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Behavioral Ecology
53
Behavioral Ecology
• Introduction
• Responding to Environmental Variation
• The Evolution of Animal Societies
• Behavioral Ecology, Population Dynamics, and
Community Structure
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Introduction
• Ecology is the science that deals with all kinds of
biological interactions.
• Individuals of all species interact in various ways
with individuals of their own and other species
and with their physical environment.
• The term environment includes both abiotic
(physical and chemical) and biotic factors (all
other organisms living in an area).
• Behavioral ecology is the study of how animals
make “decisions” that influence their survival and
reproductive success.
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Responding to Environmental Variation
• Throughout their life, all organisms must make
many decisions and life choices in a changing
environment.
• Some environmental changes, such as the
approach of danger, require an immediate
response; others allow for a more gradual
response.
• Some plants detach their leaves in a wind storm
and regrow leaves afterward.
• Lizards bask in the morning sun and then move to
the shade when it gets too hot.
Figure 53.1 Plants Can Respond to Environmental Changes
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Responding to Environmental Variation
• Organisms may evolve life cycles that anticipate
cyclical environmental change.
• Insectivorous birds leave high latitudes in autumn
for more favorable wintering grounds; grazing
animals may migrate to follow the rains.
• Other animals hibernate until environmental
conditions have improved.
Figure 53.2 Migration Is a Response to Predictable Seasonal Changes
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Responding to Environmental Variation
• The environment in which an organism normally
lives is called its habitat.
• For choosing a habitat, an animal seeks food,
resting places, nest sites, and escape routes from
predators.
• Numerous factors influence how animals choose
environments, but in general, habitat selection
cues are good predictors of general conditions
suitable for survival and reproduction.
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Responding to Environmental Variation
• Habitat selection by red abalone involves
chemical cues.
• The abalone begins its life as a motile larva and
swims in the open ocean until its yolk sac is gone
(about seven days).
• Guided by chemical signals from coralline algae
(the abalone’s food source), the abalone settles
on the seafloor and metamorphoses into an adult.
• Only the coralline algae produce the chemical, so
the larvae always settle onto suitable habitat.
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Responding to Environmental Variation
• Many animals use the presence and success of
already settled individuals as an indication that
the habitat may be good ground.
• When collared flycatchers arrive on their breeding
grounds in the spring, they peer into the nests of
other individuals.
• In studies in which broods were artificially
enlarged by researchers, birds settled
preferentially in those areas.
Figure 53.3 Flycatchers Use Neighbors’ Success to Assess Habitat Quality
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Responding to Environmental Variation
• An animal may leave an area temporarily or
permanently if the population has grown too large
to be supported there.
• When a colony of the ant Lepidothorax albipennis
has grown too large for its nest site, recruiter ants
look for a new site.
• A recruiter returns to the nest and releases a
pheromone that attracts a second recruiter, and
both visit the new site. Then a third recruiter is
brought in, and so on.
• Once a threshold number of workers has been
recruited to a site, the recruiters begin carrying
eggs and larvae from the old to the new site.
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Responding to Environmental Variation
• The most common way for an animal to improve
its survival and reproductive success is to
establish an exclusive territory.
• Advertising and defending the territory takes time
and energy.
• To understand the evolution of these types of
behavior, ecologists use a method called cost–
benefit analysis, based on two assumptions:
 An animal has a limited amount of time and
energy to devote to any particular activity.
 Animals generally do not perform behaviors
whose total costs are greater than the sum of
their benefits.
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Responding to Environmental Variation
• The cost of behavior has three components:
 The energetic cost is the difference between
the energy the animal would have expended
had it rested and the energy expended in
performing the behavior.
 The risk cost is the increased chance of being
injured or killed as a result of performing the
behavior.
 The opportunity cost is the sum of benefits
the animal forfeits by not being able to perform
other behaviors during the same time interval.
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Responding to Environmental Variation
• An experiment conducted with Yarrow’s spiny
lizards demonstrates all three costs.
• Male lizards with implanted testosterone patrolled
their territories more, performed more displays,
and expended more energy than control males
did (energetic cost).
• They had less time to feed (opportunity cost),
captured fewer insects, stored less energy, and
died at a higher rate (risk cost).
• Normally, the lizards only defend their territories
vigorously during the breeding season.
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Responding to Environmental Variation
• Foraging theory is used to predict how animals
will behave when searching for food.
• A scientist first specifies the objective of the
behavior and then attempts to determine the
behavioral choices that would best achieve that
objective.
