We may finally know the secret to how migrating birds can sense Earth’s magnetic fields: a molecule in their eyes called cryptochrome 4 that is sensitive to magnetism, potentially giving the animals an internal compass.
The process may result in the animals seeing darker or lighter areas in their vision when they look in the direction of magnetic field lines, says Henrik Mouritsen at the University of Oldenburg in Germany. “You may be able to see where north is as kind of a shading on whatever else you would be seeing.”
Previous work has shown that certain species of birds, such as the European robin (Erithacus rubecula), use Earth’s magnetic fields when they migrate, as well as using visual and other cues. Some European robins migrate south every northern hemisphere winter, for instance from Scandinavia to the UK, and return in spring.
At least part of this ability is thought to lie in their eyes, because their magnetism sensing is disturbed in the absence of light. Mouritsen has previously shown that when birds are using their internal compass, the information is processed in the same parts of the brain that process vision.
Suspicion had fallen on the cryptochrome 4 molecule because it is present in the eye’s light-detecting cells and has a structure that suggests it can be affected by magnetic fields. Now Mouritsen and his colleagues have shown how the molecule reacts to magnetic fields in the lab.
The team found that in the presence of light, electrons can jump between different parts of the molecule, and between it and another molecule called flavin adenine dinucleotide (FAD), ultimately leading to the production of a compound called CRY4-FADH*. The process is suppressed by weak magnetic fields.
Changes in the level of CRY4-FADH* potentially give a way that light-sensitive cells in the eye could alter their output – making the view lighter or darker – depending on the direction and strength of the magnetic field in the bird’s field of vision, says Mouritsen.
The team also looked at cryptochrome 4 from chickens and pigeons, which don’t migrate. Each species has a slightly different version of the molecule, and the team found that these two are less affected by magnetism, suggesting that the version of the molecule in migratory birds has been fine-tuned to amplify its sensitivity.
But the group hasn’t yet demonstrated that cryptochrome 4 is being used for magnetic sensing in real life. “We only looked at this molecule in isolation, we didn’t look at it inside a bird, which is extremely difficult,” says Mouritsen.
Roswitha Wiltschko at the Goethe University Frankfurt in Germany says the case isn’t yet closed because there are other cryptochrome molecules in the eye that could also be responsible for magnetic sensing. “Most cryptochromes would in principle be able to do this,” she says.
And while pigeons don’t migrate, they have been found to be able to navigate using magnetism, suggesting that other cryptochrome molecules may play a role, she says.
Journal reference: Nature, DOI: 10.1038/s41586-021-03618-9
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