Feeding Behavior

• Forage – to look for food– Herbivore – Carnivore– Omnivore

Feeding Behavior

• Herbivores are prey to carnivores.

• Prey = an animal hunted or seized for food, especially by a carnivorous animal.

• Prey have evolved strategies and defenses to avoid being eaten.

• Carnivores are predators of herbivores

• Predator = an animal that lives by preying on other animals for food by hunting and killing them.

• Predators have evolved 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.

• Results: Large snails break most easily!

Drop different sized snails from various heights and observe if there is any

difference in breakage.

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?

• 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.

Drop large snails from heights under 5 meters, at 5 meters, and over 5 meters. Observe how

many break at each height.

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.

COSTBENEFIT

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