# Foraging and Optimal Foraging Theory

Psychology I · Ethology · https://tryals.app/learn/psychology-i/foraging-and-optimal-foraging-theory

## The Economics of Eating

**Foraging** is the behavior of searching for, capturing, and consuming food. **Optimal Foraging Theory (OFT)** predicts natural selection favors foragers maximizing **net energy intake** per unit of time, balancing energetic payoff against time spent working.

A prey item's **profitability** is its energy content divided by its **handling time** (the time needed to capture and process it). Profitability is calculated as $E / h$, where $E$ is energy and $h$ is handling time. **Search time** is the time spent actively looking for the next item or patch.

| Concept | Definition | Formula / Detail |
| --- | --- | --- |
| **Profitability** | Energy yielded per unit of effort | $E / h$ ($E$ = energy, $h$ = handling time) |
| **Search time** | Effort spent finding alternatives | Increases as high-yield items deplete |

Foragers are predicted to always prefer prey with the highest profitability available in their immediate environment.

## Patch Choice and MVT

The **Marginal Value Theorem (MVT)** extends OFT to patchy environments. As a forager consumes food in a patch, the local rate of energy gain declines because the easiest items are gone and remaining ones take longer to find.

MVT predicts the forager must leave the patch exactly when its **marginal capture rate** (the rate right now) drops to equal the **environmental average** (the overall achievable rate across the habitat, factoring in travel time to fresh patches).

| Rate Type | Meaning | Action Trigger |
| --- | --- | --- |
| **Marginal capture rate** | Current declining gain within a patch | Drops as resources are depleted |
| **Environmental average** | Overall achievable rate with travel | Dictates the exact moment of departure |

> **Common pitfall:** Assuming a forager should stay in a patch until it is empty. MVT proves leaving too late wastes time that could be spent gaining food faster elsewhere.

## Practice questions

5 of this lesson's 10 practice questions, with answers. The full set is in the app.

### 1. Select every statement that is TRUE under optimal foraging theory.

A. The most profitable prey is always simply the largest, highest-energy item.
B. The MVT leave-point depends on the environment's overall average rate.
C. Foragers are predicted to maximize net energy per unit time, not total food eaten.
D. A forager should always deplete a patch fully before leaving.
E. A big prey item can be worth less than a small one if it takes long enough to subdue.

**Answer:** B. The MVT leave-point depends on the environment's overall average rate.; C. Foragers are predicted to maximize net energy per unit time, not total food eaten.; E. A big prey item can be worth less than a small one if it takes long enough to subdue.

**Why:** The profitability formula, maximizing net energy per time, and the environment-set leave-point are all true. The largest item is not always most profitable (handling time matters), and fully depleting a patch is sub-optimal - so those two are false.

Page: https://tryals.app/practice/psychology-i/foraging-and-optimal-foraging-theory/select-every-statement-that-is-true-under-optimal-foraging-theory

### 2. Patches are moved further apart so travel time between them doubles. The Marginal Value Theorem predicts the forager should now stay LONGER in each patch. Why does a more expensive journey make it worth staying in a patch that is steadily depleting?

A. Because the longer journey leaves the forager hungrier on arrival, so it eats more before it is satisfied.
B. Because patches that are further apart are generally richer, so there is more in each one to extract.
C. Because the forager maximises intake across the whole cycle.
D. Because the energy already spent travelling has to be recouped before leaving can be justified.

**Answer:** C. Because the forager maximises intake across the whole cycle.

**Why:** Leave when the patch's yield drops to the habitat-wide average, and travel time is part of that average because it yields nothing, so doubling it lowers the bar the patch must clear. Beware the sunk-cost route to the same answer: what matters is what the *next* journey costs, not what the last one did.

Page: https://tryals.app/practice/psychology-i/foraging-and-optimal-foraging-theory/patches-are-moved-further-apart-so-travel-time-between-them-doubles

### 3. Order the sequence a forager follows under the Marginal Value Theorem.

**Answer:**

1. Enter a fresh patch where the capture rate is high.
2. Take the easiest, most profitable items first.
3. As the patch depletes, the local capture rate steadily declines.
4. The marginal rate falls to the environment's average rate.
5. Leave for a new patch, paying the travel-time cost.

**Why:** On entry the rate is high; easy items go first; the rate declines with depletion; when it hits the environmental average the forager leaves. Departure is triggered by the marginal rate reaching the average, not by the patch running dry.

Page: https://tryals.app/practice/psychology-i/foraging-and-optimal-foraging-theory/order-the-sequence-a-forager-follows-under-the-marginal-value-theorem

### 4. Optimal foraging theory predicts animals maximise energy per unit time. Real animals often take a smaller, more **certain** meal over a larger, riskier one. Does that refute the theory?

A. Yes, because the theory makes no predictions about risk and so cannot be tested
B. It refines it, since a starving animal cannot average over trials
C. No, the observation is a measurement artefact of laboratory conditions
D. Yes, any deviation from the predicted optimum falsifies the model

**Answer:** B. It refines it, since a starving animal cannot average over trials

**Why:** The currency was slightly wrong, not the method. An animal near its energy threshold is maximising survival probability, and a run of bad luck is fatal in a way the long-run average never records. Risk-sensitive foraging is what the theory looks like once you ask what the animal is actually playing for.

Page: https://tryals.app/practice/psychology-i/foraging-and-optimal-foraging-theory/optimal-foraging-theory-predicts-animals-maximise-energy-per-unit

### 5. If patches move farther apart so travel time rises, the Marginal Value Theorem predicts the forager should leave each patch sooner.

**Answer:** False

**Why:** False. Longer travel *lowers* the average rate for the whole habitat, so the forager should stay in each patch **longer** before the diminishing return falls below that average. The prediction is counterintuitive and it is what makes the theorem testable, and it holds up in the field.

Page: https://tryals.app/practice/psychology-i/foraging-and-optimal-foraging-theory/if-patches-move-farther-apart-so-travel-time-rises-the-marginal