SCOTT DETROW, HOST:
Homing pigeons rely on a variety of signals to navigate the skies, including magnetism. But it's never been quite clear how exactly the birds detect magnetic cues. A team of researchers proposes the answer might be found in a pretty unexpected place. Here's science reporter Ari Daniel.
ARI DANIEL: Drop a homing pigeon somewhere it's never been, and the bird can flap its way back home.
MARTIN WIKELSKI: You have to provide them actually a good home so that they want to return, and that means good food and nice, cozy housing.
DANIEL: To fly home, a pigeon has to figure out where it is and in which direction to go, says Martin Wikelski, who directs the Max Planck Institute of Animal Behavior in Germany. They rely on odor cues, the sun, visual landmarks and magnetic direction, which is using Earth's magnetic field to orient and navigate.
WIKELSKI: It would be as if you were put into a forest where you know my home is east. And then you have a compass. You just go east. You end up at your cabin.
DANIEL: Pigeons rely on magnetic direction under certain circumstances.
WIKELSKI: That is, either at night or during completely overcast conditions.
DANIEL: For decades, researchers have struggled to explain how these pigeons sense magnetic direction. Then some years back, Wikelski was attending an interdisciplinary meeting where he met someone he might never have encountered otherwise - immunologist Christian Kurts from the University of Bonn in Germany.
CHRISTIAN KURTS: And in the coffee break, we talked to each other. Show me one scientist who doesn't like talking about their research (laughter).
DANIEL: Kurts told him he'd been studying immune cells called macrophages when he stumbled upon something surprising. In the spleens of mice, these cells responded to a magnetic field.
KURTS: One of the jobs of these cells is to eat and degrade old red blood cells. Our red blood cells contain hemoglobin.
DANIEL: And hemoglobin contains iron. This bundle of iron atoms gives those immune cells magnetic properties, and the researchers wondered if something similar might be true for homing pigeons. So they examined cells from across the bird.
CLIVIA LISOWSKI: And then we figured out that it's actually the liver who has the strongest magnetic properties.
DANIEL: Clivia Lisowski is a cell biologist at the University of Bonn. Turns out the same kind of macrophages in the livers of homing pigeons also store oodles of iron atoms.
LISOWSKI: And then the next question was, do they really play a role in animal navigation?
DANIEL: To answer that, the researchers traveled to the south of Germany, near the Swiss border, with 34 pigeons that would need to fly 12 miles to reach home. In a little more than half the birds, the team used a drug to temporarily deplete those iron-containing macrophages when it was completely overcast, so no directional cues from the sun, says Martin Wikelski.
WIKELSKI: That's the time when the pigeons have to rely upon their magnetic sense to find the direction. And the ones without the macrophages were basically lost.
DANIEL: But the ones with the macrophages could do it. And when the sun came out, the birds that had been lost managed to find their way home.
WIKELSKI: And then I thought, well, my God, this could really be real.
DANIEL: In particular, these macrophages may be acting as a kind of internal compass that can detect magnetic direction. The findings are published in the journal Science.
CATHERINE LOHMANN: I haven't yet seen the smoking gun that says this is the way they do it.
DANIEL: Catherine Lohmann is a behavioral biologist at UNC-Chapel Hill who wasn't involved in the study. The authors do present microscope images showing the immune cells sitting alongside nerve fibers, but Lohmann isn't sure how a magnetic signal would then be relayed to the bird brain.
LOHMANN: There's no mechanism that I know of for macrophages communicating with the nervous system.
DANIEL: Still, she's open to the possibility. And the researchers are looking forward to the experiments needed to really clinch their new theory.
KURTS: One thing I have learned in many years of research - you should never be too certain that you are right. Nature's always more complicated than you think.
DANIEL: For NPR News, I'm Ari Daniel.
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