Scientists have long known that the birds navigate
using the earth’s magnetic field. Now, a new study has found subtle
mechanics in the brain of pigeons that allow them to find their way.
A
team at Baylor College of Medicine in the U.S. identified a group of 53
cells in a pigeon’s brain that record detailed information on the
Earth’s magnetic field, a kind of internal global positioning system
(GPS).
However, the study, published in journal
Science, leaves open the question of how these “GPS neurons” actually
help the birds sense the magnetic field.
“People had
reported in the past, establishing that birds do not seem to respond to
the polarity of the magnetic field, yet here we have neurons that are in
fact doing that,” study author Prof. David Dickman said.
“That’s
one of the beautiful aspects of what we have identified, because it
shows how single brain cells can record multiple properties or complex
qualities in a simple way,” he told BBC News.
For
their study, Prof. Dickman and his colleague Le-Qing Wu set up an
experiment in which pigeons were held in a dark 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
they adjusted the elevation angles and magnitude of their artificial
magnetic field, they simultaneously recorded the activity of the 53
neurons in the pigeons’ brain which had already been identified as
candidates for such sensors.
So, they measured the
electrical signals from each one as the field was changed and found that
every neuron had its own characteristic response to the magnetic field,
each giving a sort of 3-D compass reading along the familiar
north-south directions as well as pointing directly upward or downward.
In
life, this could help the bird determine not only its heading just as a
compass does, but would also reveal its approximate position, the
researchers said.
Each cell also showed a sensitivity
to field strength, with the maximum sensitivity corresponding to the
strength of the Earth’s natural field, they added.
And
like a compass, the neurons had opposite responses to different field
“polarity”, the magnetic north and south of a field, that surprised the
researchers most of all.
Several hypotheses hold that
birds’ magnetic navigation arises in cells that contain tiny chunks of
metal in their noses or beaks, or possibly in an inner ear organ.
However,
the most widely held among them was thrown into question when
researchers found that purported compass cells in pigeon beaks were in
fact a type of white blood cell.
Another theory
suggests that a magnetic sense may come about in receptors in birds’
eyes. When exposed to light, the theory says, molecules called
cryptochromes undergo a fleeting change in their atomic makeup whose
length depends on their alignment with a field.
The
new research throws this latter possibility also into question, as it
would work equally well with a north- or south-pointing field.
“We’re
leaning toward a third receptor in the inner ear, and we’re doing
experiments to try to determine whether it is in fact a receptor or
not,” said Prof. Dickman.
It’s now believed that more
than one mechanism may be at work in bird navigation — in their eyes,
beaks or ears —- and Prof. Dickman said he is looking forward to getting
to the bottom of it.