The prairie vole, a small North American grassland rodent, breeds year

agree with E,

D would be wrong because just because the winter is harsh does not mean that will cause the prairies to die

I say E.

The argument implies that the variation in group size is related to the mortality rate in young voles. The groups are smallest from spring to early autumn. If snakes that eat young voles are only active from spring to early autumn, this would cause smaller groups during that time due to the fact that young voles are dying (being eaten). Therefore, this would support the argument.

Correct answer will provide the reason why young voles die more often from spring through early autumn. (This is the explanation offered.)
Proportions within the communities of voles are irrelevant.

Does it explain why more than usual voles die from spring through early autumn?

This piece of information weakens the explanation: If there is more food to feed on, the voles should proliferate. That would result in an increased number of voles.

The temperatures in winter are irrelevant. Many ways to circumvent this hint.

Does it explain why more than usual voles die from spring through early autumn?

Actually E is the only answer which explains a difference in mortality. However, I do not understand one thing here.

If snakes are active from spring through early autumn, probably killing a lot of young voles, how can the group be bigger in winter (after the snakes killed the voles)?
I think that's a flaw in the answer...
The other answers are not better, but this made me stuck...

Reasoning: Here's a causal reasoning in that the mortality among young voles -> seasonal variation in group size. Correct answer do one of the following: 1)eliminate alternative cause; 2) cause doesn't occur effect doesn't occur; 3) cause does occur effect occurs 4) the relationship is not reversed 5) validate study

Which of the following, if true, provides the strongest support for the explanation offered ?

A. It is in the spring and early summer that prairie vole communities generally contain the highest proportion of young voles. Wrong - Opposite (Weakens). If the spring and early summer contains the highest proportion of young voles, then mortality among young does not explain the seasonal variation.

B. Prairie vole populations vary dramatically in size from year to year. Wrong - Neutral. This does nothing to the conclusion.

C. The prairie vole subsists primarily on broad-leaved plants that are abundant only in spring. Wrong - Weaken. If the vole "subsists on plants that are abundant in spring", the the vole would be more numerous in the spring.

D. Winters in the prairie voles' habitat are often harsh, with temperatures that drop well below freezing. Wrong - Weaken. If the temperatures cause the seasonal variation in young voles, then the argument is weakened.

E. Snakes, a major predator of young prairie voles, are active only from spring through early autumn. Correct - strengthens the cause and effect relationship. Besides, if the major predator are active only from spring through early autumn, then the mortality rate of the young vole is likely the cause of the seasonal variation.

IMO E.

I too am stuck with the reasoning behind the answer option.

I picked D with a reasoning somewhat similar to that of jaituteja. It would be great if someone could explain why this line of reasoning is wrong.

1. As mentioned, we are talking of a difference in group size, NOT in population. Most of the reasoning in this thread is assuming that they are equivalent.
2. It has been stated that we are looking at Infant Mortality as the reason behind change in group size.

I reasoned that in higher social animals such as these, groups will form with a purpose. In times where harsher conditions / elements would cause higher infant mortality, groups will form to better protect infants against those conditions.

A, B, C are obvious candidates for elimination.
D. Harsh winters would naturally cause higher mortality and would affect infants more than it would adults. Larger groups could huddle together for warmth and thus protect the infants. So they are preferable. The timings are in conformity, so this is a potential answer.
E. Larger groups could also form to protect against predators as groups of adults might be able to fend off snakes. Sounds promising, but the timing is wrong. Snakes come out at one time, groups are larger at other times. Eliminate.

In my mind, the connection between Group size and Infant mortality is a 2-step one. Where the common answer here suggests that Group size is a direct indication of Infant mortality, I posit that Groups are forming to mitigate high mortality - which kind of reverses the causality. I continue to find this more reasonable than assuming that Group size and Population size are one and the same thing.

Would somebody please point out where I might be going wrong?

Thanks

The prairie vole, a small North American grassland rodent, breeds year around, and a group of voles living togather consists primarily of an extended family, often including two or more litters. Voles commonly live in large groups from late autumn to winter; from spring thru early autumn, however, most voles live in far smaller groups. The seasonal variation in groups size can probably be explained by a seasonal variation in mortality among young voles.

Which of the following, if true, provides the strongest support for the explanation above?

