Question 165·Hard·Command of Evidence
Feeding Preferences of Merriam's Kangaroo Rat for Seeds of Five Desert Plants
| Plant species (common name) | Percentage of offered seeds eaten | Contains capsaicinoid | Seed-coat thickness (µm) |
|---|---|---|---|
| Fireberry | 22% | yes | 80 |
| River pepper | 18% | yes | 65 |
| Golden oat | 62% | no | 40 |
| Desert flax | 12% | no | 90 |
| Prairie millet | 70% | no | 35 |
Biologists investigating the foraging behavior of Merriam's kangaroo rats proposed that the rodents avoid seeds containing capsaicinoid, a bitter-tasting compound found in some desert plants. A student reviewing the data, however, argues that capsaicinoid alone cannot fully account for the rats’ seed preferences.
Which choice best describes data from the table that support the student’s argument?
For “command of evidence” questions with tables or graphs, first restate the claim you need to support or challenge, then decide what pattern in the data would logically do that (for example, a mismatch between prediction and results). Next, scan the table for numbers or categories that show that pattern, especially by grouping rows with the same key variable (here, capsaicinoid: yes/no). Finally, test each answer: eliminate choices that merely restate or strengthen the original hypothesis, and choose the one that clearly shows the hypothesis does not fully match the data.
Hints
Restate what each side is claiming
The scientists think capsaicinoid explains why the rats avoid some seeds. The student says capsaicinoid by itself does not explain all of the rats’ choices. What would data showing that look like?
Look for patterns that don’t match capsaicinoid content
Scan the table for seed types that have the same capsaicinoid status (both yes or both no) but different percentages eaten, or seed types with different capsaicinoid status but similar percentages eaten.
Pay attention to the non-capsaicinoid seeds
Among Golden oat, Desert flax, and Prairie millet (all “no” for capsaicinoid), compare the percentages eaten. Do the rats treat these seeds similarly or differently?
Ask which option truly challenges the original hypothesis
Eliminate any answer that fits nicely with the idea that rats simply avoid capsaicinoid. The correct answer should show that something else must also be influencing their seed choices.
Step-by-step Explanation
Clarify the scientist’s claim and the student’s counterclaim
The biologists’ idea: Merriam’s kangaroo rats avoid seeds that contain capsaicinoid, so capsaicinoid explains which seeds they prefer.
The student’s argument: capsaicinoid alone cannot fully explain the rats’ preferences. That means some pattern in the data doesn’t line up perfectly with capsaicinoid content, implying other factors are influencing eating behavior.
Identify what evidence would support the student
To support the student, look for a mismatch between capsaicinoid status and consumption. For example:
- Seeds without capsaicinoid that are still eaten at very low rates, or
- Seeds with capsaicinoid that are eaten at high rates, or
- Large differences in consumption within a group that shares the same capsaicinoid status (all yes or all no).
Any of these would suggest capsaicinoid by itself doesn’t fully account for the preferences.
Check whether the non-capsaicinoid seeds are treated similarly
Focus on seeds where capsaicinoid = no:
- Golden oat: 62% eaten
- Desert flax: 12% eaten
- Prairie millet: 70% eaten
If capsaicinoid were the only driver, these seeds (all lacking capsaicinoid) would be expected to be treated more similarly. Instead, the rats eat some of these seeds a lot and one of them very little.
Connect the mismatch to the student’s claim
Because there is a large gap in the percentages eaten among seeds that all lack capsaicinoid (for instance, one “no-capsaicinoid” seed is eaten at a very low rate while another is eaten at a much higher rate), the rats’ preferences cannot be explained by capsaicinoid alone. This is exactly the kind of evidence that supports the student’s argument.
Select the answer choice that states this evidence most directly
The choice that explicitly points out a much lower consumption rate for one seed than another even though neither contains capsaicinoid is:
The rats consumed a much lower percentage of Desert flax seeds than of Golden oat seeds, even though neither seed type contains capsaicinoid.