It has been debated for nearly four decades but no one has yet been able to prove it is chemically possible. Now good evidence suggests that birds can actually “see” the lines of the Earth’s magnetic field.

Klaus Schulten of the University of Illinois, proposed forty years ago that some animals – including migratory birds – must have molecules in their eyes or brains which respond to magnetism. The problem has been that no one has been able to find a chemical sensitive enough to be influenced by Earth’s weak geomagnetic field.

Now Peter Hore and colleagues at the University of Oxford have found one.

Cryptochromes are a class of light-sensitive proteins found in plants and animals, and are thought to play a role in the circadian clock, in regulating plant growth, and timing coral sex. A few years ago, Henrik Mouritsen of the University of Oldenburg in Germany showed that they were present in the retinal neurons of migratory garden warblers, and that these cells were active at dusk, when the warblers were performing magnetic orientation.


Weak but radical

Cryptochromes have not yet been made in the lab and obtaining them is difficult, but Hore’s team has now shown that a related molecule – a carotenoid-porphyrin-fullerene triad – with similar chemical properties to cryptochromes is sensitive to weak magnetism.

Like cryptochromes, the CPF molecule, made by Devens Gust of Arizona State University, US, is stimulated by light of specific wavelengths to produce two free radicals. Hore found that he could control the concentrations of each free radical in a solution of CPF by applying a magnetic field.

Hore says most chemists would have predicted that the extremely weak magnetic field could not possibly have an effect on molecules because the electromagnetism would be completely swamped by the molecules’ inherent energy. “They are just tiny, tiny perturbations,” he says.

But how could this influence the direction taken by a migrating bird? Birds appear to orientate at dusk, and cryptochromes form their pair of free radicals when “activated” by the blue light typical of dusk.

Hore suggests that dusk might activate the birds’ magnetic sense, producing the radical pair. The concentrations of each free radical would be controlled by the Earth’s magnetic field, which is known to vary with latitude. As a result, he speculates, the radicals would bind in varying degrees with other signalling molecules, depending on how far north or south the animal is.

Between the lines

How birds decode their “magnetic sense” is another topic of debate. Mouritsen believes they have an additional layer to their vision, which when switched on allows them to visually “see” the Earth’s magnetic field. The situation would be similar to “head-up displays” in fighter jets and some cars, where transparent screens displaying information are built into windscreens.

“Having that on all the time would be distracting, so you can see why it would be desirable for the system to switch on and off,” says Hore.

Thorsten Ritz, of the University of California, Irvine, welcomes this “first-ever demonstration of an earth-strength magnetic field effect on photochemical reactions”. “It beautifully matches the theoretical expectations,” he says.

Another group suggests that a quantum effect could underlie the birds’ magnetic sense.

Journal reference: Nature (DOI: 10.1038/nature06834)