LR Question 23 Explanation, by LSATHacks
QUESTION TEXT: Zoologist: Plants preferentially absorb heavy nitrogen from…
QUESTION TYPE: Strengthen
CONCLUSION: Prehistoric European cave bears weren’t pure herbivores.
REASONING:
- Rain ➞ plants absorb nitrogen ➞ herbivores eat plants, get nitrogen ➞ carnivores eat herbivores, get even more nitrogen
- A study compared: bone samples from ancient European cave bears vs. blood samples from modern bears that were fed a lot of meat.
- The heavy nitrogen levels in the bone and blood samples were the same.
ANALYSIS: This is a dense stimulus. I’ve tried to summarize the key facts. It’s important to note that:
- We’re comparing bone from dead animals to blood from live animals.
- The heavy nitrogen levels in bone and blood were the same.
I missed the second point when I did this, and got this question wrong as a result.
By the way, the point of comparing the dead bears to modern bears which ate meat was to establish a baseline. Modern bears aren’t full herbivores, so if the blood samples match then ancient bears likely weren’t herbivores either.
___________
- It doesn’t matter if plants have other sources of heavy nitrogen. All that matters is that they do get a lot of nitrogen from rain.
- This is a trap answer. It sounds like it’s talking about relevant things, but it has a slightly shifted emphasis, and therefore talks about the wrong things.
1. We don’t care how fast herbivores got their blood nitrogen. We only care about the absolute level of nitrogen they have when other animals eat them.
2. It doesn’t matter what herbivore bone tells us about herbivore blood. The stimulus is making a claim about animals that eat meat. And it’s possible that bone/blood nitrogen levels are hard to assess in herbivores, but easier to assess in meat-eaters. - It doesn’t matter exactly how many samples were taken. We only care whether enough samples were taken, and whether those samples were accurate.
- CORRECT. The stimulus compared bone to blood. It’s not obvious that bone and blood levels of nitrogen would normally be equal.
This answer shows that in modern bears we do expect bone and blood levels to be equal. Therefore, the fact that old bear bone has equal levels of nitrogen as new bear blood becomes more significant. - This is tempting. It certainly shows that a meat enriched diet is the same as….eating meat. But, how does that help? The main problem with the stimulus is that we don’t know if it’s important that bone levels of nitrogen and blood levels of nitrogen are identical.
This answer doesn’t address that problem. It just tells us that a meat enriched diet produces the same effect as eating meat. Which, come to think of it, isn’t that surprising?
(I will admit, it’s a bit odd that a modern meat enriched diet exactly matches the nitrogen levels the bears got from their diets back then. But it’s not the central issue. The blood-bone disparity is what’s central)
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To be honest I found (B) a bit more tempting than (E).
Anyhow, the way how I eliminated (E) was for two reasons:
1.) We don’t know the heavy N-level in the bones of the modern meat-eating bear. Some would say “but we know the heavy N-level in the bones of the Ice-Age bear, therefore we can infer the heavy N-level in the bones of the modern bear as well, since they’re the same, according to the answer choice.”
But, (un)fortunately, WE CAN’T! The above reasoning some people would make is based on the hidden assumption that the Ice-Age bears are meat-eating as well. We can’t assume this though, since this is exactly the conclusion of the argument we want to strengthen! We would basically assume the conclusion we want to prove!
Since we don’t know for sure that the argument is valid (that’s why the stem asks us to strengthen it), it would be naive to assume the conclusion of it to be correct (=the Ice-Age bears were indeed not exclusively herbivorous) in order to choose answer (E).
2.) Who says that the level of heavy-N in the bones of meat eating bears is not also exactly the same to exclusively herbivorous bears? If so, this fact would be not very useful.