Some British robins, not all of which migrate, fly up to 200 miles a night as far as southern Spain or Portugal to spend the winter. Just like humans use compasses, these birds can feel the earth’s tiny magnetic field. But how they do this remains uncertain.
The idea that radical pair reactions could be the basis of the magnetic field sensitivity of birds was first proposed in 1978 by the physicist Klaus Schulten. Radical pairs are field sensitive because the unpaired electron of each radical has a magnetic moment that results from its spin. The pair has a singlet state in which the two spins are antiparallel and a triplet state in which they are parallel. A reaction in a magnetic field can influence the intermediate stages of the radical pairs and thereby change the product distribution.
The research community began to draw attention to Schulten’s suggestion in 2000 after he and his colleague Thorsten Ritz suggested that cryptochromes could be responsible for the magnetoreception of birds. In vertebrates, for example, cryptochromes play a role in maintaining a 24-hour clock cycle. In other organisms they enter into photochemical radical pair reactions – this is how they regulate the growth of plants.
Now researchers in the UK, Germany and China have for the first time detected a radical pair reaction in laboratory-synthesized cryptochrome 4, a protein found in the eyes of night-migratory European robins. The cryptochrome binds the oxidized form of a flavin adenine dinucleotide (FAD) cofactor. When exposed to blue light, FAD can be reduced to a radical, which then forms a radical pair. It can then either be reoxidized or undergo another reaction – the relative probability of this depends on the spin state of the radical pair.
The product of this second response could be a “signal state” that stimulates neurons in the birds’ brains. “The net result is that as soon as all radicals have either reacted back to signal state or passed into signal state, you get a lot of the signal state of the protein that depends on the presence, strength and direction of the field during the lifetime of the radical pairs,” explains the physical chemist Peter Hore of Oxford University, one of the researchers who led the study, cryptochrome 4 found in non-migrating pigeons and chickens did not show this magnetic field-dependent response.
“The cryptochromes appear to be in the cone cells, which are mainly used for color vision – and birds like humans see no color in poor light,” says Hore. “These are nocturnal migratory birds, so it is possible that the visual signaling pathways will be taken over by the magnetic signals at night when the birds have to use their compass. This is just a speculation we’re investigating, but it is a very attractive idea that the birds literally see the magnetic field. “
Eric Warrant from Lund University in Sweden is impressed. ‘The [researchers] have shown that three of the biophysical properties of cryptochrome that enable it to function as a magnetosensory molecule are actually fulfilled, ”he says. “The first thing they have shown very clearly is that the radical pair in the robin is very magnetically sensitive … The second is that the reaction products are formed in sufficient yield to be realistically useful as signaling substances, and third,” that they are be produced long enough – at least on the order of milliseconds – to be able to activate a neuron … It has not yet been proven [the mechanism] entirely, but it really got us very, very close. ‘
Behavioral biologist Roswitha Wiltschko from Goethe University Frankfurt is skeptical, however. As a proponent of the cryptochrome radical pair hypothesis, she argues that several aspects of the researchers’ cryptochrome-4 mechanism are inconsistent with studies in live birds. For example, she says, it takes blue light, and yet birds can perceive magnetic fields in narrow-band green light. “I have nothing against the study itself, that’s probably fine,” she says. “But the interpretation that this is now the magnetosensor is definitely incorrect.” Instead, she advocates an alternative hypothesis with cryptochrome 1.
Researchers hope live animal testing is possible, but it will be difficult, Hore says, because wandering robins cannot be bred in captivity.