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

GMAT Critical Reasoning : (CR) Questions

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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.

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

It is the spring and in the early summer that prairie vole communities generally contain the highest proportion of young voles.

Prairie vole populations vary dramatically in size from year to year.

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

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

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

Solution

Passage Analysis:

Text from Passage	Analysis
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.	What it says: Prairie voles breed all year and live in family groups with multiple litters What it does: Sets up basic facts about vole breeding and family structure What it is: Background information Visualization: Family group = parents + 2-3 litters (maybe 15-20 voles total)
Voles commonly live in large groups from late autumn to winter; from spring through early autumn, however, most voles live in far smaller groups.	What it says: Group sizes change seasonally - big groups in winter, small groups in warmer months What it does: Introduces the key pattern that needs explaining What it is: Observational data Visualization: Winter groups = 50-80 voles, Spring/summer groups = 10-20 voles
The seasonal variation in groups size can probably be explained by a seasonal variation in mortality among young voles.	What it says: Changes in group size are likely due to young voles dying at different rates in different seasons What it does: Provides the author's proposed explanation for the seasonal pattern What it is: Author's hypothesis

Argument Flow:

The argument starts with background facts about vole breeding and family life, then presents an observed seasonal pattern (large winter groups vs. small summer groups), and finally offers a hypothesis that mortality rates among young voles vary by season to explain this pattern.

Main Conclusion:

The seasonal variation in vole group sizes can probably be explained by seasonal differences in mortality rates among young voles.

Logical Structure:

The argument uses an explanatory structure where observed data (seasonal group size changes) is linked to a proposed cause (varying mortality rates in young voles). The conclusion is tentative ('probably') and relies on the logical connection that if young voles die at different rates seasonally, this would naturally affect overall group sizes.

Prethinking:

Question type:

Strengthen - We need to find information that makes the author's explanation more believable

Precision of Claims

The key claim is about seasonal variation in mortality among young voles explaining group size changes (large groups in winter, small groups in spring/summer)

Strategy

We need to find evidence that supports the idea that young vole mortality varies by season in a way that would explain the group size pattern. Since groups are larger in winter and smaller in spring/summer, the explanation suggests that either:

fewer young voles die in winter, allowing groups to grow larger, or
more young voles die in spring/summer, causing groups to shrink. We should look for biological, environmental, or behavioral factors that would create this mortality pattern.

Answer Choices Explained

It is the spring and in the early summer that prairie vole communities generally contain the highest proportion of young voles.

This tells us that spring and early summer have the highest proportion of young voles in communities. However, this doesn't help explain why groups are smaller during these seasons. If anything, having more young voles should make groups larger, not smaller. This doesn't support the mortality-based explanation and actually seems to contradict the observed pattern.

Prairie vole populations vary dramatically in size from year to year.

Population variation from year to year doesn't address the seasonal pattern within each year. We need to understand why groups change size from season to season, not why populations fluctuate annually. This is irrelevant to the seasonal mortality explanation we're trying to support.

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

Information about food abundance in spring doesn't directly connect to young vole mortality patterns. While food availability might affect survival, this choice doesn't establish the connection between seasonal food patterns and the specific mortality variation needed to explain group size changes.

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

Harsh winters with freezing temperatures might seem relevant, but this actually works against the explanation. If winters are harsh, we'd expect higher mortality in winter, which would mean smaller winter groups. But we observe the opposite - larger groups in winter. This doesn't support the proposed explanation.

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

This directly supports the explanation by providing a concrete biological mechanism. Snakes are major predators of young voles and are only active during spring through early autumn - exactly when vole groups are smaller. This means young voles face higher predation pressure (higher mortality) during the seasons when groups shrink, and lower predation pressure (lower mortality) during winter when groups are larger. This creates the precise seasonal mortality pattern needed to explain the observed group size variation.

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