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Foraging for resources. Getting food is a critical part of every animal’s existence. No animal can survive to reproductive age without knowing how to forage, so natural selection should favor efficient foraging strategies. All evidence suggests that this is in fact the case. Foraging.

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foraging

Getting food is a critical part of every animal’s existence.

No animal can survive to reproductive age without knowing how to forage, so natural selection should favor efficient foraging strategies.

All evidence suggests that this is in fact the case.

Foraging
ant foraging story

About 50,000,000 years ago (that’s 50 MILLION) ants began cultivating their own food by entering into a mutually beneficial relationship with certain species of fungi.

Ants promote the growth of the fungi by controlling the environment (temperature, moisture content, etc.), which is good for the fungi.

The Ants then eat the vegetative mycelium which are produced by the fungi (which is good for the ants).

Aside from humans, ants are one of the few groups on the planet that grow their own food.

In 1999, Cameron Currie found another piece to the puzzle, some ants associated with fungal care have a white crusty substance growing on them. This substance was found to be a mass of Streptomyces bacteria. This bacteria is known to kill bacteria that harm fungi, and it is hypothesized that the ants use it to protect their crop.

Ant foraging story
cashing food how do they remember

Many birds and mammals cash their food (hide it underground for later use) Squirrels, Foxes, Corvid birds (Crows, Magpies, Jays, and Jackdaws) are but a few common species that exhibit this behavior. Their ability to remember where thousands of bits of food are hidden (spatial memory) is amazing. How do they do it?

In 1992, Susan Healey and John Krebs studied spatial memory in 7 species of Corvid bird. 2 of these species did not cash food at all. 4 species did some food cashing and 1 species (the European Jay) fed almost exclusively on cashed food, and had to remember the location of between 6,000 and 11,000 seeds for up to 9 months.

What Healey and Krebs found was a strong correlation between the volume of the bird’s hypocampus (a small region of the hind brain) and the ability to remember where food stores were hidden (the larger the hypocampal volume, the more cashes that could be remembered)

Cashing food (How do they remember?)
questions about foraging

So, what questions do we need to ask about foraging:

    • How does foraging theory predict where and what animals eat?
    • What role does learning play in foraging decisions?
    • How does group social dynamics affect foraging?
    • How does the organism’s anatomy and physiology affect it’s foraging behavior?
Questions about Foraging
optimal foraging theory oft

Optimal Foraging Theory models are models that answer the following questions:

      • What food items should a forager eat?
      • How long should a forager stay in a certain food patch?
      • How is foraging affected when certain nutrient requirements are in place?
      • How does variance in food supply affect a forager’s decision about what food types to eat.
Optimal foraging theory (oft)
what to eat where to eat it

One of the most basic problems of foragers is what items to include in a diet and what to exclude.

Let’s suppose an animal can potentially forage for and consume food types A, B, and C. Should the forager eat all three? Only type B? Perhaps two out of the three, but if so, which subset (A&B, B&C, A&C).

Scientists have developed mathematical optimality models to predict the ESS.

What to eat & where to eat it
basic oft what to eat

Let’s consider the simplest possible case; choosing between two food types.

It does not matter what the food type is (prey, seed, etc.) each food type will have a caloric value (how much energy the forager will get from eating it), an encounter rate (how often the forager will find it), and a handling time (how long it takes the forager to acquire it) associated with it.

As an example: one prey type may be encountered every three minutes (encounter rate). It may take the forager 2 minutes to kill and eat it (handling time), and it may get 300 calories from ingesting it (energy).

Basic OFT (What to eat)
more basic oft

Let’s define the profitability of the food item as Energy divided by Handling Time (E/HT).

The greater the E/HT ratio, the greater the profitability of going after that item.

If we assume that the prey item with the highest profitability is always taken (we’ll call it item A), then the question is should item B ever be taken, and if so when?

A little bit of higher order math shows the following surprising result:

More basic OFT
simplest case results

What is surprising is that the math shows, it is not the availability of food item B that matters, it is the availability of food item A that determines whether it’s an ESS to take both items or just item A.

If item A is encountered often enough, then it doesn’t matter how many item B’s are available, the forager should only take item A, but if the availability of item A falls below a certain threshold, then the forager should make item A and item B part of its diet.

Simplest case results…
basic oft where to eat

Another critical decision a forager needs to make is how long to stay in a patch of food.

  • For example:
    • How long should a hummingbird stay sucking nectar from one flower, given that there are other flowers available?
    • How long should a bee spend extracting pollen from one flower before it moves on to the next?
Basic OFT (Where to eat)
where to eat cont d

In 1979 Eric Charnov developed the MarginalValueTheorem.

Imagine a forager is feeding in an area with different patches of food.

As the forager feeds, it is depleting the patch of food it is currently on, causing the rate of consumption to slow down.

Other less depleted patches become more attractive.

In order to get to a new patch, the forager must pay some cost (lost time foraging, increased predation pressure, etc.)

