Feeding Behavior
• Forage – to look for
food
– Herbivore
– Carnivore
– Omnivore
Feeding Behavior
• Herbivores are prey to
• Carnivores are
carnivores.
predators of herbivores
• Prey = an animal hunted • Predator = an animal
or seized for food,
that lives by preying on
especially by a
other animals for food
carnivorous animal.
by hunting and killing
them.
• Prey have evolved
strategies and defenses • Predators have evolved
to avoid being eaten.
strategies that enhance
their hunting skills.
Feeding Behavior
• Evolve = To change over time.
• Due to natural selection (survival of the fittest)
predator and prey are slowly changing to
become more successful at what they do.
• However, each of the prey’s new adaptations
(a trait that makes an animal more successful)
for defense is followed by the predator’s new
counter-adaptation for hunting.
Foraging Behavior
• Foragers can make four types of decisions:
– where to eat (location of most likely food supply)
– when to eat (time of day or night)
– what to eat (berries or salmon)
– how to eat (individually or in groups)
Stages in Foraging
• The first stage is a generalized search for
appropriate habitat. For example, deer may
have learned, or instinctively know, that richer
forage is found near streams and rivers.
Foraging Strategies
• The second stage is a
search strategy to find the
actual food items.
Searching strategies can
be divided into two basic
types:
– sit and wait (carnivores
only)
– active searching
(carnivores or herbivores),
such as the hunting
behavior of coyotes, and
the foraging behavior of
ungulates (deer-like)
Foraging Behavior
• Once food is discovered animals may find
hoarding is a beneficial component of foraging
behavior.
Foraging Behavior
• The final stage is consumption.
– an herbivore consumes the right plant when it is
found
– a carnivore must kill its prey before consumption.
Foraging in Groups
• Problems with foraging in groups:
– Uninvited guests. These can be members of the
same or different species. When one animal
observes another eating, it is attracted and
attempts to share the food.
Foraging in Groups
• Problems with Foraging in Groups
– The animal that finds the food must choose a
balance between defense and consumption.
– For the guest, the advantages are numerous
• it eats without searching
• avoids the risks involved in capturing and killing prey
Foraging in Groups
• Benefits of Group Foraging
– Cooperative hunting. Animals may cooperate to
trap elusive prey, or in taking down larger prey.
– Social bonds derived from kinship or mating may
enhance cooperation.
– However, once the prey item is secured,
competition for food among the hunters can be
severe and may be regulated by dominance
relationships.
Cost-Benefit Analysis of
Feeding Behavior
• Example:
– Researcher observes crows eating snails
– Researcher divides all of the available snails into
three groups by size: small, medium and large
– He observes
• Crows pick up only large snails
• Crows fly up 5 m and drop snails.
• Snail-shell breaks and crows eat snails.
Research Questions
1 - Why are only large
snails eaten and small
and medium snails
ignored? Answer in
group.
Large snails
break more easily
than small snails.
Researcher’s Hypotheses
• How do you test hypothesis 1? Answer in
group.
Drop different sized snails from various heights
and observe if there is any
difference in breakage.
• Results: Large snails break most easily!
Research Questions
2 - Why does the crow fly 5m high instead of
lower or higher? Answer in group.
Drops less than 5 m do not break
snails. Drops greater than 5m do
not increase snail breakage.
Researcher’s Hypotheses
• How do you test hypothesis 2?
Drop large snails from heights under 5 meters,
at 5 meters, and over 5 meters. Observe how
many break at each height.
• Results:
– As the height of the drop increased up to 5meters
the snails broke more easily.
– Above 5 meters there was no improvement in
their breakage.
The behavior of the crows is optimal
• Crows do not waste their time (and calories)
by foraging for small or medium-sized snails.
• Crows do not waste their time by flying at
heights above or below 5 meters.
• Crows reap the highest benefit with the
lowest cost.
COST
BENEFIT
Environmental Factors Affect
Feeding Behavior
• Example:
– Researcher observes in California:
• Inland garter snakes eat fish and frogs (few slugs available)
• Coastal garter snakes eat banana slugs (slugs are plentiful)
Research Question
• What controls the feeding behavior of garter
snakes?
– Is it controlled by genetics? OR
– Is it a function of available food?
• Hypothesis:
If inland snakes and coastal snakes are
raised in the same environment then they
will both readily eat slugs.
(Food availability.)
Researcher’s Hypothesis
• How do you test this?
– Pregnant female snakes
are taken from both
environments and kept
under identical
conditions.
– The baby snakes born in
this new environment
are offered slugs to eat.
Observations and Conclusions
• Observations
– The offspring of coastal garter snake ate slugs
– The offspring of inland garter snake wouldn’t eat
slugs
• Conclusion
– The feeding behavior of garter snakes is controlled
by genetics NOT food availability
– Therefore the researcher’s hypothesis was
incorrect
Extension of Garter Snake Experiment
• Experimenter mated the inland snakes with
the coastal snakes. (Garter snakes usually
bear 10-30 live young*.)
– Of the young, some would eat slugs and others
would not.
– This reinforced the conclusion that “slug-eating” is
controlled by genetics and shaped by natural
selection.
Conclusions
• Snakes that “liked” slugs were fit on the coast.
This trait was naturally selected for in a
coastal environment.
• Eating slugs would not make an inland snake
more fit. This trait was NOT selected for in an
inland environment.
Game Theory
• Game theory: the payoffs to individuals
associated with one behavioral tactic are
dependent on the types and frequencies of
behaviors exhibited by other animals in the
population.
Game Theory Activity
Evolution of Two Types of Feeding
Behavior
• Example: Roseate tern – two different
behaviors found in a population (controlled by
genetics!)
– Fish hunting behavior (bird does NOT have the
stealing behavior)
– Fish stealing behavior (bird does NOT have the
hunting behavior)
Evolution of Two Types of Feeding
Behavior
• Is it possible for the stealing birds to evolve
without the hunting birds in the population?
• Is it possible for the hunting birds to evolve
without the stealing birds in the population?
• Which phenotype evolved first?
Evolution of Two Types of Feeding
Behavior
• Imagine all roseate terms in a population hunt
for fish*
• What would happen if a mutation occurred in
which a bird didn’t hunt, but instead stole fish
from others? Would it survive?
• What would happen if the young of this one
mutant bird were all stealers? Would they go
hungry?
Evolution of Two Types of Feeding
Behavior
• What would happen if the proportion of
stealing birds in the population continued to
increase?
• Would the stealing birds go hungry?
• What would happen to this behavior in the
population? Increase or decrease?
• What happens to the number of hunters if the
population of stealers decreases?
Populations of Roseate Terns
• Scientists have found about
– 35% of roseate terns are stealers
– 65% are hunters.
– These numbers represent an equilibrium.
• If the % of stealers goes up, the % of hunters
goes down.
• Then the % of stealers decrease and the % of
hunters increase.
Conditional Foraging Strategies
• Each animal in a population is able to use
more than one method for finding food. The
behavior is NOT genetically predetermined
• Each animal alters its behavior depending on
the conditions
Conditional Foraging Strategies
• Example: small
sandpiper
• Birds forage for small
invertebrates on beach
• Dominant birds
monopolize patches of
seaweed. They flip it
over looking for food.
This is the best source
of food on the beach.
Conditional Feeding Strategies
• Subordinate birds are left to probe in the mud
and sand for food.
• If they go near the seaweed, the dominant
birds chase them away.
• However, if there is a lot of seaweed, all of the
birds forage among the seaweed.
• Therefore the birds choose their foraging
behavior depending on the condition.
Feeding Behavior and Skulls