What is the Marginal Value Theorem and how does it apply to foraging behavior?

The Marginal Value Theorem predicts that foragers should leave a patch when the capture rate in that patch drops below the average capture rate across all patches. In this experiment, foragers test MVT predictions by comparing their prey capture rates and giving-up times across buckets with different prey densities, revealing whether their foraging decisions match theoretical predictions.

How do you calculate capture rate in a foraging experiment?

Capture rate is calculated by dividing the number of prey captured by the total time spent foraging plus travel time in each patch. This metric reveals foraging efficiency and helps determine whether foragers spend more time in high-density patches where capture rates are higher.

What does giving-up time measure in foraging studies?

Giving-up time is the interval between when a forager acquires the last prey item and when they leave a patch. It indicates how long a forager continues searching after finding prey becomes difficult, reflecting their decision to abandon a depleting resource patch.

Why is scaled giving-up time used instead of raw giving-up time?

Scaled giving-up time normalizes individual differences by dividing each bucket's giving-up time by the group's mean giving-up time. This standardization allows class data to be pooled without individual forager variation skewing results, enabling meaningful comparison across different foraging groups and experimental conditions.

What hypotheses are tested in the optimal foraging simulation?

The experimental hypothesis predicts foragers will catch the most prey in high-density buckets and spend the most time there. The null hypothesis states foragers will capture equal prey numbers and spend equal time in all buckets regardless of prey density.

How does prey density influence forager patch selection?

Foragers should preferentially select and remain longer in patches with higher prey density, where they encounter prey more frequently and achieve higher capture rates. This behavior reflects optimal foraging strategy, as staying in productive patches maximizes energy gain relative to time invested.

What data should be recorded during the foraging simulation?

Recorders must document the time each prey item is found, arrival and departure times for each bucket, and total foraging and travel times. This detailed timing data enables calculation of capture rates and giving-up times, which are then analyzed to test whether forager behavior aligns with predictions.

Optimal Foraging

Concepts Instructor Prep Student Protocol

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Often in life, the things an individual needs may not be within immediate grasp. So, organisms need to forage. Foraging is defined simply as the process of searching for food or more generally resources. Although food is probably the first example that comes to mind, organisms must forage for other resources like shelter or mates and animals often go to great lengths to get what they need. However, the act of foraging itself requires energy. Essentially, individuals must balance burning calories with obtaining them. To put it simply, foraging requires a strategy. Ecologists created the optimal foraging theory to model the circumstances in which an organism meets the optimum balance between energy expended foraging, i.e., two acorn's worth and the potential energy it can acquire like six new acorns.

One characteristic of foraging is that resources are not always spread out evenly within a habitat. More typically, resources are contained in patches that foragers must move between. So how exactly should foragers navigate these patches of resources? In 1976, evolutionary ecologist Eric L. Charnov came up with the marginal value theorem or MVT to explain how organisms optimize their foraging. The MVT has five main predictions for optimal foraging. First, it states that foragers should capture more prey in patches with high prey density than in low prey density patches. Second, the theory predicts that foragers should spend more time hunting for resources in high prey density patches. Next, the forager should have a higher prey capture rate in dense environments with patches that are closely packed together than in sparse environments with patches that are spread further apart. The fourth prediction states that foragers will spend more time foraging in these dense environments than in sparse environments. Finally, a forager should leave a patch when the capture rate has declined to the average rate of all patches. And this is where a concept called giving up time or GUT comes in. To calculate the GUT simply subtract the time that the final prey item was acquired from the time that the individual leaves the patch. Therefore, the GUT should be the same for all of the patches a forager visits.

In this laboratory, you will complete foraging simulations in several different artificial resources patches and then perform calculations to gauge how your class performed as foragers and whether you adhered to the rules of the marginal value theorem.

Print Procedure Steps

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Simulating Foraging with Artificial Habitats and Prey
- NOTE: The foragers will hunt for prey represented by pinto beans in four buckets of rice with varying prey densities. Without knowing what these densities are, foragers must obtain as many prey items as possible in as little time as possible.
- HYPOTHESES: In this experiment, the experimental hypothesis could be that foragers will catch the most prey in the higher prey density bucket and also spend the most time foraging there. The null hypothesis would then be that foragers will catch an equal number of prey in each bucket and that foragers will spend an equal amount of time foraging in each patch.
- To begin the foraging activity, record the names and role of each participant at the top of the student data collection sheet. NOTE: Before starting the experiment, the forager is allowed to become familiar with the foraging process – the following steps are a practice round.
- The forager should kneel or sit in front of the average density bucket.
- Once the timer says start, the forager may begin looking for the pinto beans representing the prey. The forager can spend as much time as they like foraging in this bucket.
- Once a prey item is located, the forager must signal that one has been found and place the prey in the cup next to the bucket.
- Once they believe they have acquired most of the prey in the patch, they should tell the timer to stop the time and the practice round is over.
- To begin the simulation proper, the forager should sit or kneel in front of bucket A.
- The timer must be ready to start the stopwatch.
- The recorder should be ready to record the time each prey item is located, in addition to the time that the forager arrived at and left each bucket.
- When all parties are ready, the timer should start the stopwatch and say “start”, and the forager should begin foraging. NOTE: The stopwatch will run through the rest of the round for buckets B and C.
- As the forager signals finding pinto beans and places them in the cup next to the bucket, the timer should tell the recorder the time.
- The recorder should then write the time in the student data collection sheet. Click Here to download Table 1
- Once the forager has decided to move on to the next bucket, they should tell the timer “done”, and begin moving to the next patch.
- The recorder should record the time the forager left and the time they arrived at bucket B in the student data collection sheet.
- Repeat the same procedure for buckets B and C. Once the forager has completed the activity, it is time for the next forager to begin using the same procedure.
Results
- Before performing any data analysis, review the data collection sheet and compile your individual data for captured prey, foraging time, and travel time for each bucket in your student calculations table. Click Here to download Table 2
- After this, calculate your individual capture rate for each bucket by dividing the number of prey captured by the sum of the foraging time and travel time and record these values in your table. Individual capture rate = Number of prey captured / (foraging time + travel time)
- To calculate your Giving-Up-Time or a GUT for each bucket, subtract the time that the last prey item was acquired from the time you left the bucket. GUT = time forager left bucket - time last prey item acquired
- Next, calculate the mean GUT for your foraging group and record the values in your table.
- Finally, calculate your scaled GUT for each bucket by dividing the bucket GUT by your group's mean GUT. Scaled GUT = Bucket GUT / Group mean GUT NOTE: The scaled GUT will allow class data to be pooled later on without differences in individual foragers skewing the data.
- The instructor should now compile the data into the classroom spreadsheet. Click Here to download Table 3
- Plot a bar graph of the average number of prey collected in each bucket. On the x-axis, A should represent the low prey density bucket, B should represent the medium prey density bucket, and C the high prey density bucket. Plot the y-axis as the number of prey items caught.
- Next, plot the graph of the class average scaled GUT, where the x-axis represents the buckets with varying levels of prey density and the y-axis shows the values for the average scaled GUT.
- Plot a third graph of the average time spent foraging in each bucket.
- Now review the Marginal Value Theorem (MVT) from the concepts video and examine the three graphs that you generated from your class data. Take note of whether the graphs conform to the predictions of MVT.