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

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

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27 Nov 2014, 21:27
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 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 a 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.
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Re: The prairie vole, a small North American grassland [#permalink]

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28 Nov 2014, 04:56
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 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 a 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 - If this is the case then population must me large in spring instead of small.

B. Prairie vole populations vary dramatically in size from year to year - Already sated in passage.

C. The prairie vole subsists primarily on broad-leaved plants that are abundant only in spring - If this is the case then population must me large in spring instead of small.

D. Winters in the prairie voles' habitat are often harsh, with temperatures that drop well below freezing - If this is the case then population must me small in winters instead of large.

E. Snakes, a major predator of young prairie voles, are active only from spring through early autumn - Correct. Snakes, predator of young voles, are active during spring-autumn hence population be small.

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

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15 Jan 2015, 13:36
D is my take. "Voles commonly live in large groups from late autumn through winter; from spring through early autumn, however, most voles live in far smaller groups".

from late autumn through winter--->live in large groups
from spring through early autumn--->most voles live in far smaller groups

Thus they are getting killed gradually on the late autumn through winter time. D tells us that winter is rash and drops below freezing, which could kill the young ones. So the population of voles will be less at the end of winter. And sequntial time of spring through early autumn, the population will be lower.

Correct me if I am wrong..
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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?

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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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The prairie vole, a smal [#permalink]

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05 Jun 2015, 05:05
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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 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 a 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.
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Re: The prairie vole, a small North American grassland rodent, [#permalink]

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06 Jun 2016, 21:36
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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