FORAGING FOR RESOURCES

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.

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.

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)

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?

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.

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.

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).

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:

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.

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?

WHERE TO EAT CONT’D In 1979 Eric Charnov developed the Marginal Value

Theorem. 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?

MARGINAL VALUE THEOREM

8 5 5 2

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.

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.

# of resources available

# of

reso

urce

s ta

ken

ultim

ate

gene

ralist

ultimate specialist

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

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

Always exploit the same narrow range of resources

Always exploit the same wide range of resources

Exploit only a few items, but these change over time (seasonally)

Exploit a wide range of resources which change over time (seasonally)

Specialist

Generalist

Stereotyped

Plastic

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”

Spring/Summer

Fall/Winter

# of Mayflies in the environment

# of

May

flies

in th

e di

et

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?

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…

MOOSE’S DIET PLANin

take

of a

quat

ic pl

ants

intake of terrestrial plants

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

Energy Constraint

Rumen Constraint.

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

The Moose’s stomach can only hold this much.

DECISION VARIABLES Any foraging animal has a bunch of decisions to make. Decision matrix…

successful?

attack?

item encountered

resume search

No

No

No Yes

Yes

Yes Recognition time

Pursuit & Kill time

Consumption time

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

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

Tim

e

Number of patch types (different kinds of food)

1 2 3 4 5 6

Time to travel to nearest patch

Time spent hunting for food once on a patch.

These lines cross closest to 4

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.

SUB-DECISIONS WITH THE PATCH MODEL Here are some sub-decisions a forager needs to think

about, within the patch model:1. What is the quality of the prey/food like?2. What is the abundance of the prey/food like?3. How long should the forager stay in the area?4. How should the forager search for the prey/food (systematically,

randomly)?5. How does the forager protect the prey/food it has already foraged?6. What competition for prey/food is there?

From its own species From other species

7. 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…)

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.

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).

oo

PATCH CHOICE OF STICKLEBACKS…

low medium

highPopulation density of Zooplankton

Atta

ck st

rikes

by

Stick

leba

cks

No Kingfisher present

Kingfisher present