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# Birds of various species, from pigeons to swallows to larger birds, ca

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Re: Birds of various species, from pigeons to swallows to larger birds, ca [#permalink]
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mehro023
SajjadAhmad

OE for Q3 please ?

Thanks

-k

Hi mehro023,

Not the OE, but let me know if this helps.

3. Which of the following would be most useful to know in order to evaluate the retinal theory of navigation?

A. whether the absence of magnetites in the retina is the reason that robins fail to detect north when their cluster N are destroyed
B. whether other species' retinas are identical to the retinas of robins
C. whether quantum mechanics is a popularly understood theory
D. whether cryptochrome processes magnetic information
E. whether cluster N is found in wandering macrophages

Explanation: can be inferred from the lines: "Birds' eyes do not contain magnetite grains, however. Rather, some advocates of the theory that birds navigate by retinal interaction believe that a retinal protein known as cryptochrome processes magnetic information within the cluster N." Thus whether cryptochrome processes magnetic information would be most suitable to evaluate the retinal theory of navigation.

Thanks.
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Re: Birds of various species, from pigeons to swallows to larger birds, ca [#permalink]
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mehro023
SajjadAhmad

OE for Q3 please ?

Thanks

-k

Official Explanation

3. Which of the following would be most useful to know in order to evaluate the retinal theory of navigation?

Difficulty Level: 700

Explanation

We are asked for something "useful to know" in evaluating the second theory. The most useful thing to know will always be something that is critical to know, if there is such a thing, so we can always start with that. We can evaluate each answer choice with that question: is this critical to whether or not theory 2 is correct?

(A) mixes magnetite grains and retinas, details from theory 1 and theory 2, in a jumble. So (A) is out.

(B) may be critical, or at least relevant.

(C) is not critical to whether the theory is correct; it may impact whether the theory is understood, but the passage concerns only whether the theory is correct. So (C) is out.

(D) has promise.

(E) is another jumble, because wandering macrophages are not relevant to theory 2, only theory 1.

We are down to (B) and (D). Which is more critical? It's not critical that other birds have retinas that are identical to those of robins, only whether they are identical in the relevant aspect. Their retinas could be of a different color, or larger, but so what? And that leads us back to the cryptochrome. The passage says that "scientists believe" that cryptochrome processes magnetic information. If that's not true, the theory will fall apart. If it is known to be true, then the theory is a little stronger.

The correct answer is (D).

Hope it helps
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Re: Birds of various species, from pigeons to swallows to larger birds, ca [#permalink]
Here's my two cents on why (D( should be the correct answer for Q1 instead of (B) (which is the official correct answer):

The passage outlines two prevailing theories about how birds navigate long distances using a magnetic sense: one based on magnetite grains in their beaks and the other based on a chemical reaction in their retinas influenced by the Earth's magnetic field.

The critics of the first theory, which posits that magnetite grains in the beak help birds navigate, argue against this idea. They suggest that the grains are not concentrated in a way typical for a sensory organ but are found in wandering macrophages, implying that these grains may not function as hypothesized in navigation. The passage also notes that critics of this theory do not support the idea of magnetite grains being responsible when they are outside the beak either, as no supporting evidence from beak dissections or severed trigeminal nerves exists for such a position.

Instead, the critics propose the second theory: that birds navigate using a mechanism in their eyes involving a retinal protein known as cryptochrome. This theory is supported by evidence that destroying part of a robin's retina known as cluster N eliminates its ability to detect north.

Thus, according to the critics of the first theory, the factor enabling a migrating bird to navigate is D. The function of the portion of the retina known as cluster N. This choice aligns with the alternative theory they propose based on retinal interactions rather than magnetite grains.

Hence, the correct answer choice in this case should be (D) and not (B).
Re: Birds of various species, from pigeons to swallows to larger birds, ca [#permalink]
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