A) It is the spring and in the early summer that prairie vole communities generally contain the highest proportion of young voles. -->This doesn't support or provides any explanation as why there is seasonal variations due to mortality.
B) Prairie vole populations vary dramatically in size from year to year. --> This doesn't strengthen at all. It says that by default the population varies and as such there are no seasonal variation factors.
C) The prairie vole subsists primarily on broad-leaved plants that are abundant only in spring. --> okay plants are abundant in spring. But we don't have any information if due to scarcity are the rodents dying ? no information given.
D) Winters in prairie vole's habitat are often harsh, with temperatures that drop well below freezing.-->Okay temperatures drop in winter, but no information given to conclude whether due to low temperature are they dying ?
E) Snakes, a major predator of young prairie voles, or active only from spring thru early autumn. --> Perfect ! This option explains why the rodents are dying in different different seasons because of snakes which are active in certain period and predate on the rodents !

Premise: The voles are in abundance from autumn to winter, but are very few in number from spring through autumn. This is explained because of the mortality in your voles.

To support the reason given, we need to find something that justifies the mortality of the young voles.
Of the given option, option E tells us just the same. It gives us a reason why the young voles do not survive. because they are hunted by the snakes.

Correct option: E

Bro, I cant assure you that my strategy will work for you but you can give it a try ( As suggested by eGMAT & MGMAT CR ), here is my approach ( And I am finding it works for me )

1. Read the first Line ( This introduces the Topic/Background ) & try to pre think ( What is going to follow next )
2. Form Mental Images ( If Scientists are described , visualize men working in Science LAB in white LAB Coats )
3. Read / Re Read the conclusion ( Find what the authors believes to be true and what he/she convinces us to believe)
4. While reading the Stimulus predict the question that might follow ( Generally works well for Logical Flaw Questions )
5. Try to find the Reasoning used by the author for arriving at a specific Conclusion


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GMATNinja souvik101990

Can it be argued that the groups start larger in winters but because of its intensity not many young ones survive and that is why by the time it is spring the group size decreases.

If that were the case, we would expect the group size to decline throughout the winter. We are told that "voles commonly live in large groups from late autumn through winter." This explanation would only make sense if a large proportion of the voles suddenly died towards the end of the winter.

Regardless, don't worry about inventing your own explanations. Your job is to analyze the choices given!

Hi, option E] does not say anything on '' from late autumn to winter'' why rodents are in large groups in this period?

How should we approach?

himanshu0123
Since the voles breed year-round, their numbers would naturally increase when predation declines. So the idea would be that during snake season, numbers drop as voles get gobbled up. Then, as numbers recover, the voles would end up gathering in larger groups.

(A) It is the spring and in the early summer that prairie vole communities generally contain the highest proportion of young voles. - Incorrect, Opposite to what is given

(B) Prairie vole populations vary dramatically in size from year to year. - incorrect, year to year is out of scope.

(C) The prairie vole subsists primarily on broad-leaved plants that are abundant only in spring. - Incorrect, Opposite of what is given ( Smaller groups )

(D) Winters in prairie vole's habitat are often harsh, with temperatures that drop well below freezing. - Incorrect, Opposite ( Large group )

(E) Snakes, a major predator of young prairie voles, are active only from spring through early autumn.­ - Correct, During this time small group , reasoning.

Official Answer :

Argument Evaluation
Situation
Prairie voles live in groups made up of relatives. Because they breed year-
round, there are generally young voles in each group. These groups are larger
in late fall and winter, while group size decreases in spring, summer, and early
fall, probably because of a seasonal difference in the mortality rate of young
voles.
Reasoning What provides support for the claim that increased mortality among young voles
causes the annual reduction in vole group size observed in spring? If vole groups
are large in late autumn and winter and much smaller during the rest of the
year, then something must be causing the reduction in group size that occurs
each spring through early autumn. The explanation states that increased
mortality in young voles probably accounts for the decline in group size.
Therefore, identifying a likely cause of such increased seasonal mortality
would support the explanation.
A. A higher proportion of young voles in spring would undermine the suggestion that their
increased mortality causes a decline in group size at the same time.
B. There is no evidence that annual variations in overall prairie vole populations are related
to the seasonal reduction in vole group size each spring.
C. An abundance of food in spring would not contribute to increased mortality among
young voles at the same time.
D. Harsh winters would not account for increased mortality among young voles in spring,
given the information in the passage that vole familial groups are large through winter.
E. Correct. If snakes that eat young voles are active in spring through early autumn, then
that would support the explanation that increased mortality among young voles during
the same period causes the seasonal decline in vole group size.