Homing in on animal magnetism.
If "Hansel and Gretel" had been written by a salamander, there would have been no story at all. Bread crumbs or no, the little newts would have followed the variations in the earth's magnetic field homeward ... poor wicked witch. That, at least, might be the scenario according to recent work on magnetoreception, which for the first time provides clear evidence of a magnetic component in a terestrial vertebrate's ability to navigate.
It is widely accepted that birds use the earth'se magnetic field for general oreintation on long migratory flights. But the role of magnetorectpion in true navigation has been unclear. "there's a good bit of evidence that, at least on their first migration, birds just head south and fly a certain distance," says John Phillips, who reports his work in the Aug. 15 SCIENCE. "For that kind of orientation behavior, all you need is a compass. But if you're trying to home and you're in unfamiliar territory, you don't go north unless you're south of home; you need to have map sense as well." The earth's magnetic field, the researcher says, might provide some of that information.
Phillips, who did this research at Cornell University, worked with migratory salamanders known to have two orienting responses: In their home pond, they oreint according to the shore, while away from their home they navigate toward the pond itself. He kept the newts outdoors, exposed to the local magnetic field.
On the day of the test, he raised the temperature of some of the holding tanks in a single sharp step to motivate the newts to get out of the water. When he moved those newts into the magnetically controlled laboratory for testing, they headed in the same direction as the shore in their home pond. This is a compass response: In the wild the newts stay in a closely bounded area of the pond, so the shore is always in a given direction from the water.
But other newts were exposed to a period of widely fluctuating water tempeatures before the testing began; these newts oriented not in the direction of shore, but as though they were trying to return to their home pond. This, Phillips says, indicated that the newts had a map sense, an awareness of where the laboratory was in relation to home. Both responses had a magnetic component, since both could be altered by manipulating the experimental magnetic field.
such a map sense would put much tougher demands on a magnetic receptor than does simple compass orientation. An animal using the magnetic field as a clue for close-quarters navigation must be able to cope, Phillips points out, with the fact that "if you go a kilometer across the earth's surface, the magnetic field changes only about one one-hundredth of a percent."
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|Title Annotation:||earth's magnetic field for long migratory flights of birds|
|Date:||Aug 23, 1986|
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