Release a pigeon thousands of kilometers from home, and it'll fly across seas, forests, or deserts to return. It's not sight or smell that allows this amazing navigation; migratory birds can sense the magnetic fields that vary across Earth's surface. Now, scientists have identified a collection of brain cells that let pigeons interpret these magnetic fields. They hope the findings will help reveal how the birds sense the magnetism in the first place, and shed light on this mysterious sixth sense.
"This is very exciting," says biologist John Phillips of Virginia Polytechnic Institute and State University in Blacksburg, who was not involved in the new study. "There have been very few clear-cut findings in the past."
Debate on how birds sense geomagnetic fields has largely revolved around magnetite  particles found in various parts of their heads. Scientists have hypothesized that magnetite, a form of iron that's the most magnetic of naturally occurring minerals, is the key ingredient in specialized cells that react to changes in magnetism. And the presence of magnetite in birds' beaks had led some researchers to believe that this structure was key to birds' homing abilities.
But earlier this month, a team of scientists showed that the iron in birds' beaks isn't magnetite —it's balls of another, less magnetic, form of iron accumulated in white blood cells that are cleaning toxins out of the animals' bodies."That whole story just crashed and burned," says Phillips.
At Baylor College of Medicine (BCM) in Houston, Texas, biologist David Dickman had previously found magnetite in the inner ears of pigeons, offering an alternate hypothesis for where the magnet-sensing cells are located. Last year, he discovered that four areas of the brain that are largely linked to inner ear function each showed a broad change in activity when pigeons were exposed to magnetic stimulation.
In the new study, published online today in Science, Dickman and BCM biologist Le-Qing Wu placed seven homing pigeons (Columba livia) in a dark room in the center of a cube-shaped set of magnetic coils. As the cube was rotated, the intensity of the magnetic field felt by the pigeon in the center varied. The scientists turned it in every direction, testing out the effect of various magnetic fields found on Earth. As they did this, Dickman followed the activity of 329 neurons in one of the areas of the brain he'd previously implicated. Fifty-three of the brain cells showed significant changes in activity as the coils rotated , reacting to field strength and polarity. The properties of the neurons allow them to have a unique activity pattern for every different spot on Earth, the scientists realized. Not only can the neurons allow the pigeons to pinpoint their longitude and latitude, says Dickman, but they can differentiate the Northern Hemisphere from the Southern Hemisphere and tell the pigeons which direction they're facing.
The data don't reveal which cells detect the magnetic fields, but, when combined with Dickman's previous results, they suggest that the inner ear is key. Some scientists still hold that the magnetic sensing cells will be found in the beak, or in birds' eyes, but working backward from the brain will help sort it out, says Dickman. "We now have a tool to study this with. We can go back and ask what cells and organs are feeding into this circuitry."
The new findings could apply to other animals as well, says Phillips. Sea turtles, fish, and vertebrates including mice, cattle, and deer have been found to be sensitive to geomagnetic fields. But whether it applies directly to humans is unknown, he says. "There's no evidence for that now. But there could be some kind of unconscious magnetic sense that helps us sense direction and spatial orientation."