• This approach is known as optimality modeling.
• The underlying assumption is that natural
selection has molded the behavior of animals so
that they solve problems by making the best
choices available to them.
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Responding to Environmental Variation
• In the case of food selection, for example,
optimality modeling would support the energy
maximizing hypothesis:
 If the most valuable prey type is abundant, a
predator gains the most energy per unit of time
spent foraging by taking only that prey type.
 As the abundance of the most valuable prey
type decreases, the predator adds less
valuable prey to its diet.
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Responding to Environmental Variation
• The energy maximizing hypothesis has been
tested using bluegill sunfish.
• Prey types were three sizes of Daphnia, the water
flea. They were ranked according to their energy
quantity and the energy required for the fish to
capture and ingest them.
• If all three sizes of Daphnia were present in low
numbers, the fish ate every one encountered.
• If large Daphnia were abundant, the fish ignored
the smaller ones.
Figure 53.5 Bluegills Are Energy Maximizers
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Responding to Environmental Variation
• Animals select certain foods for reasons other
than energy content. Many species of mammals
and birds, for instance, get minerals by eating
mineral-rich soil.
• One hypothesis to explain human’s taste for
spices is that spices have antimicrobial properties
against food-borne bacteria.
Figure 53.6 Mineral Seekers
Figure 53.7 Most Spices Have Antimicrobial Activity (Part 1)
Figure 53.7 Most Spices Have Antimicrobial Activity (Part 2)
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Responding to Environmental Variation
• The most basic mating decision is the choice of a
partner of the correct species.
• Decisions are made based on the qualities of a
potential mate, the resources it controls, and nest
sites.
• The reproductive behaviors of males and females
are often very different, in part because of the
costs of producing sperm and eggs.
• Sperm are cheap to produce (energetically).
Therefore, males of most species can increase
their reproductive success by mating with many
females.
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Responding to Environmental Variation
• Eggs are energetically more expensive to produce.
• To improve their reproductive success, females
need to assess the quality of potential mates,
including health, genetic quality, potential for
parental care, and quality of resources they control.
• Males use a variety of tactics to induce females to
copulate.
• Females favor signals at which males can’t “cheat”,
and have favored the selection of reliable signals.
Figure 53.8 A Male Wins His Mate
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Responding to Environmental Variation
• Experiments with bluethroats have shown that
females respond to the UV light reflected by the
bright blue throat patch of males.
• They prefer normal males to those to whom
sunscreen has been applied because intense
plumage is an indicator of a male’s health.
Figure 53.9 Ultraviolet-Reflecting Plumage Affects Female Choice
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The Evolution of Animal Societies
• Social behavior evolves when individuals who
cooperate with others of the same species have,
on average, higher rates of survival and
reproductive success than those achieved by
solitary individuals.
• Today’s animal social systems are the result of
long periods of evolution, but behavior leaves few
traces in the fossil record.
• Biologists infer possible routes of the evolution of
social systems by studying current patterns of
social organization.
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The Evolution of Animal Societies
• Three important concepts in understanding animal
social systems:
 Social systems are best explained by how they
benefit the individuals who join together, not
according to how they benefit the species as a
whole.
 They are dynamic, as individuals constantly
communicate and adjust relationships.
 The costs and benefits to specific individuals
differ according to their age, sex, physiological
condition, and status.
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The Evolution of Animal Societies
• Group living may improve hunting success or
expand the range of prey that can be captured.
• Hunting in groups, our ancestors were able to kill
larger animals than they could have if they had
hunted alone.
• Small birds forage in flocks; flocking has been
shown to provide protection against predation.
Figure 53.10 Groups Provide protection from Predators
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The Evolution of Animal Societies
• Social behavior has costs as well as benefits.
• Individuals in a group may compete for food,
interfere with one another’s foraging, injure one
another’s offspring, inhibit one another’s
reproduction, or transmit diseases to their
associates.
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The Evolution of Animal Societies
• The most widespread social system is the family,
an association of one or more adults and their
dependent offspring.
• If parental care or the breeding season lasts a
long time, older offspring may be available to help
parents care for younger siblings.
• Florida scrub jays live on territories that contain a
breeding pair and helper offspring who bring food
to the nest.
• Parental care is altruistic—it involves
tremendous costs for parents and helpers. How
has it been possible, therefore, for altruism to
evolve?
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The Evolution of Animal Societies
• Altruistic behaviors are most easily understood in
terms of close genetic relatedness.
