FORAGING

I. BEE DANCES REPRISED

Just a followup -

1. you will see and hear the videoclip again

2. Additional examples of how bees dance to indicate the direction of food along with a new handout.

II. OPTIMAL FORAGING MODELS IN BIRDS

We now turn our attention to birds where the focus is on energy models and determining whether the behavior of birds and other animals can be described as optimal. Thus, we are examining the function question.

A. Foraging and Decision Making

1. All animals must find and prepare food in order to survive. This process is referred to as foraging.

a. There are a number of decisions that foragers must make.

b. These relate to the energy required to obtain and process food.

2. example: crows dig up clams. However, they must break the shells to eat them. This is usually accomplished by flying into the air with the clam and dropping it on the ground below. Several drops may be required. Should crows ignore small clams after digging them up and go for bigger clams?

3. Answers to these questions:

1. require assessments of energy

2. Gains and losses are viewed from a cost/benefit perspective.

A. Cost-Benefit Models

1. Optimal foraging is thought of in terms of energy expended (cost) vs. energy gained (benefit)

2. Cost = energy spent searching for and processing food for ingestion which is usually measured as:

3.Benefit = calories (energy) gained

4. Optimal foraging theory predicts that animals should select:

5. Does the foraging behavior of animals actually conform to this theory (as we shall see; sometimes it does).

6. However, energy models do not always work (they need modification), and we shall see why?

B. Developing Foraging Models

1. Clam Choice by Crows

a. Description (crows dig up clams - carry them in the air some distance and drop them on the rocks below - do they select profitable clams?)

b. Size of clams (crows should select clams that maximize their caloric intake)

c. Crows can't determine clam size prior to digging them up (digging costs are the same but carrying costs are not). Bigger clams have more calories than small clams.

1. Richardson and Verbeek developed a model that predicted that crows should respond differently to clams of different sizes. According to the model, crows should respond in what way to:

small clams 2.4-2.7 cm ___________________________________

 

medium clams of 2.8- 3.3 cm _______________________________

 

large clams of 3.5- 4.0 cm__________________________________

d. Do crows behave as Richardson and Verbeek's model predicts? Draw Graph

 

 

 

 

c. But there is more to consider than clam size because crows still have to break open the shell. To do so, crows carry the clams up into the air and drop them on the rocks below. Furthermore repeated drops are required. (video clip)

1. Energy expenditure - What strategy is optimal with regard to

a. how high to fly and how many drops to make?

b. Also do the crows follow flight path A (head up when dropped) or flight path B (head down when dropped)?

2. Zach's Model: Zach dropped large clams at varying heights and recorded the number of drops required to break the clam shell. What is the optimal response according to his calculations (be sure to fill in the rest of the table and mark the most efficient strategy with a star)

Height of Drop (m)	Number of Drops	Total Flight Height = #drops x height per drop
2	55	110
3	13	39
5	-	-
6	-	-
7	-	-
10	-	-
15	4	60

c. Does the model developed by Zach predict what crows do?

 

 

 d. At what point in the flight does the crow drop the clam (on the upward trajectory with head up or on the downward trajectory with head down). Why?

 

 

e.  Strategies for breaking walnuts. How does this compare with breaking clams? (videoclip)

 

 

 

 

 

2. Mussel Choice by Oystercatchers

a. Oystercatchers eat mussels that must be pried open or hammered with their bill. Our focus will be on oystercatchers that use the hammering strategy only.

b. As is the case with clams, mussels vary in size and large mussels provide more calories than small mussels.

c. Based on what crows do, we might predict that oystercatchers should prefer large mussels.

d. testing the prediction

1. Like crows, oystercatchers dig up mussels of different size.

2. Like crows, they ignore all small mussels (under 30mm)

3. Unlike crows, they choose medium sized mussels (30-45 mm) and ignore large mussels over 45 mm

e. Two possibilities

1. The animals are not behaving “optimally”

2. Or the optimality model is incorrect or incomplete

b. Most often, the optimal foraging model falls short because it is incomplete -other factors have to be added in to the model. What factors should be considered for the oystercatcher?

 

1. Factor 1_____________________________

Does this factor alone explain the discrepancy?

