Week 5: Foraging behavior

Question 1

Foraging behavior

Answer 1

searching for and consuming food

Foraging behavior is a critical part of every animal’s existence

Many animals spends a good deal of their waking hours foraging

Question 2

Anti-fungi mutualism

Answer 2

Ant-fungi mutualism is a symbiosis seen in certain ant and fungal species

The ants promote the growth of the fungi (good for the fungi), while also eating the vegetative shoots produced by their fungal partners (good for ants)

This mutualistic relationship began about 50 million years ago

Question 3

Streptomyces bacteria

Answer 3

A whitish-grey crust found on and around many ants with fungus food gardens

A mass of Streptomyces bacteria - a type of bacteria that produces many antibiotics

Hypothesis: ants use the bacteria’s antibiotics to kill parasites that grow in their fungal gardens

Question 4

Ants use antibiotics to regulate parasitic disease

Answer 4

All 20 species of the fungus-growing ants had Streptomyces bacteria associated with them.

Ants transmit the bacteria across generations.

When male and female reproductive ants are examined, only females possess the bacteria.

The bacteria found on fungus-growing ants produce antibiotics that wipe out only certain parasitic disease. Escovopsis, a serious parasitic threat to the ants’ fungus garden.

Question 5

Starlings

Answer 5

Starlings fly from their nest to a feeding site (patch), search for a load of prey by probing in the grass, then take them home to the nestlings.

The bird is very efficient at probing. But when it has a load of leatherjackets already in its bill, it becomes less efficient. For this reason it is not necessarily the best thing to fully load its beak.

Question 6

Starling’s problem of load size

Answer 6

traveling time: the amount of traveling back and forth from the nest to a foraging site

searching time: probing the prey in a foraging site and catching it

The first couple of leatherjackets are found quickly and easily, but because of the encumbrance of the prey in its beak, the bird takes longer and longer to find each successive prey.

Within each foraging trip, diminishing returns arise because, as it loads up with prey, the starling’s efficiency at collecting further prey is progressively diminished.

Question 7

Loading curve

Answer 7

Load as a function of search time

The longer the starling has been foraging, the less likely it is to find another leatherjacket in a patch.
–> When should the starling give up on this curve?

Question 8

Diminishing returns

Answer 8

• handling time
• depletion
• prey in the patch take evasive action and become harder to catch
• the predator becomes less likely to search new areas in the patch
• the predator starts with the easy prey and then goes on to hunt for those that are more difficult to catch

—> Loading curve is a curve of diminishing returns

Question 9

Rate of intake

Answer 9

‘best” option = maximising net rate of delivery of food

rate of intake = load/ (traveling time + searching time)

Question 10

Optimal foraging theory

Answer 10

max rate of intake = max load/(traveling time + searching time)
—> the slope of the line between A and the intersection with the gain curve

Question 11

Traveling time

Answer 11

Suppose that the starling now switches to feeding at a close site with a short travel time, how should its load per trip change?

optimum is less for the shorter travel time.

Question 12

Load size as a function of round trip time

Answer 12

Kacelnik (1984): tested the prediction of the model of load size
- trained parent starlings in the field to collect mealworms
- mealworms were dropped in a wooden tray
- varied the intervals of dropping mealworms
- the trained bird waited on the wooden tray for mealworms

the load size increased as a founding of round trip time

Question 13

Marginal value theorem

Answer 13

Foraging in a patch:
- prey are distributed in patches
- prey within patches are depleted by foragers
- forager pays travel cost to reach other patch

What should a forager do to maximise the rate of gain?
- leave the patch when the rate of gain becomes marginal
- marginal value theorem (MVT)

Application of MVT:
- applicable to lots of situation in which an animal exploits a resource that occurs in discrete patches, and within a patch it experiences diminishing returns

Question 14

Optimality theory

Answer 14

• the notion that adaptations have greater benefits-to-costs ratios than the putative alternatives that have been replaced by natural selection
• currency: the costs and benefits impinging on the animal
• constraints: mechanisms of behavior and the physiological limitations of the animal

3 advantages of optimality modeling:

1. testability
   - testable, quantitative predictions
   - easy to tell whether hypotheses are right or wrong
2. explicit assumptions
   - the bird encounter only one patch at a time,
3. generality

Question 15

Bobwhite quail coveys

Answer 15

Northern bobwhite quail spend the winter months in small groups, called coveys, which ranges in size from 2 and 22 individuals.

Benefits: anti-predator defense; the larger the group is, the more safe. Then levels off around a group size of 10

Costs: Increased competition for food in larger groups, forcing more movement

Optimal covey size for northern bobwhite:
- Most are composed of intermediate size, with about 11 individuals.
- Daily survival rate are highest in coves of this group size.

Question 16

Optimal foraging theory

Answer 16

≡ Organisms forage in such a way as to maximize their energy intake per unit time.

Animals behave in such a way as to find, capture and consume food containing the most calories while expending the least amount of time possible in doing so.

Question 17

Where to eat

Answer 17

How long should a forager stay in a patch of food?

A patch is defined as a clump of food that can be depleted by a forager.

Once a forager begins feeding in a patch, the rate at which it takes in food slows down, as the more the forager eats, the less food remain in the patch.