Data revealing how magnetism guides animals
Monday, July 10th 2000, 12:00 am
By: News On 6
Animal magnetism sounds like a phrase from a bad personal ad.
But it's an ad a scientist might answer, hoping to get lucky by finally figuring out how animals use magnetism to find their way around.
For years scientists have wondered how animals from fish to fruit flies use Earth's magnetic field to navigate. Now researchers may have come up with something worth sticking on the refrigerator door.
In recent months, scientists have shown that magnetic fields may be able to influence chemical reactions inside living cells. Other research is testing sunlight's role in magnetic navigation. And research soon to be published suggests that tiny biological bar magnets are stowed inside the nose of trout.
"The picture at this point is that magnetic sensitivity is pretty near ubiquitous in the animal kingdom," said John Phillips of Indiana University.
Scientists have evidence that Siberian hamsters and naked mole rats, loggerhead turtles and rainbow trout, homing pigeons and red-spotted newts, and even the common fruit fly, just to name a few, can sense Earth's magnetic field. Birds about to embark on their yearly migration will flock to the north side of a cage, even when they can't use the sun to get clues about direction. Newts accustomed to swimming north to a shore for safety will automatically swim north even after they've been transferred to the lab.
So far, the only way for an average human to sense Earth's field is to hold a compass in hand. But some researchers suspect that if animals have a magnetic sense, then people might, too.
"If we don't have it, we have to explain why we don't," said Joe Kirschvink, a biologist at the California Institute of Technology in Pasadena. "We share their ancestry."
Earth's field emanates from deep within the planet, where a molten outer core churns around the solid inner core. The churning creates a magnetic field that passes up through the globe, out into the atmosphere, and back inside. If the field could be seen, it would look like iron filings surrounding a bar magnet.
The planet's magnetic field is huge, but it's weak, said Ken Lohmann, a biologist at the University of North Carolina, Chapel Hill.
"Probably a pretty good ballpark estimate is that a typical refrigerator magnet is 10 to 100 times stronger than the Earth's field," he said.
Such a weak field means that animals need an extra-sensitive sensing system, one that can detect magnetic fields even in the face of the natural mishmash of chemical activity inside any living creature. Some scientists have proposed that magnetic fields may actually affect how well certain chemical reactions proceed.
Not all chemical reactions are affected by magnetic fields â€“ a glass of vinegar and baking soda will bubble whether or not you drop in a refrigerator magnet. But a special kind of reaction, known as a radical-pair reaction, seems to be particularly finicky when it comes to magnetic fields.
In a recent issue of the journal Nature, scientists from MIT and the University of Chicago report that these chemical reactions could stand out among the natural fluctuations inside cells. The chemical reactions could occur within a cube less than half a millimeter on a side, said James Weaver of MIT.
"So it can fit in a bird," he said.
These finicky radical-pair reactions could come in handy for bird navigation, said Klaus Schulten, a chemist at the University of Illinois at Urbana-Champaign. Dr. Schulten and his colleagues Thorsten Ritz and Salih Adem recently proposed a theory to explain how birds could take advantage of a radical-pair reaction inside their eyes.
Radical-pair reactions can be triggered by light, Dr. Schulten said. Some evidence suggests that birds, along with other animals, need light to sense magnetic fields. Certain birds, for instance, can sense fields when they see blue light but not red. In recent years, scientists have found that animals' eyes contain pigments, called cryptochromes, that respond to blue light.
What's interesting, said Dr. Schulten, is that cryptochromes are just the right kind of molecule to trigger a radical-pair reaction. And they're also in the right place.
Cryptochromes are found in cells behind the retina, at the back of the bird's eye, Dr. Schulten said. Since the retina is curved, Earth's magnetic field lines strike different areas of the retina at different angles. Assuming that the angle of the magnetic field really does affect cryptochromes, a bird might be able to sense it.
"The bird would see a cue of the field, in its normal vision," Dr. Schulten said.
The theory is not yet proven, Dr. Schulten said. Along with Indiana's Dr. Phillips, he is going to test the idea in fruit flies. These pesky insects can also sense magnetic fields, and scientists have found fruit flies that are missing cryptochromes, as well as other parts of the visual system. Experiments on the fruit flies will start this fall, Dr. Phillips said.
Birds may not be the only animals that need light to sense magnetic fields. The eastern red-spotted newt, a type of salamander, goes off course by an even 90 degrees if a certain color light strikes it. In the case of the newt, however, it's not the eyes that are sensing the light. It seems, Dr. Phillips and his colleagues have found, that light penetrating deep inside the newt's head, to a brain structure called the pineal gland, affects the animal's ability to sense the magnetic field.
Not everyone is certain that light really aids magnetic sensing.
"The only problem is there's no experimental evidence for it," Caltech's Dr. Kirschvink noted.
And it could be that when an animal sees a different color light, it just gets confused. As Dr. Lohmann pointed out, if you woke up one day and everything was red, would you jump up and migrate as if nothing was wrong?
Dr. Kirschvink favors an idea he came up with years ago â€“ that animals actually have tiny magnets inside their brains, true built-in compasses that would put any Boy Scout to shame.
"Admittedly I'm biased toward it," he said.
Many experiments have shown that a strong pulse of magnetism can send an animal toward the opposite magnetic pole, something that could be true only if the pulse had re-magnetized actual magnets inside the animals' bodies.
But evidence for these magnets had been scant, until now.
Nosing out new evidence
In an upcoming issue of Nature, scientists from New Zealand will report finding microscopic bar magnets inside the nose of rainbow trout, fish known to sense magnetic fields.
In earlier research, the scientists, led by biologist Michael Walker, found what they suspected were tiny chains of magnetite, a magnetic mineral containing iron. The chains were nestled inside the trout's nose, in cells that appear to connect to a nerve known to respond to magnetic fields.
The case for the chains being true sensors is getting better, but still isn't iron-clad, Dr. Walker said.
"What we thought was the magnetite before absolutely is," he said. But, he added, "we can't actually nail it right down yet. It is still possible for others to disagree with us."
The scientists still need to prove that the cells that contain magnetite directly link to nerves, and that breaking the link would make the trout directionally challenged.
It's possible that animals use both the magnetite and the radical-pair detection method, Dr. Kirschvink conceded.
"I'm willing to admit that we don't understand everything that nature does," he said.
As for people using magnetic fields to find their way without a compass, more research is needed. Dr. Kirschvink has found tiny bits of magnetite in people's brains, but no one has proved that they help people get around.
Right now, the sixth sense of magnetism is just wishful thinking. But if it existed, it would be useful.
"You'd never get lost," Dr. Schulten noted. "And many fights with my wife about where is the right direction ... would be over."