Testing Bat Senses with Magnets and Darkness

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Earlier this year, scientists went into a dark barn in New Jersey, trapped some bats, and stuck them next a magnetic coil. Then, they tagged the flying mammals with a tiny electronic tag, and freed them — 20 miles from their home roost. The researchers then watched to see if the bats could find their way home. The goal: to see if bats can use invisible magnetic fields to navigate in the dark. NPR's John Nielsen has the answer.

MELISSA BLOCK, Host:

Bats are famously good at finding their way home after long nights hunting insects. How they do it has been a mystery. But now a group of scientists says bats maybe using the tug of the earth's magnetic field as a navigational aid.

Here's NPR's John Nielsen.

JOHN NIELSEN: Big brown bats make lots of noise when they're flying around inside pitch black barns like this one in Central New Jersey.

(SOUNDBITE OF BATS)

NIELSEN: They get even louder when Princeton ecologist Richard Holland catches one of them in a net, scrapes the hair off the back of its neck and glues a small radio transmitter to the bald spot.

D: This one is quite really aggressive.

NIELSEN: Holland tries to calm this angry bat by giving it some tasty mealworms. But fellow ecologist Martin Wikelski says it's not going to work.

D: And they can shout and eat at the same time.

NIELSEN: Scientists like Wikelski and Holland know that bats use squeaking sounds as a kind of sonar when they're down near the ground. That's why bats don't smash into barn walls, the insides of caves and so on. What the experts didn't know was how bats navigate when they're way up in the air and a long way from home.

Holland and Wikelski say they had a hunch that bats use the earth's magnetic field as a compass. So last spring, they started testing that hunch by catching angry bats and sticking them into big glass jars. Then, they stuck the jars into something called a Helmholtz coil. It's a device that produces magnetic fields. If these bats really did have magnetic compasses inside them, the compasses would be messed up by the coil. Holland says this change could make the bats think north was south or vice versa.

D: And then we took them 20 miles north of the barn where we caught them and released them. First of all, to find out whether they could work out that they've been displaced 20 miles north and would they want to go back home to their barn.

NIELSEN: The scientists followed the displaced bats in a plane that was equipped to pick up signals sent out by the beepers. They also chased them with a car, which wasn't easy.

D: We were going up and down in the interstate. We were driving through housing estates full of New Jersey McMansions, around schools, around airports, so there were lots of things in the way.

NIELSEN: What they saw was proof that bats really do have a magnetic compass. Holland says the bats that had been dropped into the coil all flew off in the wrong direction for a couple of hours. Then, when the effects of the coil wore off, the bats seemed to wake up from a dream.

BLOCK: Some of them at least actually seemed to - that they recognized that they were flying in the wrong direction after a journey of anywhere between maybe five and 15 kilometers depending on the bat. And then would actually start to correct, and some of them actually got back in the same night.

NIELSEN: Holland says he's certain that bats use lots of other cues to find their way around. For example, they might remember mountains or other landmarks. But ecologist Verner Bingman of Bowling Green State University says he's certain that Holland's study, published in this week's Nature, is sure to draw a lot of attention.

D: It's the first field demonstration of bats being sensitive to the earth's magnetic field. That's groundbreaking. The data are modest and there's huge numbers there, but they're quite compelling.

NIELSEN: Ecologist Roland Kays of the New York State Museum hopes this study will also draw attention to the technologies that made it possible. Tracking tags that are now small enough to stick to the backs of angry bats or even insects.

D: As people start to realize that tags are this small and (unintelligible), they realize there's always questions that they've been dying to ask in the back of their mind but never thought were even possible.

NIELSEN: For example, how do small birds and animals help move diseases like the West Nile virus around or where the migrating butterflies rest at the end of the day or how many other species of bats have magnetic compasses inside them?

John Nielsen, NPR News, Washington.

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