London, UK: The question of how birds navigate, using magnetic fields, has always remained a big puzzle. Now, researchers at Baylor College of Medicine, USA, identified a group of 53 cells in a pigeon’s brain that respond to the direction and strength of the earth’s magnetic field. David Dickman of the Baylor College of Medicine set up an experiment in which pigeons were held in place, while the magnetic field around them was varied in its strength and direction.
While the researchers 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 3D 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.
Each cell also showed sensitivity to field strength, with the maximum sensitivity corresponding to the strength of the earth’s natural field. And just like a compass, the neurons had opposite responses to different field “polarity” – the magnetic north and south of a field, which surprised the researchers most of all.
However, the study, published in journal Science, leaves open the question of how these ‘GPS neurons’ actually help the birds sense the magnetic field.
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 recently when researchers found that purported compass cells in pigeon beaks were in fact a type of white blood cell.
Another theory suggested that a magnetic sense may come about in receptors in birds’ eyes. When exposed to light, the theory stated, molecules called cryptochromes undergo a fleeting change in their atomic makeup whose length depends on their alignment with a field.
The new work throws this latter possibility 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,” Prof Dickman said.