So the question becomes, how long should the forager stay in the patch it’s depleting before moving on?

Where to eat cont’d
marginal value theorem
Marginal Value Theorem

The Marginal Value Theorem shows us 3 things:

1. A forager should only stay in an area until its ability to successfully forage is the same as the average ability to successfully forage in any local patch

Here are 4 patches and their relative forage success rates. So the first patch is best, the last is worst.

8

5

5

2

The average success rate for these 4 patches will be 8+5+5+2 = 20 / 4 or 5. So a forager in the first patch should only stay until the success rate hits 5, etc.

2. The greater the distance between patches the longer the forager should stay in one.

3. Foragers should stay longer in patches that are poor quality when they initially arrive.

specialists vs generalists

Generalists will take items in proportion to their availability (They will usually eat the most abundant food source in an area).

Specialists will take a small portion of one or a few types of food items.

Specialists vs. Generalists

ultimate generalist

# of resources taken

ultimate specialist

# of resources available

specialists

Extreme specialists will eat very few things, or only one thing.

Specialists

Specialist

Generalist

Stereotyped

Plastic

  • They are classified by the time and energy they invest in foraging.

Time Maximizers: Maximize their energy intake per unit of time (worry about calories per minute). These are typically the high metabolic rate organisms (hummingbirds, shrews, etc.).

Energy Maximizers: These organisms acquire as much energy as possible with out a time constraint. These are typically the grazing animals. {For example: Wild Elephants feed for 20 hours a day}

preference of food and switching
Preference of food and “switching”

The availability of Mayflies changes seasonally.

“Switching” is showing a preference for food items that are in season.

Look at this graph (of Trout stomach contents). What does it show? Does this surprise you?

Spring/Summer

# of Mayflies in the diet

Fall/Winter

# of Mayflies in the environment

There is such a thing as a Balanced Preference.

A Balanced Preference is showing a preference (at least in part) for food items that take care of a dietary constraint. For example every human society has a dish that contains some kind of legume (peas, beans, etc.). This is because humans have a dietary need for an amino acid that can only be obtained through legume consumption. So humans show a balanced preference for legumes.

the moose salt budget balanced preference

Terrestrial plants have more calories then aquatic plants, so Moose should just eat terrestrial plants.

All grazing animals have a sodium constraint (need salt), and aquatic plants have more salt than terrestrial plants, so the Moose must eat, at least some, aquatic plants.

A Moose is a big animal, so it requires a lot of energy.

Also a Moose’s rumen (stomach) is only so big, so it can only hold so much food.

So, what’s a Moose to do…

The moose salt budget (balanced preference)
moose s diet plan
Moose’s diet plan

Energy Constraint

This triangle formed between the three lines represents where the Moose must feed (how many aquatic vs. terrestrial plants the Moose must eat).

intake of aquatic plants

Rumen Constraint.

The Moose’s stomach can only hold this much.

Sodium constraint. The Moose must get at least this much sodium.

intake of terrestrial plants

decision variables

Any foraging animal has a bunch of decisions to make.

Decision matrix…

Decision Variables

resume search

Consumption time

No

Yes

successful?

Pursuit & Kill time

No

Yes

attack?

Recognition time

No

Yes

item encountered

Start Here

back to seach handling times

For all animals the profitability of a food is roughly equal to the Energy content of the food / search time + handling time

  • For Generalists the search time tends to be short (they eat just about anything so there is generally something close by to eat), but the handling time tends to be longer (they are not expert at handling any one type of food).
  • So, the ultimate goal is to maximize the average rate of energy consumption.
    • For generalists, many foods do that
    • For specialists, few foods do that
Back to seach & handling times
where to forage

McArthur and Pianca came up with the Patch Model in 1966.

Where to forage

Time to travel to nearest patch

These lines cross closest to 4

Time spent hunting for food once on a patch.

Time

Since the lines cross close to 4, optimally this species should feed at 4 different patch types (4 different types of food). That’s the ESS for this species.

5

1

4

2

3

6

Number of patch types (different kinds of food)

sub decisions with the patch model

Here are some sub-decisions a forager needs to think about, within the patch model:

    • What is the quality of the prey/food like?
    • What is the abundance of the prey/food like?
    • How long should the forager stay in the area?
    • How should the forager search for the prey/food (systematically, randomly)?
    • How does the forager protect the prey/food it has already foraged?
    • What competition for prey/food is there?
      • From its own species
      • From other species
    • What are the characteristics of the prey/food (when a predator enters a patch the prey density may be high, but may rapidly change to low as the prey items hide…)
Sub-decisions with the patch model
when is it time to leave a patch

Rules of thumb (gut rules the animal just knows, evolutionarily):

    • When the times between capture become too long.
    • If the forager is full
    • If the time in the patch has exceeded some set time
    • If the number of unsuccessful attempts at capture gets too high
    • If the number of prey items gets too small
    • If the quality of the prey is not very good (generally represented by the size of the prey)

This is all of course on top of things like how far away is the next patch, how long will it take me to get there, will I get eaten on the way there, etc.

