Question #6 asks how naked mole rats (NMR) differ from "all other known vertebrate groups." So, we are looking for something about naked mole rats that is completely unique to NMRs when compared to all other known vertebrate species.

Let's look first at (D):

In the third paragraph, the author states that "in naked mole rat societies, on the other hand, differences in behavior are related primarily to reproductive status... body size, and perhaps age." He/she then goes on to explain that the size of a particular NMR is a better indicator of what tasks it will complete than is its age -- the NMR's growth rate influences the time they stay in a given task, "regardless of age."

So, at best the information in (D) is only partially true for NMR's. As stated above, we also need proof that NMRs are the only known vertebrates that exhibit the behavior in question. There is simply no evidence in the passage that this is the case for (D). In the last paragraph, we learn that "the division of labor within social groups is less pronounced among other vertebrates than among naked mole rats," but the author doesn't elaborate on how other vertebrates divide their labor. They could split up tasks based on age, in which case (D) would not be unique to NMRs. For this reason, (D) is out.

Now take a look at (E):

Again, we need to prove that this is a unique attribute of NMRs among all known vertebrate groups.

In the last paragraph, the author states that "most cooperatively breeding vertebrates (except the wild dog, Lycaon pictus) are dominated by a pair of breeders rather than by a single breeding female." These cooperatively breeding species must somehow choose which female gets to breed. The passage doesn't tell us how this decision is made, so we cannot be sure that they do not fight to determine which female becomes the breeder. In addition, we have no idea what is going on with the Lycaon pictus -- maybe they fight to choose a single breeding female, or maybe they do not. Because we do not have this information, we cannot say that NMR's differ from all other vertebrate groups in this way. (E) is out.

Compare this to (A):

The evidence for this answer choice is found in the first paragraph: "Until recently, scientists did not know of a close vertebrate analogue to the extreme form of altruism observed in eusocial insects like ants and bees... However, such a vertebrate society may exist among underground colonies of the highly social rodent Heterocephalus glaber, the naked mole rat."

Here, we learn that scientists did not know of any vertebrates showing an "extreme form of altruism" until they studied NMRs. From this, we can conclude that NMRs are the only known vertebrate group to exhibit this behavior. (A) is the correct answer.

I hope that helps!

Hello Sir EducationAisle

I desperately need some help here, Sir.

Of the 7, I got 1 wrong, and the one I got wrong was probably the easiest of the lot, the first one. The passage states that an analogous vertebrate has been found, in the first paragraph only. However, in the subsequent paragraphs, the points of discussion have been purely about:

1. Reproduction aspects of the naked mole rat.
2. How they are different from other vertebrates.

The answer to the main point of the passage must point to the entire passage. However, here, that is not the case. How can the answer be C when only the first paragraph discusses the similarity. Due to obvious reasons, I chose D. I am starting to wonder if I should change the way I approach 'easy' questions with my GMAT 3 days away. Also, do main point or primary purpose questions always point to what the entire paragraph discusses.

Thanks a ton in advance

GMATNinja

I am bit confused here. I choose A but my reasoning was:

'and mating by subordinate females may not be totally suppressed, whereas in naked mole rat colonies subordinate females are not sexually active, and many never breed.'

this is mentioned in last sentence of last paragraph so I thought it shows that Naked mole rats exhibit an extreme form of altruism. was my inference reasonable here?