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

Question

The prairie vole, a small North American grassland rodent, breeds year-round, 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 through to winter; from spring through early autumn, however, most voles live in far smaller groups. The seasonal variation in group size can probably be explained by seasonal variation in mortality among young voles.

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

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.

(B) Prairie vole populations vary dramatically in size from year to year.

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

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

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

ID: 500405

00768

Source :  GMATPrep Default Exam Pack

Gnpth · Accepted Answer

Hi,

The key word here is "Breeds year around"- In this sentence it is stated that the voles breed throughout the year. If snakes are only active from spring to early autumn, they will not be able to kill the young voles born during late autumn or in winter.

so the group is bigger in winter.

Hope it helps.

Zarrolou · Answer

We have to provide support to the claim:
  The seasonal variation in group size can probably be explained by seasonal variation in mortality  among young voles .  
We have to connect  seasonal variation ==>  seasonal variation in mortality  somehow (also note that the conclusion talks about "among young voles").
Winter = high
Spring through early autumn = low

A. It is in the spring and early summer that prairie vole communities generally contain the highest proportion of young voles. 
This goes against the passage, who says that in spring the young voles should diminish.
 B. Prairie vole populations vary dramatically in size from year to year. 
By how much the size varies does not helps us in establishing our point.
 C. The prairie vole subsists primarily on broad-leaved plants that are abundant only in spring. 
Keep an eye on the conclusion we are trying to prove: our focus must be on the young voles.
We can infer something like "so the population is likely to increase in spring" reading C, which anyway is against the passage.
 D. Winters in the prairie voles' habitat are often harsh, with temperatures that drop well below freezing. 
Keep in mind that during winter the number are higher and during spring through early autumn the numbers are lower. If we know that despite harsh condition the population grows, we still do not have anything to connect YOUNG VOLES to the seasonal variation in mortality.
 E. Snakes, a major predator of   young prairie voles  , are active only from   spring through early autumn  . 
Look at E: it has all the key words (young prairie voles, spring through early autumn) and supports our conclusion stated above.

Snakes, a major predator of young prairie voles, are active only from spring through early autumn. => so during this period it feeds on young voles mainly, so we have connected
 seasonal variation in group size ==> seasonal variation in mortality among young voles.

terp26 · Answer

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

carlson2010 · Answer

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.

HumptyDumpty · Answer

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?

wfmd · Answer

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...

mejia401 · Answer

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.

neo656 · Answer

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

spellbind3r · Answer

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

culsivaji · Answer

The prairie vole, a small North American grassland rodent, breeds year-round, 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 through to winter; from spring through early autumn, however, most voles live in far smaller groups. The seasonal variation in group size can probably be explained by seasonal variation in mortality among young voles.

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

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 !

OptimusPrepJanielle · Answer

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

Abhishek009 · Answer

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ravjai81 · Answer

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.

GMATNinja · Answer

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!

himanshu0123 · Answer

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?

DmitryFarberMPrep · Answer

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.

MS61 · Answer

(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.

GauriAkode22 · Answer

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.

yazlal25 · Answer

I thought about this one in a slightly different manner. 
Given - they exist in smaller groups from spring to autum and in larger groups from autum to winter.

Option D says - the winters are harsh - I conclude from this that due to harsh winter some young moles do not survive the winters and hence their population comes down by the time its spring.

Option E says - Snakes which are their primary predators are active in spring - which means they hunt down some voles during spring and would lead to lowering their population by autumn.

Thus, I chose D over E. Could someone explain whats the flaw in my thinking.

GMATNinja

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

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

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615 to 715 in 15 Days (Ria's Strategies)

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