The Magnetic Sense
Some
creatures have built-in compasses.
Imagine that you are sitting in a canoe on a dark and cloudy
night in the middle of a big lake. You are completely lost.
You have no idea of which way to paddle or where you might
get by paddling in any one direction.
While you are worrying about what you might do, think about the many other creatures that would have no problem at all. A long list of animals as different as birds, bees, fish, turtles, and whales would know right off what direction would take them home.
For more than twenty years scientists have been seeking to understand the mystery of this “sixth sense” of direction. By trying out ideas and solving problems one by one, they are now getting close to an answer.
One idea that is fun to think about is that animals might have a built-in compass. Our Earth is itself a big magnet. So a little magnetic needle that swings freely will line itself up with the big Earth magnet to point north and south.
When people discovered that idea about a thousand years ago and invented the compass, it became a new high-tech gadget. It allowed sailors to navigate on ocean voyages, even out of sight of land and under cloudy skies.
Birds
Find Their Way
Actually the idea of an animal compass came just from observing
animals in nature. Many birds migrate between their summer
homes and their winter homes twice a year.
Some of them fly for thousands of miles and mostly at night. Experiments have shown that some birds can recognize star patterns. But they can keep on course even under cloudy skies. How can they do that?
A common bird that does not migrate but is great at finding its way home is the homing pigeon. Not all pigeons can find their way home. Those that can are very good at it, and they have been widely studied.
One neat experiment was to attach little magnets to the birds’ heads. The idea was to try to block the magnetic sense—just as a loud radio can keep you from hearing a call to supper.
On sunny days, that did not fool the pigeons. Evidently they can use the sun to tell direction just as people can.
But on cloudy days, the pigeons with magnets could no longer find their way. It was as if the magnets had blocked the birds’ magnetic sense.
Similar experiments with the same kind of results were done with honeybees. These insects also seem to have a special sense of direction.
In spite of these experiments, the idea of an animal compass seemed pretty far out. How would an animal get the magnetic stuff for a compass?
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A chain of magnetic particles is visible inside this bacterium, shown here about 33,000 times larger than its actual size. This simple compass keeps the microscopic organisms always swimming north. |
Living
Compasses
An
answer came from an unexpected source. A scientist was studying
bacteria that live in the muds of ponds and marshes. He
found little rod-like bacteria that all swam together in
one direction—north.
More study showed that each little bacterium had a chain of dense particles inside. And the particles proved to be magnetite, a magnetic form of iron oxide.
The bacteria had made themselves into little magnets that could line up with the Earth’s big magnet.
The big news was that a living thing, even a simple little bacterium, can make magnetite. That led to a search to see whether animals might have it.
Fortunately, there is a very sensitive instrument called the magnetometer, which is for detecting tiny amounts of magnetism. By using it, scientists were able to find magnetite in bees, birds, and fish.
In each animal except for the bee, the magnetic stuff was always in or close to the brain. The idea of a built-in animal compass began to seem reasonable.
The
Trout’s Compass
Now a team of scientists in New Zealand have brought us
another step closer to understanding the magnetic sense.
They
found that a fresh-water fish, the trout, would respond
to a magnetic field. They chose the trout for further study
because they already knew a lot about its nervous system.
They found magnetite, all of it located in a small spot in the fish’s head and close to its nose. That spot has nerve fibers that become part of a big nerve on its way from the nose to the brain.
But did these nerve fibers really give the brain information about magnetic fields? When the scientists made a change in the magnetic field around the fish, that change set off nerve messages that were carried to the brain.
The team had pinpointed the location of the trout’s magnetic sense. They had found the fish’s compass.
This chain of discoveries still has one more step to go. We know now that an animal can have a built-in compass. But we do not know how it works to send nerve messages to the brain.
We know how the eye senses light and how the ear senses sound. Now we can look forward to the last chapter about how the animal compass works: how it senses a magnetic field.