 

 

2. Factor 2_____________________________

C. Constraints and Optimal Foraging models

1. optimal foraging theory consists of maximizing the net energy gained while minimizing the energy expended to find and process food.

2. Not all models of energy maximization appear to explain foraging decisions; Thus there may be constraints like those encountered for the oystercatcher that require modifications to optimal foraging models. Here we broaden our discussion of what those constraints might be.

2. Types of constraints (other factors that influence foraging decisions)

a._____________________

 

b._____________________

 

c.______________________

3. Examples: Predation Risk in kangaroo rats

4. Constraints and Different Strategies for overcoming Toxicity (howler monkeys, pocket mice, and macaws)

a. Not all food items are safe; animals often encounter toxicity in the foods they consume

b. How might animals overcome plant toxins:

1. ________________________

 

2. Alter plant parts to make them more edible

 

3.____________________________

c. Why do howler monkeys eat only on rarer smaller leaves rather than abundant larger leaves?

 

 

d. How does the pocket mouse overcome toxicity? (video clip)

 

 

 

e. What strategy do macaws use to overcome toxicity? (video clip)

5. Constraints: Nutritional Needs

What is the example?

D. Social Factors and their infuence on Foraging Behavior 

Up to this point, we have examined foraging behavior in individual animals, but it should not surprise you that there can be enormous social influences on animals that live in social groups (think of the work with honey bees and also rats learning food preferences from smelling the breath of conspecifics). Here we question what other kinds of information may be gained from others.

1. Success of Foraging: If animals live in groups, can they gain information about the success of foraging from other animals?

2. Information Center Hypothesis

a. This hypothesis asks whether some animals can use other animals to determine a location of food?

b. well established in insects (e.g., honeybees, ants)

c. What about social nesting birds, do they monitor other birds in the colony?

1. Prediction 1 - Foraging birds should leave nesting colonies together and fly off in the same general direction.

Does this happen in barn swallows__________________?

 

Does this happen in black headed gulls________________?

 

2. Prediction 2 - If some birds are leaders and other followers, then followers should be unsuccessful birds that are now following previously successful birds

 

Does this happen for barn swallows________________________?

 

Does this happen for black headed gulls_____________________?

Although this prediction is not confirmed for either, what happens in ospreys?

 

 

 

 

 

3. Watching the foraging activities of others (what do birds learn?)

 a. Setup

1. This study by Templeton asked whether starlings could make foraging decisions after observing other starlings and if so what kinds of information were they most sensitive to.

2. Thus, there were demonstrators and observers.

3. Demonstrators were initially trained to remove white lids with and without small black stickers to uncover a food reward. During testing, the demonstrator birds were exposed to different colored lids, one of which contained a black sticker. Thus, demonstrator birds reliably removed the lid containing the black sticker.

4. The black sticker was not visible to the observer bird, nor could the observer bird see the food in the food well.

5. The only information available to the observer bird was the color of the lids and whether the demonstrator bird ate something.

6. The experiment was set up to provide different kinds of information to the observer bird.

NEGATIVE INFORMATION - The demonstrator bird always chose the incorrect lid and the observer had to choose the opposite lid in order to receive a reward

POSITIVE INFORMATION - the demonstrator bird always chose the correct lid and the observer had to choose the same lid in order to receive a reward

MIXED INFORMATION - The demonstrator bird chose the correct lid 50% of the time; the observer had to select the same lid when the demonstrator was correct and the opposite lid when the demonstrator was incorrect.

Did the observer birds learn from the demonstrators?

 

Did the kind of information matter?

 

 

How do you explain the results?

 

 

E. Marginal Value Theorem (finding/staying problem)

1. Many species forage in areas that are called ____________ because food is generally not uniformly distributed in the environment

2. Thus, animals must make decisions about when, where, and how long to forage in a particular _______ before moving on to the next one

3. What is the marginal value theorem?

4. What does gut represent?

LEARNING GOALS (Foraging Behavior)

In addition to knowing the factual information presented on the general topic of Foraging, you should also:

1) understand the Wenner/Von Frisch controversy and be able to explain how it is that "both men were right."

2) know which animals appear to follow a simple energy model of optimal foraging and those that do not

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