Migratory birds sense earth's magnetic field
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Light-absorbing molecules present in the retina of a bird's' eye help in sensing the earth's magnetic field.
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Migratory birds use the magnetic field as a source of compass information to fly in the appropriate migratory direction.
MIGRATORY BIRDS, as well as many other animals, are able to sense the magnetic field of the earth, but how do they do it?
``A fascinating possibility is that they may actually see the earth's magnetic lines as patterns of colour or light intensity superimposed on their visual surroundings,'' said John B. Phillips, associate professor of biology in the College of Science at Virginia Tech. The results of more than two decades of research allow him to let such an image cross his mind.A paper in Nature, reports evidence that the earth's magnetic field is sensed by light-absorbing molecules in the retina of a bird's' eye.
Thorsten Ritz, a postdoctoral associate in the famous Phillips' lab at Virginia Tech, who is now a faculty member in the Department of Physics and Astronomy at the University of California at Irvine, co-authored the paper with John Phillips.
Any effect of the earth's magnetic field on a photoreceptor's response to light is expected to be extraordinarily weak — so weak in fact that the possibility of such effects have been largely ignored. But animals have developed specialized visual systems. ``Some animals can see ultraviolet light. Some animals can see polarized light,'' Phillips said.
How animals' nervous systems become adapted to detect different things is the subject of Phillips' research. ``As a biologist interested in specialized sensory systems, the question of whether photoreceptors have become specialised for detection of the earths' magnetic field is a fascinating topic,'' he said.
Phillip's lab has conducted research that has demonstrated that the magnetic `compass' sense involves a light-dependent mechanism in some animals.
Phillips' lab showed in earlier papers published in Nature, that directional information obtained from the magnetic compass in amphibians was altered by changing the colour of light, and that the photoreceptors responsible were not located in the eyes, but in the pineal organ, or `third eye,' located on top of the head.
A molecule, referred to as a photopigment, has to absorb light, for a photoreceptor to detect light, Phillips said. A series of biochemical events that result in a change in the electrical charge across the cell membrane is started by light energy. This neural impulse can then be communicated to other cells in the nervous system.
Several theoretical models, including models proposed by lead author Thorsten Ritz and his Ph.D. adviser Klaus Schulten at the University of Illinois at Urbana-Champaign, have suggested ways in which the magnetic field can interact with a photopigment to divert energy and make the photoreceptor more or less responsive to light, Phillips said. These changes in the response to light may depend on the alignment of the earth's magnetic field relative to the photopigment molecules in the eye, producing a `visual' pattern that could be used to obtain directional (`compass') information from the magnetic field, he said.
Migratory birds held in `orientation cages' during their normal seasonal migrations use the magnetic field as a source of compass information to hop in the appropriate migratory direction. The experiments took advantage of this fact. European robins orienting to the north during spring migration were exposed to low-level radio frequencies predicted to disrupt the energy states of any light-absorbing molecules involved in sensing the magnetic field to distinguish between the photoreceptor and magnetite mechanisms.
In the presence of the radio frequency fields, the robins were unable to orient with respect to the magnetic field. This effect also was shown to depend on the alignment of the radio frequency field relative to the earth's magnetic field, a further prediction of Ritz and Schulten's model.
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