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# The prairie vole, a small North American grassland rodent,

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Re: The prairie vole, a small North American grassland rodent, [#permalink]

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01 May 2013, 11:26
quiet888 wrote:
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?

Correct answer will provide the reason why young voles die more often from spring through early autumn. (This is the explanation offered.)
Quote:
A. It is in the spring and early summer that prairie vole communities generally contain the highest proportion of young voles.

Proportions within the communities of voles are irrelevant.

Quote:
B. Prairie vole populations vary dramatically in size from year to year.

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

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

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.

Quote:
D. Winters in the prairie voles' habitat are often harsh, with temperatures that drop well below freezing.

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

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

Does it explain why more than usual voles die from spring through early autumn?
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Re: The prairie vole, a small North American grassland rodent, [#permalink]

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01 May 2013, 16:24
C. snakes wake in spring and it small rodents.
B. Says only that there is a variation in the population not why this happens

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Re: The prairie vole, a small North American grassland rodent, [#permalink]

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22 Jul 2013, 02:49
I did pick E but C was very tempting can someone explain? Thanks
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Re: The prairie vole, a small North American grassland rodent, [#permalink]

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23 Jul 2013, 07:40
quiet888 wrote:
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 ?

A. It is in the spring and 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 the prairie voles' 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.

OA is E...Why are other options wrong??
Can someone explain...!!!!
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Re: The prairie vole, a small North American grassland rodent, [#permalink]

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23 Jul 2013, 08:06
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jaituteja wrote:
OA is E...Why are other options wrong??
Can someone explain...!!!!

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.
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Re: The prairie vole, a small North American grassland rodent, [#permalink]

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23 Jul 2013, 08:22
Thanks for the explanations... KUDOS..!!!!

I have a doubt and need clarification..

"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."

This only states that the group size was reduced.Maybe, the family consists of 10 members from late autumn through winter and got separated forming smaller 5 groups each of 2 member from spring through early autumn.. Just as we have nuclear and joint families.
This does not mean the voles were low or high... Maybe their number was same....

I need clarity on this...
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Re: The prairie vole, a small North American grassland rodent, [#permalink]

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23 Jul 2013, 08:41
jaituteja wrote:
Thanks for the explanations... KUDOS..!!!!

I have a doubt and need clarification..

"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."

This only states that the group size was reduced.Maybe, the family consists of 10 members from late autumn through winter and got separated forming smaller 5 groups each of 2 member from spring through early autumn.. Just as we have nuclear and joint families.
This does not mean the voles were low or high... Maybe their number was same....

I need clarity on this...

Your reasoning is correct, that can happen BUT if that's the case we would not be able to prove that:
The seasonal variation in group size can probably be explained by seasonal variation in mortality among young voles.
as we are asked to. Your point could work like a weakener option. We have to prove what the conclusion states (a thing that E does), not to weaken it...

Also note that we are not allowed to reduce the number as much as we like because
a group consists primarily of an extended family, often including two or more litters. So the average group consists of an extended family and litters, their number grows and then declines, and we are asked to connect this variation to the mortality of the young voles. Nothing more.

Hope I've explained myself well
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Re: The prairie vole, a small North American grassland rodent, [#permalink]

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23 Jul 2013, 08:52
Zarrolou wrote:
jaituteja wrote:
Thanks for the explanations... KUDOS..!!!!

Your reasoning is correct, that can happen BUT if that's the case we would not be able to prove that:
The seasonal variation in group size can probably be explained by seasonal variation in mortality among young voles.
as we are asked to. Your point could work like a weakener option. We have to prove what the conclusion states (a thing that E does), not to weaken it...

Also note that we are not allowed to reduce the number as much as we like because
a group consists primarily of an extended family, often including two or more litters. So the average group consists of an extended family and litters, their number grows and then declines, and we are asked to connect this variation to the mortality of the young voles. Nothing more.

Hope I've explained myself well

You have explained really well brother..!!!

I agree that we need to focus on the conclusion... I was just trying to explore the argument..

I was pre-thinking that if rodents were living in small groups, they were not able to gather much food or any other requirement,leading to death of young voles.(because of food scarcity), etc..
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Re: The prairie vole, a small North American grassland rodent, [#permalink]

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23 Jul 2013, 09:53
quiet888 wrote:
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 ?

A. It is in the spring and early summer that prairie vole communities generally contain the highest proportion of young voles. weakens argument. we want the proportion of young voles to be low during the spring

B. Prairie vole populations vary dramatically in size from year to year. year to year variations in size is irrelevant, we are look at the seasonal changes in population

C. The prairie vole subsists primarily on broad-leaved plants that are abundant only in spring. completely out of scope. what does this have to do with population size in spring? Neither weakens nor strengthens

D. Winters in the prairie voles' habitat are often harsh, with temperatures that drop well below freezing. this weakens the argument. Remember that population is highest during winter, we don't want evidence that says winter is the harshest season. This wouldn't make sense to our argument. If winter season is harsh, the population size should then be lower in winter than in spring.

E. Snakes, a major predator of young prairie voles, are active only from spring through early autumn. this is the only that explains why the population is lower in the spring than in winter.

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Re: The prairie vole, a small North American grassland rodent, [#permalink]

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04 Sep 2013, 02:03
+ E

Thanks for the explanations... KUDOS..!!!!
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Re: The prairie vole, a small North American grassland rodent, [#permalink]

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10 Feb 2014, 04:24
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.
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Re: The prairie vole, a small North American grassland rodent, [#permalink]

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21 Jul 2014, 10:17
Can you please explain why option D is wrong? Thanks in advance
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The prairie vole, a small North American grassland rodent, [#permalink]

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02 Sep 2014, 22:16
Hi e-gmat,

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.

My Analysis:

Logical structure:

PV : Breeds throughout the year.
Lives in group, primarily extended family

Late.A to Winter: Large group
Spring to Early.A: Small group

Conclusion: Variation in mortality caused seasonal variation in mortality among young voles. (Please confirm whether the passage is causal passage?)

Implicit Assumption:
There is no other cause for seasonal variation such as decrease in population of other demographic group.

Prethinking:
The population of other demographic group or the mortality of them remain constant throughout the year.

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

E. Snakes, a major predator of young prairie voles, are active only from spring through early autumn.
Now this didn't even picture in our pre thinking?

My doubts in RED.
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Re: The prairie vole, a small North American grassland rodent, [#permalink]

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01 Dec 2014, 12:19
Yes the answer is E. Only this choice strengthens the conclusion.
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The prairie vole, a small North American grassland rodent, [#permalink]

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05 Mar 2015, 18:01
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
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Re: The prairie vole, a small North American grassland rodent, [#permalink]

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30 Mar 2015, 04:28
quiet888 wrote:
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 ?

A. It is in the spring and 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 the prairie voles' 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.

Source: GMATPrep CR PDF
Need to find support for why the population differs in summers than winters. Hence E.
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Re: The prairie vole, a small North American grassland rodent, [#permalink]

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06 Jun 2016, 21:36
Expert's post
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
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Re: The prairie vole, a small North American grassland rodent, [#permalink]

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08 Jun 2016, 11:20
During the season in which the group numbers are low E offers a strengthener explaining why the mortality may occur - activity of predatory snakes!
Re: The prairie vole, a small North American grassland rodent,   [#permalink] 08 Jun 2016, 11:20

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# The prairie vole, a small North American grassland rodent,

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