Bird migration is one of the most astounding feats in the animal kingdom. Each y

Question

Bird migration is one of the most astounding feats in the animal kingdom. Each year, birds fly thousands of kilometers to their breading grounds in the spring and then again back to their wintering grounds in the autumn, driven primarily by availability of food. Whereas some birds migrate in large flocks (in which first time migrants hatched during the summer can follow more experienced birds), most songbirds migrate alone at night. Several mechanisms have been proposed to explain how a songbird can navigate over great distances to a specific location it has never previously visited; these include sense of smell, sensitivity to infrasound (sounds of frequencies below 20 Hz). And reliance on celestial cures such as the sun and stars. However, none of these mechanisms fits the observed anatomical physiological and behavioral data as well as magnetoreception, the innate ability of birds and other organisms to detect the earth’s magnetic field and navigate with respect to it.

2. The earth generates a magnetic field analogous to that of a bar magnet running roughly between its geographical North and South Poles, except that the poles are reversed (the magnetic south pole of the earth corresponds to the geographic: North Pole). Thus the earth’s magnetic field loops outward from Antarctica and inward around the Arctic. In northern Europe and similar latitudes, the magnetic field vector has a magnitude of about 50 ? T and points about 25o north of down.

3. In one experiment designed to test migratory songbirds’ magnetoreception, specimens were placed in round indoor enclosures surrounded by two Helmholt; coils. A Helmholtz coil consists of a pair of parallel, tightly wound helical wires, or solenoids, of radius R, separated by a perpendicular distance R. When a constant current is run through the wires, a Helmholtsz coil produces a near-constant magnetic field along its alignment axis. In the experiment, one Helmholtz coil was aligned along a north-south axis and the other was aligned along a vertical axis, so that the magnetic field vector within the enclosure could be altered by superposition of the applied fields from the coils and the earth’s magnetic field. The results showed that the specimen birds’ magnetoreception was axial rather than polar: that is, the birds were sensitive to the angle between the magnetic field vector in the enclosure and the local gravitational vector. The defined “north” as the direction on the side of the acute angle made between those vectors. By contrast, a typical magnetic compass aligns its arrow with the magnetic field vector’s poplar component (parallel to the surface of the earth), and thus consistently points north (toward the earth’s south magnetic pole). Helmholtz was a pioneer in the scientific study of human vision and audition. He coined the term "psychophysics," to capture the distinction between the measurement of physical stimuli and their effect on human perception. For example, the amplitude of a sound wave can be varied, causing the sound to appear louder or softer, but a linear step in sound pressure amplitude does not result in a linear step in perceived loudness. ?

4. It is not known precisely how birds’ magnetoreception works, but two theories seem to hold the most promise. The magnetite hypothesis holds that thin iron oxide crystals within proteins align with an external magnetic field to open or close pressure sensitive ion channels. The cryptochrome hypothesis states that photosensitive proteins called cryptochromes found in birds’ eyes are bound to the photoreceptors in the retina. Blue-light absorption by these proteins produces Flavin-tryptophan radical pairs, such that subsequent chemical reactions are influenced by the interaction of the external magnetic field and these radial pairs (depending upon whether they are in the singlet or triplet state). The different protein configurations could therefore cause birds to perceive the visual field as brighter (or otherwise different) to the north or south than in other directions.

1.. Scientists seek to determine whether songbirds’ magnetoreception is polar or axial using the experimental apparatus described in paragraph 3. Supposing the experiment is conducted in northern Europe, which of the following extend fields provided by the Heimhotltz coils should be used to confirm the birds’ magnetoreception is not polar?

A. The vertical Helmholtz coils provide an external field of B = 50 sin 25o upward

B. The vertical Helmholtz coils provide an external field of B = 50 cos 25o upward .

C. The vertical Helmholtz coils provide an external field of B = 50 sin 25o south

D. The vertical Helmholtz coils provide an external field of B = 50 cos 25o south

Explanation / Answer

in the experiment to prove that the birds magnetoreception is not polar but axial, we have to remove the polar component of magnetic field of the earth

which is 50*sin(25) to north

hence the helmolts coil shoudl gemerate 50*sin(25) to south

option c.) he vertical Helmholtz coils provide an external field of B = 50 sin 25o south

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