After a really fun time with dengue fever in Venezuela and Brazil... well, so I'd happily rub millipedes all over myself if I thought it would help get rid of the little turds. I wish that capuchin monkeys had taught me this trick back in the day.

Anyway, let's start with the conclusion, which is that "monkeys probably rub millipedes into their fur only because doing so helps protect them against mosquitoes." How does the author arrive at this conclusion?

• We know that capuchin monkeys often rub a certain type of millipede into their fur.
• We also know that this rubbing behavior is rare except during the rainy season. So even though they do this OFTEN, it is uncommon outside of rainy season.
• Rainy season is also when mosquito populations are at their peak.
• The millipedes secrete two chemicals that are potent mosquito repellents.
• Mosquitoes carry parasites that debilitate the monkeys. So the monkeys would have an incentive to use the mosquito repellent, if possible.

The author reasons that since the monkeys only rub the millipedes into their fur during the rainy season when mosquitoes are at their worst, it is likely that the monkeys are only doing this for mosquito protection. If there were OTHER reasons for rubbing the millipedes into their furs, then the monkeys would probably do it all year round.

For example, if the millipede secretions were also good for the monkeys' skin and fur, then the rubbing behavior would probably continue regardless of whether mosquito populations were high or low. But, according to the author, since the monkeys only do this during peak mosquito season, the mosquitoes are probably the only motivation for the rubbing.

Which of the following choices would help us evaluate the author's argument and conclusion? (And if you prefer your explanations in video form, check out this YouTube CR webinar.)


How the millipedes use those chemicals is irrelevant. We only care to explain why the MONKEYS might have an interest in those chemicals. Why do they rub the millipedes and why do they only do so during the rainy season? Choice (A) does not help us answer either of those questions, so it can be eliminated.


We are only concerned with explaining the rubbing behavior of the capuchin monkeys in Venezuela. Regardless of whether the millipedes are found in other parts of the world, why do these monkeys only exhibit the rubbing behavior during the rainy season? Choice (B) is irrelevant and can be eliminated.


Again, we are trying to explain why the capuchins only rub mosquitoes into their furs during the rainy season. The rubbing behavior of other animals is irrelevant, so (C) can also be eliminated.


We know that the rubbing is rare outside of rainy season. The author explains this by suggesting that the rubbing is not needed when the mosquitoes aren't so bad. But what if there is another explanation? What if the monkeys would LIKE to rub the millipedes into their furs outside of peak mosquito season but simply cannot do so because the millipedes are not readily available? If that were the case, it would suggest that the monkeys have OTHER reasons for wanting to continue the rubbing behavior. Wanting to repel mosquitoes during the rainy season would probably not be the ONLY reason. This would contradict the author's argument and conclusion. Thus, choice (D) would be useful to determine in order to evaluate the argument.


We are told that the rubbing behavior is rare outside of rainy season, and we can infer that the monkeys often rub the millipedes into their fur during the rainy season. Sure, they might be doing the same thing with some other insects, but why is the rubbing behavior rare outside of rainy season? The author provides one explanation, and choice (E) does not provide an alternative. Choice (E) has no impact on the author's argument and can be eliminated.

Choice (D) is the best answer.