In late summer, the swallows of northern Europe get restless. They gather in twos and threes, then clusters of a dozen or more. Then – suddenly – they’re gone, winging south with an unerringly accurate sense of direction.
That birds, and many other migratory animals, navigate by the Earth’s magnetic field is clear. But how they do this is not understood, despite decades of investigation. In the latest twist, Can Xie and his colleagues at Peking University in Beijing, describe a candidate protein, found in migratory birds’ eyes, that help birds to ‘see’ south. They report their discovery in Nature Materials in November.
“It’s a really cool result – a really interesting molecule,” says Sönke Johnsen, a biologist at Duke University. But so far, he adds, Xie has only studied the protein’s magnetic sensitivity in a test tube. “The jury is still out on whether this occurs in any animal.”
Half a century ago, behavioural scientists discovered birds could see what to us is invisible – the Earth’s magnetic field. Migratory birds such as European robins, trapped in cages, aligned their bodies with their migratory path, swapping direction if the magnetic field was artificially reversed.
Initially, researchers focused on magnetite – a magnetic mineral found in traces in the beak of certain migratory birds. Then, in the 1990s, researchers discovered the sense of magnetism was linked to light perception – several species of birds can only detect magnetic fields in ultraviolet to green wavelengths.
“Perhaps they see some kind of pattern superimposed on their normal vision, a certain shadow,” says Klaus Schulten, a biophysicist at the University of Illinois in Urbana.
It was Schulten who, in 2000, first suggested a role for cryptochromes – proteins found in the retinas of many migratory birds. Photosensitive molecules, such as cryptochromes, spit out electrons in response to light, and Schulten had noticed subtle differences in this response when he altered the magnetic field. What’s more, fruit flies modified to lack cryptochrome no longer sense magnetic fields.
Xie reasoned that if cryptochromes were involved they would need help from a second molecule, most likely one that contained iron – one of the most magnetic materials on Earth, and the stuff of compass needles. To discover what that molecule might be, his team searched the genome of fruit flies for proteins that both bound iron and latched on to cryptochromes. The genes also had to be turned on in the same cells where the cryptochrome gene was turned on.
They pulled out several candidate molecules. But in tests one out-performed the rest: they called it “MagR”.
MagR is in bird retinas, the researchers discovered. In a test tube, it combines with cryptochromes to form crystals that are so magnetic they stuck to the researchers metal tools. If a magnet is rotated in their vicinity, the crystals rotate neatly in response.
Finally, electron microscopy revealed the structure of those crystals: rods containing neat rows of iron atoms, like a nano-sized compass needle.
The field of magnetoreception is littered with false starts, and Xie is the first to admit that how these needles might work – including the role of the photosensitive component – is still unknown.
But Schulten for one is keen to see if the cryptochrome-MagR behaviour is replicated in the eye of a living animal. “What are they doing?” he says. “Let’s find out.”