• An individual contributes to its own individual
fitness by producing offspring and may also help
relatives in ways that increase their fitness.
• By helping its relatives, an individual can increase
the representation of some of its own genes in the
population. This process is known as kin
selection.
• Together, individual fitness and kin selection
determine the inclusive fitness of an individual.
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The Evolution of Animal Societies
• Occasional altruistic acts may eventually evolve
into altruistic behavior if the benefits of increasing
reproductive success of relatives outweigh the
costs in terms of the individual’s own reproductive
success.
• Breeding pairs of white-fronted bee-eaters are
assisted by nonbreeding adults who help them
incubate their eggs and feed nestlings. Helpers
choose nests with young that are most closely
related to them.
• This behavior likely evolved through kin selection.
Individual birds do not gain anything other than
inclusive fitness, and nests with helpers produce
more fledglings than nests without helpers.
Figure 53.11 White-Fronted Bee-Eaters are Altruists
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The Evolution of Animal Societies
• Species such as ants, bees, and wasps, whose
social groups include sterile individuals, are said
to be eusocial.
• In these species, worker females defend the
group against predators or bring food to the
colony, but they do not reproduce; only a few
females, known as queens, reproduce.
• Some ant species have soldiers with large
defensive weapons.
Figure 53.12 Sterile Workers are Extreme Altruists
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The Evolution of Animal Societies
• Genetic factors may facilitate the evolution of
eusociality.
• Among the Hymenoptera, a diploid egg hatches
into a female and a haploid egg hatches into a
male.
• Therefore, if a female mates with only one male,
her daughters share all of their father’s genes but
only half of their mother’s.
• Because workers are more genetically similar to
their sisters than they would be to their own
offspring, they increase their own fitness by caring
for their sisters rather than by reproducing
themselves.
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The Evolution of Animal Societies
• Eusociality may be favored if establishment of a
new colony is difficult and dangerous. Nearly all
eusocial animals construct elaborate nests or
burrow systems within which their offspring are
reared.
• High predation rates may account for the
eusociality of naked mole rats, who live
underground in colonies of 70–80 individuals and
restrict breeding to a single queen and several
kings.
• Inbreeding could help explain the evolution of
eusociality among many hymenopteran species in
which queens mate with many males, and among
termites and naked mole-rats, in which both sexes
are diploid.
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Behavioral Ecology,
Population Dynamics, and Community Structure
• The ways in which organisms make decisions
about habitats, food, and associates may have
important implications for the structure and
function of ecological systems.
• First, animals with complex social organizations
often achieve high abundances.
• Second, the decisions animals make about the
above three matters may influence the range of
habitats and foods used by a species.
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Behavioral Ecology,
Population Dynamics, and Community Structure
• For ants and termites, living in colonies allows
them to harvest vital resources from other
organisms.
• Most of the biomass of arthropods in the canopies
of tropical rainforests are social ants.
• Termites, which live in dense colonies, are the
primary consumers of plant tissues in the savannas
of Africa.
• Both actively cultivate fungi that break down
difficult-to-digest plant tissues, including wood.
• Some ants tend phloem-sucking aphids and other
insects that provide the ants with the
carbohydrates they need.
Figure 53.13 Termite Mounds Are Large and Complex
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Behavioral Ecology,
Population Dynamics, and Community Structure
• Social living also enables organisms to use
temporally and spatially patchy foods. An example
is the wildebeest, which travels in large herds and
is the most abundant wild mammal in Africa.
• Social organization allows humans to live in high
densities and to specialize in different activities.
Figure 53.14 Social Organization Allows Humans to Live at High Densities
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Behavioral Ecology,
Population Dynamics, and Community Structure
• Despite the “rule of thumb” accuracy of the
optimality modeling approach, interspecific
interactions may prevent animals from living in
those environments in which they would do best.
• Individuals of a behaviorally dominant species
may be able to exclude individuals of a
subordinate species from its preferred foraging
areas.
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Behavioral Ecology,
Population Dynamics, and Community Structure
• Hummingbirds extract nectar from flowers and
defend flower patches.
• In an experiment in Arizona, investigators set up
feeders with artificial nectar, some rich in sucrose
(blue), others containing a dilute solution (yellow).
Hummingbirds quickly learned which were the
high-quality feeders.
• Larger male blue-throated hummingbirds kept
smaller male black-chinned hummingbirds away
from the rich feeders.
• Nevertheless, the smaller hummingbirds achieved
about the same amount of energy from the dilute
feeders because they were able to feed longer at
them.