We may finally know the secret of how migratory birds can sense Earth’s magnetic fields: a molecule in their eyes called cryptochrome 4, which is magnetically sensitive and may give the animals an internal compass.
The process can cause the animals to see darker or lighter areas in their field of vision when looking in the direction of the magnetic field lines, says Henrik Mouritsen of the University of Oldenburg in Germany. “You might be able to see where north is as a kind of shading over what you would otherwise see.”
Previous work has shown that certain bird species such as the robin (Erithacus rubecula), use the Earth’s magnetic fields as they migrate, as well as visual and other cues. Some robins migrate south in the northern hemisphere each winter, for example from Scandinavia to Britain, and return in the spring.
It is believed that at least part of this ability resides in their eyes, as their perception of magnetism is impaired in the absence of light. Mouritsen has previously shown that when birds use 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-sensing cells and has a structure that suggests it can be affected by magnetic fields. Now Mouritsen and his colleagues have shown in the laboratory how the molecule reacts to magnetic fields.
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 CRY4-FADH* levels may mean that light-sensitive cells in the eye could change their output – making the vision brighter or darker – depending on the direction and strength of the magnetic field in the bird’s field of vision. says Mouritsen.
The team also studied cryptochrome 4 from chickens and pigeons, which do not 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 enhance its sensitivity.
But the group has yet to show that Cryptochrome 4 is used for magnetic sensors 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 of Goethe University Frankfurt in Germany says the case is ongoing because there are other cryptochrome molecules in the eye that could also be responsible for magnetic perception. “In principle, most cryptochromes would be able to do this,” she says.
And while pigeons don’t migrate, they’ve been found to be able to navigate using magnetism, suggesting other cryptochrome molecules might play a role, she says.