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

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

Read again with a different perpective. Guess what? It does make sense now...
Thanks ..

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

I am sorry but the logic here is seems flawed to me. If vole groups are smaller from during spring and summer time, they cannot become larger in the winter. If snakes are the cause of vole mortality, than the group size should decline overtime starting at high group size, and slowly decrease to winter. Now if voles split up into smaller groups as a defense tactic, the smaller group sizes are a result of that tactic, not the snakes

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 !

Can someone please explain to me meaning of this line "The seasonal variation in groups size can probably be explained by a seasonal variation in mortality among young voles" , doesn't this mean , rodents are dying because of seasonal changes , which is supported by choice D. Please help me understand how E is correct. Would really appreciate your help.

Thanks
Megha

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


Apart from these I am of the opinion that eGMAT / MGMAT and these notes ( I found here at GMATclub ) will definitely help you , coz 700+ score getters have been relying on these excellent materials for years now ...

All the best, hope these help you...

I agree. This is a poorly written item. Snakes don't reduce the population overnight. If snakes start killing in spring, then the populations would be largest at the start of spring, and then decline to lowest in early autumn.

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!

The argument is that season variations in group size can be attributed to season variation in mortality rates among young voles.
This is based on the premises/ fact that group size fluctuates between Autumn to winter, when groups are larger, and Spring to Autumn, when group sizes are smaller.

We need a statement that strengthens this conclusion

A is incorrect as it is a general fact that is not conducive to the argument
B is incorrect for the same reason
C is incorrect for the same reason as well.
D explains a condition that could lead some of the animals to their death, but we are told that the group sizes are still larger during this time, so to assume that this statement explains the drop in group size between spring-autumn is illogical and contradicts our premises.
E is correct - an abundant predator during Spring-Autumn logically explains why mortality rates increase during this time and thus why group sizes drop.

The prairie vole, a small North American grassland rodent, breeds year around, and a group of voles living together 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 through 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.

---->The conclusion aims to dictate the reason why the mortality in young voles due to seasonal variation. So, the strengthening choice should point out new info that can refer the mortality in young voles and seasonal variation.

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.--->No affect answer.

(B) Prairie vole populations vary dramatically in size from year to year.--->Irrelevant info

(C) The prairie vole subsists primarily on broad-leaved plants that are abundant only in spring.--->Incorrect answer.

(D) Winters in prairie vole's habitat are often harsh, with temperatures that drop well below freezing.-->No affect choice.

(E) Snakes, a major predator of young prairie voles, are active only from spring through early autumn.--->Correct

Hi
I have a doubt in this question, per the conclusion seasonal variation of mortality among young voles is the possible reason behind seasonal variation of group size. So to me, the strengthener should increase the belief that indeed that is the cause behind the effect. However correct answer choice only tells that the cause merely exists. Since the passage inherently implied this when it was mentioned that seasonal variation of mortality is the possible cause, I am confused what additional information the correct answer choice is giving to establish the causal relationship. Please help me where is the gap in my understanding.
Thanks

Posted from my mobile device

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.