Vision, hearing, taste, smell, and touch are the five senses that humans use to perceive the world around them. Many other creatures can detect the Earth’s magnetic field as well. For some time, chemists, biologists, and physicists located at Oxford (UK) and the Universities of Oldenburg (Germany) have been amassing evidence that migratory birds like European robins have a magnetic sense based on a special light-sensitive protein in the eye. This team shows that the protein cryptochrome 4, which is found in the retinas of night-migratory songbirds, is sensitive to magnetic fields and could be the long-awaited magnetic sensor in the issue of Nature.
Another significant breakthrough was the discovery of the mechanism that causes this sensitivity. CRY4’s magnetic detecting abilities are thought to be activated when blue light hits the protein, according to scientists. This light triggers a chain of events that shuttle one electron around the protein, resulting in two unpaired electrons in different parts. Because of a quantum feature of electrons called spin, those lone electrons behave like tiny magnets.
Cryptochrome proteins are made up of chains of amino acids, with 527 in robin cryptochrome 4. Oxford physicist Peter Hore and Oldenburg physicist Ilia Solov’yov used quantum mechanical calculations to support the theory that four of the 527 proteins, known as tryptophans, are required for the molecule’s magnetic properties. According to their estimates, electrons bounce from one tryptophan to the next, forming magnetically sensitive radical pairs. To demonstrate this experimentally, the Oldenburg team created radically updated versions of the robin cryptochrome, in which each tryptophan was swapped by a different amino acid to prevent electrons from moving.
The Oxford chemistry teams were able to experimentally prove that electrons move within the cryptochrome as expected in the predictions — and that the produced radical pairs are required to clarify the observed magnetic field effects — using these modified proteins.
The magnets of the two electrons might either point in the same direction or different directions. The electrons, however, do not settle on either arrangement, according to quantum physics. Instead, they exist in a state known as quantum superposition, which merely describes the likelihood of finding electrons in either arrangement.
Cryptochrome 4 was also produced in chickens and pigeons by the Oldenburg team. The magnetic field did not influence CRY4 detected in nonmigratory chickens and pigeons, according to the researchers. According to biophysicist Thorsten Ritz of the University of California, Irvine, a migratory bird’s higher sensitivity to the magnetic field in CRY4 might suggest that maybe there is actually something unusual about the cryptochromes of migratory birds that use this for a compass.
The proteins of these non-migratory birds show similar photochemistry to that of the migratory robin when tested in Oxford, but are far less magnetically sensitive. However, Ritz and Muheim also point out that laboratory testing with chickens and pigeons has proved that these animals can detect magnetic fields. It’s unclear whether the robin CRY4’s enhanced sensitivity in lab testing is due to evolutionary pressure for migratory birds to have a better magnetic sensor.
The fact that tests on isolated proteins do not match the circumstances in birds’ eyes makes interpretation of the data more difficult. Scientists believe the proteins in the retina, for example, are aligned in one direction, according to Xu. Future experiments on real retinas will be conducted to gain a literal birds-eye perspective of the process, the researchers hope.
Mouritsen explained that these findings are significant because they indicate for the first time that a molecule from a migratory bird’s visual apparatus is responsive to magnetic fields. However, he points out that this isn’t conclusive evidence that cryptochrome 4 is the magnetic sensor the team is seeking for. The researchers looked at isolated proteins in the lab in all of their research. The magnetic fields utilized were also more powerful than the magnetic field of the Earth. “It therefore still needs to be shown that this is happening in the eyes of birds,” Mouritsen emphasizes. Technically, such research is not possible at this time.
The authors believe that proteins in their native environment can be substantially more responsive. Protein is likely to be fixed and aligned in the cells of the retina, enhancing its sensitivity to the magnetic field. Furthermore, they are likely to be linked to other proteins that could increase sensory inputs. These still undiscovered interaction partners are currently being sought by the team.
Hore adds that If he and his team can demonstrate that Cryptochromium 4 is the magnetic sensor, they will have shown an essentially quantum mechanism that makes animals a million times weaker than before sensitive to environmental inputs.
Xu, J., Jarocha, L.E., Zollitsch, T. et al. Magnetic sensitivity of cryptochrome 4 from a migratory songbird. Nature 594, 535–540 (2021). https://doi.org/10.1038/s41586-021-03618-9