Pigeons Create GPS from Earth's Magnetic Field
The birds have what amounts to an internal compass in their brain.
Pigeons may not look like a high tech device, but they can function like one.
Certain neurons in pigeon brains encode the direction and intensity of Earth's magnetic field, providing the common birds with an internal global positioning system, according to a new study.
The findings in the study, published in the latest Science Express, likely apply to other birds as well.
"We have found cells in the (pigeon) brain that signal the direction, intensity and polarity of an applied magnetic field," co-author David Dickman, a Baylor College of Medicine neuroscientist, told Discovery News. "These three qualities can be used by the brain to compute heading information, like a compass, and latitude on the Earth surface (location between the magnetic North and South Poles)."
"It is possible that magnetic intensity could also be used to give the bird longitude (East-West location) through learned associations of differing regional variations along the Earth surface," he added. "Together, these cells could form the basis of determining heading direction and position according to a brain representation of a magnetic Earth surface map."
For the study, Dickman and colleague Le-Qing Wu placed 7 pigeons in a pitch-black room and used a 3D coil system to cancel out the planet's natural geomagnetic field and generate a tunable, artificial magnetic field inside the room.
While the researchers adjusted the elevation angles and magnitude of their artificial magnetic field, they simultaneously recorded the activity of neurons in the pigeons' brains that had already been identified as candidates for processing such magnetic signals. The scientists identified 53 specific brain stem neurons that exhibited significant responses to changes in their artificial magnetic field.
Prior research found that magnetic receptors in the retina, nose, inner ear and possibly the beak of birds receive and interpret magnetic field information, which then goes to the brain for processing.
In addition to the findings applying to other birds, they could also apply to bacteria, honeybees, fish, turtles and even a few mammals, such as the blind mole rat. All of these organisms are documented as being able to sense and use the Earth's magnetic field.
In the case of pigeons, the ability allows them to travel hundreds of miles. Dickman reminded that "the ancient Romans used pigeons to carry messages home from their battles." Up to a certain point, the GPS is useful, but when honing in on a specific location, he believes pigeons rely more upon other orientation cues, such as vision and smell.
Humans, on the other hand, still have to ask for directions.
"It is not currently believed that humans possess a magnetic sense," Dickman said. "However, humans do have a very elaborate spatial mapping system that helps us navigate our daily routes, like finding the kitchen from our bedroom, or the grocery store when driving from home."
Most of the related brain activity comes from the hippocampus. When this is damaged, through things like inner ear problems or dementia, people may then have a diminished, or even completely absent, spatial orientation capacity.
The new study is receiving rave reviews from other experts in the field.
Kenneth Lohmann, a University of North Carolina professor of biology, told Discovery News that the study "is the most thorough investigation of the magnetic sense so far, in terms of neurobiological approaches, and it will no doubt inspire much additional work in the future."
John Phillips, another professor of biology, but at Virginia Tech, said, "The findings are convincing and a very important contribution to this field of research."
The biggest cheer, however, came from Joseph Kirschvink, a California Institute of Technology professor of geobiology. In his PhD thesis, published in 1981, he proposed that animal magnetoreception was based on biological magnetite. At the time, the idea was very controversial, but the new study suggests Kirschvink and co-author James Gould were right all along.
Now the mystery concerns where these magnetic cells are precisely located, how they are structured, and exactly how they work.
"Unlike most other sensory systems, the magnetoreceptor cells cannot be located close to each other in one specialized structure," Kirschvink said. "If 2 cells containing magnets were too close, the magnets would stick together!"