When is it time to leave a patch?
what can behavior tell us about patch choice

So birds (at least sometimes) visit the poorest patches to check on conditions (they don’t spend all their time at the best patches).

Let’s look at a system involving zooplankton (producer), Three-Spine Stickleback (fish/1 consumer), and the Belted Kingfisher (bird/2 consumer).

What can behavior tell us about patch choice?

o

o

patch choice of sticklebacks
Patch choice of sticklebacks…

No Kingfisher present

Kingfisher present

Attack strikes by Sticklebacks

low

medium

high

Population density of Zooplankton

patch choice

Probably the biggest issue affecting patch choice is the organism’s willingness to take risks (like risking starving, or risking being eaten).

  • For risk prone organisms essentially there are two alternatives:
    • If the next patch is far away, or in poor condition, then you will have to stay.
    • If the next patch is close or in good condition, you will likely take the risk, and move on.
Patch Choice
more on patch choice

Food Characteristics may influence the choice of patch as well:

    • Food density
    • Whether food resource is renewable, if so, how frequently.
    • Availability
  • If renewability is quick enough and on a scale which will support the organism, it’s likely to spend a longer time at that patch.
More on patch choice
patch search patterns

When an organism enters a patch, How does it search?

    • Random pattern (lots of turns, takes time)
    • Systematic pattern (most predators do this)
Patch Search patterns
encounters

Prey that is sedentary and conspicuous

Encounters

Indicator of time when prey is unavailable. Determines the length of stay in the patch

Conspicuous, social prey

encounters cont d

Cryptic Prey

Fast Moving Conspicuous prey

Encounters cont’d

This type of prey becomes unavailable quickly, but also becomes available quickly because it has a high metabolic rate and must feed itself.

foraging tactics

Central Place Foraging

    • Predator collects food and brings it back to the same spot all the time to consume.
      • Squirrels do this.
      • Any parent feeding young does this.
      • Humans tend to do this
    • If food is brought back to be eaten, then travel time must be considered.
      • 1. travel time (eats into profitablility)
      • 2. since you are not eating food, you are incurring costs without an immediate payoff
      • So preferable patch is close with lots of good quality food.
Foraging tactics
more on foraging tactics

3. How much food should the organism bring back?

    • The farther the patch, the more items they should bring back…
    • For European Starlings, the farther they have to travel, the more they bring back.
  • Now suppose the organism brings food back to be stored.
    • This is called front end loading (incur foraging costs up front)
    • The benefits though, are back end loaded.
  • Squirrels, need to decide whether to eat or store a seed.
    • 4 properties of acorns
      • 1. perishability 2. handling time
      • 3. fat content 4. tanin (acid) content
  • Pin Oak vs. White Oak.
    • They tend to store PO and eat WO because PO is less perishable than WO.
    • Tend to store nuts with more fat content and more tanins
More on foraging tactics
other costs of storing food

Pilfering (influenced by different storage patterns.

    • Larder Cache
      • This puts all stored resources in one place
      • Easy to remember where it is
      • If pilfered, risk loosing it all
    • Scatter Hoarding
      • This puts resources in many different places
      • Expend LOTS of energy to hoard
      • May have so many site organisms won’t/can’t defend them all.
      • Harder to remember where an individual cache is.
Other costs of storing food
clark s nutcracker

Discovered by Clark of Lewis & Clark fame.

Eats pine seeds and is the SUPREME scatter hoarder

Store them over winter for themselves and their young

Store 6-8 seed in as many as 25,000 cache sites (per bird)

Use visual landmarks to remember where the cache sites are.

Clark’s Nutcracker

25

11 days after storage

20

% of correct hits

15

285 days after storage

10

5

0

2

3

1

informed patch choice

Sometimes individuals rely on other organisms to give them information on patch availability.

  • There are two methods for this:
    • Social Facilitation:
      • When individuals follow other individuals (that have been successful foragers) to a patch.
      • If this happens the individuals in question are usually member of the same population, and the population is considered an “information center” in terms of foraging.
    • Local Enhancement:
      • Finding individuals who are already feeding and mimicking their behavior
      • This occurs by chance, and the individuals involved are not necessarily in the same population, or even the same species.
Informed Patch Choice
turkey vulture vs black vulture
Turkey Vulture vs. Black Vulture

Turkey Vulture

Black Vulture

turkey vulture vs black vulture cont d

Both the Turkey and Black Vultures are seen in the Eastern United States and both are endemic to Hunterdon County.

They somewhat compete for resources

Turkey Vultures have been proven to have a sense of smell, and usually feed alone or in small groups.

Black Vultures have no sense of smell, and feed in large groups.

Black Vultures use Turkey Vultures to find out where to feed.

Black Vultures don’t follow Turkey Vultures, but will cue in on Turkey Vultures that are already feeding.

So what kind of foraging strategy is this an example of?

Turkey Vulture vs. Black Vulture cont’d

Local Enhancement