Homing neurons. Pigeons can navigate the globe by sensing Earth's magnetic fields, an ability controlled by a newly discovered set of brain cells. Credit: Zhuda/Shutterstock
by Sarah C. P. Williams on 26 April 2012, 1:16 AM
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 [ http://news.sciencemag.org/sciencenow/2009/11/12-02.html?ref=hp ] 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 [ http://www.nature.com/nature/journal/v484/n7394/full/nature11046.html ]—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 [ http://www.sciencemag.org/content/early/2012/04/25/science.1216567 ], 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."
Pigeons may ‘hear’ magnetic fields The physiology behind pigeons' remarkable navigational abilities has long been a source of debate. The science of birds’ magnetic sense grows more complicated. 26 April 2012 http://www.nature.com/news/pigeons-may-hear-magnetic-fields-1.10540 [no comments yet]
What’s behind the Bangui Magnetic Anomaly? Some experts see an ancient meteor strike
By Graeme Wood June 22, 2014
Until the last year, when the Central African Republic’s civil war became a humanitarian crisis too dire to ignore, most Americans thought little about the country at all. It has a low global profile in part because it is exceedingly poor, with four out of five people living on less than $2 a day. It has some natural resources, but because it is landlocked by other troubled countries—Chad, Sudan, Congo, and Cameroon—even if a lull in the war allowed it to extract those from the ground, it would still face formidable problems in exporting them.
But for one group, the Central African Republic is anything but ignorable, and in fact is home to an enduring scientific mystery. Geophysicists who map the earth’s magnetic fields have identified a disturbance in the earth’s natural magnetic fields within the Republic. They still have few clues about what causes it, but at least some think it could be key to understanding one of the most dramatic events in the history of the planet.
When geophysicists look at the globe, they don’t see national borders. Instead they see geological features like fault lines and tectonic plates, or, if they study geomagnetism, zones where the earth’s normal, needle-points-north magnetic field seems to go haywire. They map these anomalies by satellite and with ground surveys. When they look at the Central African Republic, something strange appears in the center of the country: a massive aberration known as the Bangui Magnetic Anomaly, named for the country’s capital. At 600 miles across, it is one of the largest such anomalies on earth.
“If you were on the ground there and you had a magnetic compass, you’d need to correct for it,” says Patrick T. Taylor, a NASA geophysicist who has studied the anomaly closely. “The compasses would go berserk.”
The earth’s magnetic field is generally consistent, which means hikers and sailors have been able to use it to avoid getting lost since the invention of the navigational compass a thousand years ago in China. Compasses point to magnetic north, a point in the Arctic near the geographic North Pole. But a handful of anomalies are large enough to alert scientists that something unusual is happening in the earth’s crust. When the anomalies can’t be explained, they serve to show just how poorly we understand certain details of earth’s underlying structure. (Arthur C. Clarke and Stanley Kubrick used this mystery to great effect in “2001: A Space Odyssey,” which featured a magnetic anomaly in a crater on the moon that turns out to have been planted by extraterrestrials as a beacon.)
Some large anomalies on earth have obvious origins: The Kursk anomaly in far western Russia is clearly the result of an iron deposit. Not so the Bangui anomaly. First identified 60 years ago by French mappers, it continues to perplex scientists. Iron is usually the explanation for a huge positive anomaly, Taylor says, and one theory explains the Bangui anomaly by proposing that the culprit is a large, dense upwelling of magnetic material, possibly iron-rich, from the earth’s mantle a few miles beneath the surface. But the Central African Republic also has a very negative gravity anomaly—things weigh less there—which suggests that the country is sitting on rock less dense than a huge upwelling of the mantle.
In 1991, Taylor, R. W. Girdler, and J. J. Frawley pointed out features of the Central African Republic that hinted at another origin, first proposed in 1976. They found that the surface of the country resembled a huge, shallow crater, with a circular rim that mapped closely onto the area with strange magnetism and gravity.
The center of the bull’s-eye, Taylor says, may be the site of the largest meteor strike still visible on the surface of the earth, from over a billion years ago. “When meteorites strike the earth, you get a crater and a rim,” he says. And at this size, this one would be “bigger than any other known impact, by an order of magnitude.” Under this theory, a huge meteor would have hit, heating up the impact site, making the iron there more magnetic, and leaving a great pock on the earth that hasn’t disappeared after a billion or more years of erosion.
“Every solid body in the solar system—every moon, every planet, even comets—are covered by impact structures, except for earth,” Taylor says. That’s because the earth has water and an active atmosphere that eventually wash away the evidence. “It’s like they’ve been erased.”
As a result, geologists have become sympathetic to the hypothesis that many geological features that are not obviously craters are, in fact, sites of ancient impacts. “We’ve been slow to recognize the number of craters on the earth,” says Raymond Jeanloz, a planetary geophysicist at the University of California at Berkeley. “We’ve probably overlooked a few dozen.” Jeanloz points out that the most famous such site, the Chicxulub site in the Yucatan peninsula, was found and explored only in the last 35 years or so. It is now thought to be the strike that killed the dinosaurs.
Jeanloz classifies the Bangui meteor hypothesis as “interesting but unproven.” Only a handful of geophysicists have examined the Bangui anomaly in any detail, and all of them seem to agree that more work would be necessary to advance the hypothesis.
One hint, proponents believe, might be found in one of the Central African Republic’s few exports. The country has a small diamond industry—its total annual product could fit in a single suitcase. But in addition to gemstones, it boasts a peculiar type of industrial diamond. Taylor and colleagues note that both the Central African Republic and Bahia, Brazil—areas that were once close to each other before plate tectonics shifted the continents into their present positions—produce “carbonados.” Most other diamonds are found in pipes of a rock called kimberlite. But carbonados seem to be scattered, and now Taylor and others believe they might be the remnants of a meteor made mostly of diamond. The carbonados could, Taylor says, be part of the “splatter” of the strike.
The simplest way to check this hypothesis would be to go rock-hunting in the Central African Republic. Taylor would like to visit the areas that look like the rim of the crater, and study rocks for signs that they were deformed in a meteor strike. “That would be the smoking gun.” But for the moment he’ll have to wait. The Central African Republic was never an easy country to do research in—it has few paved roads and no scheduled commercial domestic plane routes. Now, the war has made research almost unthinkable. “It’s very hard to get in there and do proper work,” Taylor says.
Even if Taylor makes it to the Central African Republic, he’ll be examining the coldest of geological cold cases. Over a billion years have passed since the event he thinks happened. And we simply have very little idea what a huge meteor crater looks like a billion years after the fact. “What do you look for, when you’re looking for ancient impacts?” Taylor asks. “We don’t really know.”
Graeme Wood is a contributing editor at The Atlantic.
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