and short stalks
over 13 generations, Wilkinson and
Reillo (1994) showed that extreme
male eyestalk lengths are maintained
by sexual selection.
Female mating selection also evolved
differently in the two populations.
Figure 8.17 Mating Preferences of Female Stalk-Eyed Flies
Concept 8.3
Mating Behavior
Females may benefit from selecting
males with long eyestalks because
their male offspring will be attractive to
the next generation of females, which
supports the sexy son hypothesis.
But, eyestalk length in male flies is
correlated with overall health and vigor
(David et al.1998), supporting the
handicap hypothesis.
Concept 8.3
Mating Behavior
Females and males often differ in the
amount of energy and resources they
invest in their offspring.
Females are usually more choosy than
males in mate selection.
In anisogamous species, the female
invests a lot more to produce a large
egg than the male does to produce
sperm.
Concept 8.3
Mating Behavior
Females often continue to invest more in
the offspring, (e.g., incubating eggs,
caring for young, etc.)
Because of these costs, males often
produce more offspring during their
lifetimes than females.
Table 8.1
Concept 8.3
Mating Behavior
Selection should favor different mating
behaviors:
• It should be advantageous for a male
to mate with as many females as
possible.
• A female should “protect” her
investment by choosing males that
provide ample resources or appear
to be of high genetic quality.
Concept 8.3
Mating Behavior
There are exceptions: in some species
females compete for males.
In these cases, we would expect that
males would provide more parental
care than females, leading to
competition among females for the right
to mate with choosy males.
Concept 8.3
Mating Behavior
Females in these types of species have
higher reproductive potential than
males do.
Red phalarope females abandon their
nests once eggs are laid, and search
for other males.
The males incubate the eggs.
Concept 8.3
Mating Behavior
Pipefish males have a special pouch in
which they protect and nourish the
fertilized eggs while the female mates
with other males.
Males select the largest, most highly
ornamented females, who produce the
most eggs.
Concept 8.3
Mating Behavior
Ecological factors also affect mating
behavior.
Mate choice can be altered by factors
such as number and locations of
potential mates, mate quality, food
availability, and presence of predators
or competitors.
Concept 8.3
Mating Behavior
Ecological factors can also influence
mating systems: number of mating
partners and patterns of parental care.
Diverse mating systems result from the
behaviors of individuals striving to
maximize their reproductive success or
fitness (Emlen and Oring 1977).
Table 8.2
Concept 8.3
Mating Behavior
Polygyny can occur if females show a
clumped distribution; a male can
monopolize them.
The brushtail possum is monogamous in
habitats where food and nest sites (and
hence females) are widely separated,
but polygynous in habitats where food
and nest sites (and hence females) are
clumped.
Figure 8.18 Ecological Factors Can Affect the Potential for Polygyny
Concept 8.3
Mating Behavior
Monogamy usually occurs in mammalian
species where it is difficult for males to
defend access to more than one
breeding female.
Concept 8.4
Living in Groups
CONCEPT 8.4
There are advantages and disadvantages
to living in groups.
Concept 8.4
Living in Groups
Benefits of group living:
• Higher reproductive success—
especially when males hold highquality territories.
• Group members may share feeding
and care of young.
• Reduced risk of predation—individuals
can band together to prevent attacks;
predators may be detected earlier.
Figure 8.19 A Formidable Defense
Concept 8.4
Living in Groups
Dilution effect: as the number of
individuals in a group increases, the
chance of being the one attacked by a
predator decreases.
Group members may respond to a
predator by scattering in different
directions, making it difficult for the
predator to select a target.
Concept 8.4
Living in Groups
Group members may have better foraging
success.
Lions, killer whales, wolves, and many
other predators may coordinate their
attacks, such that actions of one
predator drive prey into the waiting jaws
of another.
Herbivores may also forage more
effectively when in groups.
Concept 8.4
Living in Groups
Costs of group living:
As group size increases, the members
deplete the available food more rapidly;
more time may be spent in moving
between feeding sites.
Figure 8.20 Safety in Numbers
Concept 8.4
Living in Groups
Competition for food can become more
intense.
In groups with a dominance hierarchy,
subordinate members can spend much
time and energy on interacting with
group members.
Concept 8.4
Living in Groups
Members of a large group may live closer
together or come into contact with one
another more often than in a small
group.
As a result, parasites and diseases often
spread more easily.
Concept 8.4
Living in Groups
Group size may reflect a balance between
costs and benefits.
Optimal size should be the size at which
net benefits to the members are
maximized.
But unless group members can prevent
other individuals from joining once
optimal size is reached, observed group
size may be larger than the optimal.
Figure 8.21 Traveling in a Group
Concept 8.4
Living in Groups
It may be advantageous for individuals to
belong to groups that are larger than
optimal, but not so large that a new
arrival would do better on its own.
An intermediate-sized group might be
large enough to reduce risk of predation,
but small enough to avoid running out of
food.
Figure 8.22 Should a New Arrival Join the Group?
A Case Study Revisited: Baby Killers
The males of many species kill the young
of their potential mates—examples
include langur monkeys, horses,
chimpanzees, bears, and marmots.
DNA analysis showed that male langurs
were not related to the infants they
killed, but were related to the females’
subsequent offspring (Borries et al.
1999).
A Case Study Revisited: Baby Killers
In some species, females commit
infanticide, such as giant water bugs and
wattled jacanas.
In these species, the males provide most
or all of the parental care, and the
females have higher reproductive
potential.
A Case Study Revisited: Baby Killers
Female fruit flies sometimes lay eggs in
foods with high alcohol content.
Exposure to alcohol kills wasps that lay
their eggs on fruit fly larvae, thereby
increasing the overall chance that the
larva will survive.
A Case Study Revisited: Baby Killers
Kacsoh et al. (2013) showed that adult
female fruit flies altered their egg-laying
behavior in response to the presence of
wasps.
When female wasps were present, fruit
flies laid over 90% of their eggs in highalcohol foods.
This behavior increased survival of fruit fly
larvae exposed to wasps.
Figure 8.23 Fruit Flies Medicate Their Offspring
Connections in Nature: Behavioral Responses to Predators
Have Broad Ecological Effects
Individuals often change their behavior
in response to predators.
When exposed to recordings of
predators, song sparrows fed their
young less often, built nests in less
desirable areas, and spent less time
incubating eggs (Zanette et al. 2011).
Connections in Nature: Behavioral Responses to Predators
Have Broad Ecological Effects
The sparrow offspring lost body heat
more rapidly and weighed less than did
offspring of sparrows exposed to
recordings of nonpredators, and
number of offspring produced per year
declined.
Fear of predation can alter behavior and
result in reduced fitness.
Connections in Nature: Behavioral Responses to Predators
Have Broad Ecological Effects
Behavioral responses to predators can
also affect ecosystem processes.
Hawlena et al. (2012) found that
presence of spider predators initiated a
series of events in their grasshopper
prey that ultimately slowed the
decomposition of plant litter in the soil.
Connections in Nature: Behavioral Responses to Predators
Have Broad Ecological Effects
When spiders were present,
grasshoppers were physically stressed
and required more energy for
maintenance.
This altered foraging behavior, leading to
consumption of high-carbohydrate
foods that were low in nitrogen.
Connections in Nature: Behavioral Responses to Predators
Have Broad Ecological Effects
The altered carbon:nitrogen ratio in
decomposing bodies of grasshoppers
influenced the ratio in the soil, which
affected the community of soil
microorganisms that decompose
leaves and other plant matter.