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

Question

New Project RC Butler 2019  -  Practice 2 RC Passages Everyday   
Passage # 292, Date : 26-Aug-2019
 This post is a part of New Project RC Butler 2019 .  Click here for Details

Birds of various species, from pigeons to swallows to larger birds, can navigate long distances on Earth, across continents and hemispheres. That they can traverse these distances thanks to a magnetic sense has been demonstrated through tests in which birds fitted with magnets have lost their navigational capability. Precisely what biological mechanism enables birds to orient in this way is still something of a mystery, however, with two theories prevailing. 
 
One theory is that birds possess magnetic sensors in the form of grains of magnetite, which is an easily magnetized form of iron oxide. Such magnetite grains are common not only in animals but even in bacteria, where they have been established as a component enabling magnetic orientation. In the case of birds, magnetite grains are numerous in beaks, as dissections of pigeons have confirmed. Moreover, in another experiment, the trigeminal nerve, which connects the beak to the brain, was severed in reed warblers; the affected birds lost their sense of magnetic dip, which is critical to navigation. 
 
Critics of the theory have pointed out that the abundance of grains in the beak are not concentrated, as would be expected in a sensory organ, but rather found in wandering macrophages. And while an alternative explanation for birds' sensory abilities might posit magnetite grains outside of the beak, such an explanation would be supported neither by the beak dissections nor by the tests of severed trigeminals. Critics of magnetite-centric theories suggest a second theory: that the magnetic field of the Earth has an influence on a chemical reaction in birds, specifically in a bird's retina. Experiments have demonstrated that destroying the portion of a robin's retina known as cluster N eliminates the bird's ability to detect north. 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. Surprisingly, the mechanism by which cryptochrome could detect magnetic orientation depends on quantum mechanics: when hit by light, the cryptochrome would create a pair of particles, one of which subsequently presents information to the eye, in the form of a spot, when it is triggered a corresponding particle after that particle has traveled some distance.   1 
 D 
 1. According to the author, critics of the first theory presented in the passage believe which of the following to be a factor enabling a migrating bird to navigate?

A. Wandering macrophages containing magnetites
B. The abundance of grains in the beak
C. The function of the trigimenal nerve
D. The function of the portion of the retina known as cluster N
E. The possibility of magnetite grains in parts of the bird other than the beak

2 
 B 
 2. According to the passage, both theories described by the author have been developed to explain how

A. birds can navigate across oceans without the use of landmarks
B. birds use magnetism to navigate long distances
C. birds of various species navigate differently
D. birds fitted with magnets have lost their navigational ability
E. birds are able to navigate long distances using magnetites

3 
 D 
 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

4 
 A 
 4. The author is primarily concerned with

A. discussing different theories
B. analyzing different techniques
C. defending two hypotheses
D. refuting established beliefs
E. revealing new evidence

Source:  GMAT Free  (2)
Difficulty Level:   650

Sajjad1994 · Answer

Hello all  AcetheGMAT2019   AkshdeepS

AjiteshArun  please comment if i am wrong.

The answer to the first question is B according to the official source and official explanation but both are wrong, after reading the passage i have changed the answer to the first question from B to D. Here is my 2 cents on Passage map and question #1.

Passage Map

Topic:   Birds and their navigational power. (Science)

Para 1:   Introduces the topic in brief.

Para 2:   Provides  theory 1  and describes its details.

Para 3:   Starts with criticism on  theory 1  provide some logic on criticism and then provides  theory 2  and its details. Author want to say implicitly that  theory 2  is better than  theory 1  as based on few experiments described in the para, author didn't explicitly preferred  theory 2 on theory 1.

Explanation

1. According to the author, critics of the first theory presented in the passage believe which of the following to be a factor enabling a migrating bird to navigate?

Explanation

(A)  The information in A is rather  opposite ,  lines 3-5 in para 1  make this option opposite. It is discussed in  para 2  also but it didn't meant for what is required by the question.

(B) This is also  opposite  what is asked in the question, in start of the  para 3  this option belongs to theory one and not what critics believe to be a factor enabling a migrating bird to navigate.

(C) This option is discussed in the last of  para 2  but it has no concern with the question.

(D)  Lines 13-16 of last paragraph   (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.)  supports option D as answer. (PS: I am not satisfied with this option also but if we go with Process of elimination (POE) This is best choice among the given.)

(E)  Same a C.

PS: I have discussed the passage map and question 1 yet i am going to add  question number 1  as debatable one.

Hope it helps

mehro023 · Answer

SajjadAhmad

OE for Q3 please ?

Thanks

-k

bM22 · Answer

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.

Sajjad1994 · Answer

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

siddhantvarma · Answer

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).

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

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

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615 to 715 in 15 Days (Ria's Strategies)

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