How do birds sense the Earth’s magnetic field?
We may have finally cracked a question that has been with us for a long time- how do birds sense the Earth’s magnetic field? Researchers from the University of Oldenburg and Oxford have demonstrated that cryptochrome 4, found in the retina of migratory European robins is sensitive to magnetic fields, and could potentially be the answer to how birds navigate.
It has been known for a while that the ability to sense magnetic fields lies in the eye because magnetic sensing is affected in the presence of different wavelengths of light. It has also been shown that when birds use their internal compass, the information is processed in the same part of the brain that processes vision.
Cryptochromes are a type of protein called flavoprotein that bind to chromophores like FAD, and absorb photons of blue light when oxidized. Cryptochrome 4 (CRY4) was thought to be the prime suspect involved, as it is present in the cone cells of the retina and reacts to magnetic fields. Moreover, CRY4 has been found only in animals with well documented magnetically guided migratory behaviour like birds and fish. Furthermore, the levels of CRY4 expression in migratory robin retinas increase with the advent of the migratory season, compared to no increase in non-migratory chickens, suggesting a possible role in magneto-perception.
The researchers in this study (Xu et al) were able to clone the gene coding for cryptochrome 4 into bacteria, produce it in large quantities, and then study the magnetic sensitivity of the protein using various spectroscopic techniques. More importantly, the researchers were also able to decipher the mechanism by which cryptochrome 4 senses magnetic fields.
Xu et al demonstrated that the high magnetic sensitivity of CRY4 was due to the formation of radical pairs. Cryptochrome 4 has 4 trypotophans essential for magnetic activity, and in the presence of blue light, radicals jump between the tryptophans of the protein onto FAD, leading to the production of a cryptochrome 4- FAD complex (CRY4-FAD). Radicals are molecules with an unpaired electron, and a radical pair consists of two radicals that are formed simultaneously. Radicals are magnetic, because of a property of electrons called ‘spin’, allowing unpaired electrons to act as microscopic magnets. If the spin of the radicals in the pair is antiparallel (↑↓), they are called a singlet state, and if they are parallel (↑↑), they are called a triplet state. Now, the radical pair can rapidly switch between the singlet state and the triplet state, and both states will give rise to the production of a reaction product — Cryptochrome 4-FAD complex (CRY4-FAD). However, the singlet state can also revert back to the original unexcited state, thus not forming the CRY4-FAD complex. (Now as to why this interconversion happens, all I can say is that it is due to a quirk of quantum physics. Researching the explanation led me down a rabbit hole of quantum physics that left me with a headache and more confused at the end of it. I’ll link a couple of articles, here and here.) The amount of time spent between the triplet and singlet state, and thus the subsequent yield of the reaction product is directly influenced by the direction of Earth’s magnetic field. The amounts of the CRY4-FAD affect the output of the retinal cone cells. This would result in the view of the birds being lighter or darker, depending on the strength and direction of the Earth’s magnetic field.
The researchers also compared the CRY4 of non-migratory birds- chickens and pigeons. They found that the cryptochrome 4 of these non-migratory birds were less magnetically sensitive under identical in vitro conditions, suggesting the molecule was optimized in migratory birds to amplify its sensitivity.
However, these studies were only conducted in vitro, and the molecule should ideally be studied inside the bird. That being said, Xu et al have brought us extremely close to answering the holy grail of sensory biology- how do birds navigate using the Earth’s magnetic field. The answer? Quantum physics evidently. Whoever said biologists shouldn’t have to study quantum physics obviously hasn’t met the migratory European robin. I’ll leave you with a picture of this dorky lil, physics lovin’ birb below.
