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| Pigeons in flight |
By James Gorman, The New York Times, April 26, 2012
Birds are famously
good navigators. Some migrate thousands of miles, flying day and night, even
when the stars are obscured. And for decades, scientists have known that one
navigational skill they employ is an ability to detect variations in the
earth’s magnetic field.
How this magnetic
sense works, however, has been frustratingly difficult to figure out.
Now, two researchers at Baylor College of Medicine, Le-Qing Wu and
David Dickman, have solved a central part of that puzzle, identifying cells in
a pigeon’s brain that record detailed information on the earth’s magnetic
field, a kind of biological compass.
“It’s a stunning piece of work,” David Keays of the Institute of
Molecular Pathology in Vienna wrote in an e-mail. “Wu and Dickman have found
cells in the pigeon brain that are tuned to specific directions of the magnetic
field.”
Their report appeared online in Science Express on Thursday. Kenneth Lohmann
at the University of North Carolina at Chapel Hill, who also studies magnetic
sensing, said in an e-mail that the study was “very exciting and important.”
Navigating by magnetism includes several steps. Birds have to have a
way to detect a magnetic field, and some part of the brain has to register that
information; it seems likely that another part of the brain then compares the
incoming information to a stored map.
The Baylor researchers have offered a solution to the middle step.
They identified a group of cells in the brainstem of pigeons that record both
the direction and the strength of the magnetic field. And they have good, but
not conclusive, evidence to suggest that the information these cells are
recording is coming from the bird’s inner ear. Dr. Dickman said this research
“is still something we want to pursue.”
They did not work on the third step, but Dr. Dickman said a good
candidate for the location of that map was the hippocampus, the brain region
involved in memory of locations in both birds and humans.
A well-known and often-mentioned study of London taxi drivers showed
that experienced drivers with a mental map of London had a hippocampus larger
in one area than people without their experience. In some birds that hide seeds
and return later to their caches with astonishing accuracy, the hippocampus
grows and shrinks seasonally, presumably as they map their hiding spots.
Efforts to understand the magnetic sense in birds have gone in several
directions. Some researchers have offered evidence for chemical reactions in
the eyes sensitive to magnetic signals, while others have looked at neurons in
the beak containing minute amounts of magnetite, a mineral that is affected by
magnetic fields.
Just a few weeks ago, Dr. Keays and colleagues reported in the journal Nature that the idea
of neurons in the beak was a nonstarter.
The Baylor researchers did a kind of step-by-step tracking of what
areas in pigeons’ brains were responding to variations in an artificial
magnetic field that they created. They focused on activity in the brainstem,
one of the most primitive parts of the brain, partly because in earlier work
they had shown that this area of the brain received signals from a part of the
inner ear.
By looking at specific neurons in this part of the brain, the
researchers found that the bird’s orientation determined which neurons were
active. Each neuron was tuned to respond to signals from one direction. The
neurons also registered the strength of the magnetic field.
Other brain regions are also active in response to magnetic
stimulation and may be involved in the magnetic sense, Dr. Dickman said. And
although he does not provide an answer to how birds detect magnetism, the
research clearly falls on one side of a debate over whether magnetite is
involved, or whether chemical reactions in the eye may be the key.
Dr. Keays said the research gave strong support to the magnetite idea
and the hypothesis that “a population of undiscovered magnetoreceptive cells
reside in the pigeon’s ear.”
As Dr. Lohmann said, the discovery “will no doubt inspire much
additional work in the future.”
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