#5 - During its caterpillar stage, the leopard magpie moth

[Amrita22]

Hello Powerscore team, thank you for this explanation. I think I may be overthinking this question, so any insight you can provide would be appreciated. First, my understanding of a necessary assumption was for the argument to be valid, the assumption HAS to be true. (Argument = Valid) (Assumption = True). But, I don't see how A HAS to be true. For example, let's just say Natal Grass Cycad is not the only means by which the leopard magpie can make itself unpalatable, it's just the easiest. Perhaps a different type of leaf also has the same effect, but it requires climbing a tree that often tires the caterpillar. Then, the leopard magpie can still be in danger of extinction. I get that it's not black and white. Answer A, if negated, still weakens the logical force of the argument. But doesn't B do that too, if negated?

[Beatrice Brown]

Hi Amrita! Thanks so much for this question.

Your understanding of an assumption is correct: an assumption is an unstated premise, and the Assumption Negation Technique can be used to determine whether an answer choice is actually necessary for the conclusion to be true.

Answer choice (B) is not necessary for the conclusion to be true. Be careful when negating this answer choice! The negation of answer choice (B) would read: the leopard magpie moth has the speed or agility to escape from some of its potential predators. This would still allow the conclusion to be true because maybe one of its potential predators would still be able to catch the moth and cause its extinction. Another way to think of this is how the author of the stimulus would respond if you offered the negation of answer choice (B) as a rebuttal to their argument. The author could reply to you that sure, the leopard magpie moth may still be able to outrunsome of its predators, but the major predator would still be able to catch it so without having the toxin, the moth is in danger of extinction from this major predator.

Answer choice (A), however, is necessary for the conclusion to be true. If we negate answer choice (A), we get the following: Feeding on the Natal grass cycad is not the only means by which the moth can make itself highly unpalatable to predators. If there are other ways for the moth to become unpalatable to predators, then they won't face extinction, weakening the force of the conclusion.

Here's another way to think of answer choice (A): Remember that the evidence that the author relies upon here is that the Natal grass cycad is facing extinction and that the moth needs the toxin from this grass to become unpalatable to predators. But there's also a gap in the author's reasoning: what if there are other ways for the moth to become unpalatable to predators, such that it may not face extinction? If we add answer choice (A) to the argument, this defends the argument, making answer choice (A) a Defender Assumption.

[alistotle]

Isn't a more fundamental assumption that the argument is making that the moth being unpalatable makes it less likely to be eaten by predators. The argument simply assumes that unpalatable = not a food source. But maybe predators eat it anyway, because food is food? In that case A does not seem to be correct, because even if there was a myriad of other plants that could make the caterpillar unpalatable, it would still face extinction because the predators would not care.

[Luke Haqq]

Hi alistotle!

You raise a fair point in thinking about the meaning unpalatable. Just because something isn't pleasant to taste doesn't on its own imply that predators will not under any circumstances eat it. In the end, though, we don't know one way or another whether the argument "simply assumes that unpalatable = not a food source," even if that seems likely.

We are given the following conditional reasoning in the stimulus:

NGC Toxin

We might more fully describe this conditional reasoning as:

NGC Toxin Deters predators

Because "the Natal grass cycad is now endangered and facing extinction," the author of the stimulus concludes that "the leopard magpie moth is also in danger of extinction." In other words, the author assumes:

NGC Deter predators

It's not clear why the elimination of the grass means a risk of extinction, however, unless the author is assuming that the grass is the only means by which the magpie develops the mentioned toxin.

To test answer choice (A), we can apply the Assumption Negation technique, negating the answer choice and plugging it back into the argument to see if the argument falls apart. Answer choice (A) states, "Feeding on the Natal grass cycad is the only means by which the leopard magpie moth can make itself highly unpalatable to predators." Negating this would result in: "Feeding on the Natal grass cycad is [not] the only means by which the leopard magpie moth can make itself highly unpalatable to predators." If this were true, then there is no necessary reason why the magpie would face extinction because of the absence of the Natal grass cycad. Thus this negated answer choice would make the argument fall apart, confirming that it is an assumption on which the argument depends.

[mtdaniel]

Although one of the stated premises is that "toxin makes magpie unpalatable to predators", the argument must then make the assumption that "being unpalatable is the only way to defend against predators", correct?

If this is the case, then isn't (B) also a required assumption? That is, saying that "being unpalatable is the only way to defend itself" compatible with "does not have speed or agility to defend itself"?

I understand that (A) is a better choice based on the assumption negation test, but maybe I confused myself by assuming "being unpalatable is the only defense".

[Jeff Wren]

Hi mtdaniel,

There have been several explanations above on Answer B, so I will only add one point that may not have been clear.

The argument begins "during its caterpillar stage, the leopard magpie moth feeds on a plant...." The implication is that the moth only needs to be unpalatable during this stage, while it is presumably vulnerable. Not to stereotype caterpillars, but they generally aren't super fast. Later, once the moth is actually a moth that can fly around, it may very well have the speed and agility to avoid predators.

The problem is that it won't make it to adulthood if it's eaten while still a caterpillar. By not specifying which stage they are talking about, Answer B seems to be implying adult moths. It would be like if I made a general statement that "cheetahs are very fast," but technically newborn cheetahs aren't very fast yet.