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Rat Radar: Rodent Uses Natural "GPS"

John Pickrell in England
for National Geographic News
January 29, 2004
 
Hikers trekking through unfamiliar territory are well advised to carry a
compass, if not a GPS unit, to stay on course. Other animals are lucky
enough to have complex navigational equipment in-built. New research
reveals that Israel's blind mole rat (Spalax ehrenbergi) uses the
Earth's magnetic field on long journeys, much like a compass, to
continuously monitor and maintain its course.

But that's not where the burrowing rodent's abilities end. The mole rat also has an uncanny habit of burrowing around obstacles—such as ditches or concrete blocks—without ever coming in to physical contact with them.


"One of the most interesting characteristics of this unique subterranean mammal is that it is [totally] blind," said Tali Kimchi, who studies the brain and behavior at Tel Aviv University in Israel. "This created a strong evolutionary pressure on the mole rat to develop specialized, non-visual mechanisms of orientation that enable it to find its way underground."

Making navigational mistakes underground is expensive business too. Excavating soil uses between 360 and 3,400 times as much energy as moving the same distance above ground.

Internal Map

Israel's blind mole rats live most of their lives underground in pitch-black, complex tunnel systems. They have to dig over great distances when foraging for bulbs and roots, and then have to find their way home again.

Like other rodents, dogs, even people, mole rats can predict where they are simply by keeping track of their own balance and movement, said Kimchi. But over long distances in a mish-mash of winding tunnels, that internal map may not be enough, she said.

Sighted animals use visual landmarks to keep correcting mistakes, but the blind mole rat—which may have lost its eyes in an irreversible evolutionary step up to 30 million years ago—had to be using something else, said Kimchi.

Researchers already knew that a related species, the Zambian mole rat (Cryptomys anselli), could detect the Earth's magnetic field. So, Kimchi along with biologists Joseph Terkel, also in Tel Aviv, and Ariane Etienne of the University of Geneva in Switzerland decided to test if the Middle Eastern species might be using that ability to deftly navigate in the dark.

As they reported online January 21 in the science journal Proceedings of the National Academy of Sciences, the team put wild mole rats into two types of laboratory maze.

One maze was wheel-shaped and made up of eight radial spokes with a central hub. The animals were required to find their way from a feeding place to a nesting box by the most direct route. When the maze was placed between two massive electromagnets—which had the effect of turning the Earth's magnetic field by 90°—the mole rats were much more likely to lose their way.

In a second rectangular maze, the mole rats were tested on their ability to use their internal map along with the magnetic compass to find new shortcuts to a food reward. Again, once the magnetic field was unexpectedly turned by 90°, they were much less likely to discover a shortcut.

The mole rats lost their way less often under the altered magnetic field when they only had to travel short distances, noted Kimchi, suggesting that they switch to using the Earth's magnetic field as a reference point when navigating over long distances.

Rat Radar

While some birds, fish, amphibians, insects, and several other rodents can use the Earth's magnetic field to assess direction when they begin an excursion, the blind mole rat is the first animal to be shown to use it to regularly correct errors as it travels.

"The change in navigation strategy [over long distances] is very interesting indeed," agreed Roswitha Wiltschko, behavioral physiologist at the J.W. Goethe University in Frankfurt, Germany. Though she says that the idea that mole rats switch from an internal map to an external reference such as the Earth's magnetic field has been suggested before but not tested.

Another study by Kimchi and Terkel, published in the November 2003 issue of the science journal Animal Behaviour, shows that blind mole rats have additional radar-like abilities to detect obstacles before they come into contact with them. Another talent human hikers would kill for.

The pair found that when they blocked wild mole rats' tunnels, the animals carefully dug out the shortest route around the obstacles to reconnect them. Furthermore, they left a safe margin of 10 to 20 centimeters (4 to 8 inches) when the obstacle was a hollow ditch, but closely followed the shape of solid concrete obstacles by just 3 to 8 centimeters (1 to 3 inches). Ingeniously, when an obstacle was placed asymmetrically across the tunnel, the mole rats always detoured it on the shorter side.

The scientists believe that the animals could be using seismic waves generated by banging their heads against the earth, much like radar, to detect any dangers that lie ahead. "It's totally clear that mole rats drum their heads to communicate and have a good sense of hearing low frequency sounds," commented Pavel Nemec, zoologist at Charles University in Prague, the Czech Republic. "And it is quite possible that they use vibrations to test proximal surroundings."

"It is amazing that mole rats live in a permanently dark environment and are able to navigate so well," added Nemec. "Other animals use sight to correct their mistakes. Mole rats use the Earth's magnetic field."
 

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