Birds can see Earth's magnetic field

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. 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.