Networks of blood vessels in bats' wings serve only to disperse heat generated in flight. This heat is generated only...

GMAT Critical Reasoning : (CR) Questions

Source: Official Guide

Critical Reasoning

Assumption

HARD

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Notes

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Networks of blood vessels in bats' wings serve only to disperse heat generated in flight. This heat is generated only because bats flap their wings. Thus paleontologists' recent discovery that the winged dinosaur Sandactylus had similar networks of blood vessels in the skin of its wings provides evidence for the hypothesis that Sandactylus flew by flapping its wings, not just by gliding.

The argument in the passage relies on which of the following assumptions?

Sandactylus would not have had networks of blood vessels in the skin of its wings if these networks were of no use to Sandactylus.

All creatures that fly by flapping their wings have networks of blood vessels in the skin of their wings.

Winged dinosaurs that flapped their wings in flight would have been able to fly more effectively than winged dinosaurs that could only glide.

If Sandactylus flew by flapping its wings, then paleontologists would certainly be able to find some evidence that it did so.

Heat generated by Sandactylus in flapping its wings in flight could not have been dispersed by anything other than the blood vessels in its wings.

Solution

Passage Analysis:

Text from Passage	Analysis
Networks of blood vessels in bats' wings serve only to disperse heat generated in flight.	What it says: Blood vessel networks in bat wings have one job - getting rid of heat made during flight What it does: Sets up a key fact about how bat wing blood vessels work What it is: Scientific observation about bats
This heat is generated only because bats flap their wings.	What it says: The heat comes specifically from the flapping motion, not from anything else What it does: Connects flapping directly to heat production, building on the previous fact What it is: Additional scientific detail Visualization: Flapping wings → Heat production → Blood vessels disperse heat
Thus paleontologists' recent discovery that the winged dinosaur Sandactylus had similar networks of blood vessels in the skin of its wings provides evidence for the hypothesis that Sandactylus flew by flapping its wings, not just by gliding.	What it says: Since Sandactylus had the same blood vessel setup as bats, this supports the idea that it flapped rather than just glided What it does: Uses the bat facts to draw a conclusion about dinosaur flight behavior What it is: Author's main conclusion Visualization: Bats have blood vessels + flap wings → Sandactylus has same blood vessels → Sandactylus probably flapped too

Argument Flow:

The argument starts with facts about bats - their wing blood vessels cool down flight heat, and this heat only comes from flapping. Then it jumps to a dinosaur discovery and uses the bat facts to conclude something about how the dinosaur flew.

Main Conclusion:

Sandactylus probably flew by flapping its wings rather than just gliding, based on having similar blood vessel networks to bats.

Logical Structure:

This is an analogy argument. We know bats have blood vessels because they flap and generate heat. Since Sandactylus had the same blood vessels, the argument assumes it must have had the same heat-generating behavior (flapping). The logic depends on the idea that these blood vessels only exist when there's a heat problem to solve.

Prethinking:

Question type:

Assumption - We need to find what the argument must assume to be true for the conclusion to logically follow. This is something that, if false, would make the argument fall apart.

Precision of Claims

The argument makes very specific claims about function (blood vessels serve ONLY to disperse heat), causation (heat is generated ONLY because bats flap), and comparison (Sandactylus had SIMILAR networks). These precise claims create gaps that need assumptions to bridge them.

Strategy

Since this is an assumption question, we need to identify ways the conclusion could be falsified while respecting the facts given. The argument jumps from 'bats have blood vessels for heat from flapping' to 'Sandactylus had similar blood vessels, so it flapped too.' We need to find what must be true for this comparison to work - essentially, what could break this reasoning if it weren't true?

Answer Choices Explained

Sandactylus would not have had networks of blood vessels in the skin of its wings if these networks were of no use to Sandactylus.

This captures exactly what the argument must assume to work. The argument uses the presence of blood vessel networks as evidence that Sandactylus flapped its wings. But this only makes sense if these networks wouldn't exist unless they served a purpose. If blood vessel networks could exist without being useful, then finding them tells us nothing about flight behavior. The argument treats their presence as meaningful evidence, which requires assuming they exist because they're needed. This assumption is necessary - without it, the argument falls apart.

All creatures that fly by flapping their wings have networks of blood vessels in the skin of their wings.

This reverses the logical relationship the argument needs. The argument doesn't require that ALL flapping creatures have blood vessels - it only needs the presence of blood vessels to indicate flapping. We could have creatures that flap without blood vessels (maybe they handle heat differently), and the argument would still work. What matters is that when we DO find these specific blood vessel networks, they indicate flapping behavior. This choice confuses a sufficient condition with a necessary one.

Winged dinosaurs that flapped their wings in flight would have been able to fly more effectively than winged dinosaurs that could only glide.

This introduces a comparison about flying effectiveness that's completely irrelevant to the argument. The argument only cares about whether Sandactylus flapped versus glided, not whether flapping was more effective than gliding. The presence of blood vessels is used as evidence for the type of flight, not the quality of flight. Whether flapping was better, worse, or equal to gliding doesn't affect the logic connecting blood vessels to flight behavior.

If Sandactylus flew by flapping its wings, then paleontologists would certainly be able to find some evidence that it did so.

This choice misunderstands the argument's structure. The argument doesn't assume that evidence of flapping WOULD be findable - it's responding to evidence that HAS BEEN found (the blood vessel networks). The paleontologists already found their evidence, so the argument doesn't need to assume they'd be able to find evidence. This choice puts the cart before the horse by treating the discovery as hypothetical when it's already established fact.

Heat generated by Sandactylus in flapping its wings in flight could not have been dispersed by anything other than the blood vessels in its wings.

This goes too far by claiming blood vessels are the ONLY way to disperse heat. The argument doesn't require this extreme position. Sandactylus might have had other heat-dispersal methods too - what matters is that the blood vessels indicate heat generation from flapping. Even if other heat-dispersal mechanisms existed, the presence of blood vessel networks would still serve as evidence of the heat-generating flapping behavior. The argument works fine without requiring blood vessels to be the exclusive heat-dispersal method.

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