Human beings perceive the world around us with five senses: sight, hearing, taste, touch and smell; But other animals, such as migratory birds, can also ‘feel’ the Earth’s magnetic field.
For some time now, a group of biologists, chemists and physicists from the universities of Oxford (United Kingdom) and Oldenburg (Germany) have accumulated evidence that the magnetic sense of nocturnal migratory birds, such as the european robin (Erithacus rubecula), is based on cryptochromes, a type of light-sensitive protein in the eye.
Now the same team presents in the journal Nature a study showing that one of these photoreceptor proteins located in the retina of birds, the cryptochrome 4 (CRY4), is sensitive to magnetic fields, so it could well be the much sought after magnetic sensor of these animals.
Cryptochrome 4, a photoreceptor protein found in the eyes of robins and other birds, is sensitive to magnetic fields, so it could be the much sought after magnetic sensor that aids them in their migrations.
Furthermore, they have found that the CRY4 of robins is more sensitive to magnetism than that of chickens and non-migratory pigeons, further supporting its role as a magnetic sensor.
The teacher’s group Henrik Mouritsen In Oldenburg, he first succeeded in extracting the genetic code for cryptochrome 4 in these small songbirds and then produced the protein in large quantities using bacterial cell cultures of E. coli.
Subsequently, researchers from the Chemistry department at the University of Oxford applied a novel range of magnetic resonance and optical techniques to study this protein, thus demonstrating its marked sensitivity to magnetic fields.
The authors also shed light on the mechanism by which this sensitivity arises, based on reactions of electron transfer triggered by the absorption of blue light. Proteins like cryptochrome are made up of chains of amino acids. Specifically, the robin’s cryptochrome 4 has 527.
Quantum Mechanics in Action
The chemist Peter hore Oxford and the physicist Ilia Solov’yov from Oldenburg performed quantum mechanical calculations that support the idea that four of the 527 amino acids – of a type known as tryptophan– are essential for the magnetic properties of the molecule. According to his calculations, the electrons jump from one tryptophan to another generating the so-called radical pairs, which are magnetically sensitive.
Quantum mechanical calculations support the idea that four of the 527 amino acids of this protein are essential to endow it with its magnetic properties
To test this experimentally, the German team produced slightly modified versions of the robin’s cryptochrome, in which each of the tryptophans was replaced with a different amino acid to block the movement of electrons.
For their part, using the modified proteins, the Oxford groups were able to elucidate the role of different pairs of radicals in the observed effects of the magnetic field.
Thus it has been discovered that CRY4 has the potential magnetic properties necessary to act as a magnetic compass. The protein has a light-driven chemical reaction that triggers quantum effects that could amplify magnetic signals.
Although robins migrate at night, bearing this light-sensitive eye protein, birds normally fly above cloud level and therefore have access to starlight
Regarding the fact that robins and other birds migrate at night when there is no sunlight, Hore explains to SINC. “The night is never totally dark. If it were, the birds would not be able to see and would not fly. We still can’t say for sure if the cryptochrome mechanism is sensitive enough to work in low-light conditions at night. What we do know is that birds normally fly above cloud level and therefore should always have access to starlight. “
For his part, Mouritsen highlights: “We believe that these results are very important because they show, for the first time, that a molecule of the visual apparatus of a migratory bird is sensitive to magnetic fields”. But this is not, the team stresses, definitive proof that cryptochrome 4 is the magnetic sensor.
Future in vivo experiments
So far, in all the experiments the researchers have examined isolated proteins in the laboratory and applied magnetic fields stronger than Earth’s. “Therefore, it remains to be shown that this occurs in the eyes of birds,” warns Mouritsen, and, at the moment, these studies are not technically possible.
If we can prove that cryptochrome 4 is the magnetic sensor, we will have demonstrated a quantum mechanism that makes animals sensitive to environmental stimuli a million times weaker than previously thought possible.
Peter Hore (U. of Oxford)
However, the authors believe that the proteins involved could be much more sensitive in their native environment. In retinal cells, proteins are probably fixed and aligned, which increases their sensitivity to the direction of the magnetic field. In addition, it is likely that they are also associated with others that could amplify sensory signals.
Hore points out the following steps: “Cryptochrome experiments with these interaction partners in an attempt to study conditions more similar to in vivo, measurements of the effects of the magnetic field in oriented proteins to investigate their operation as directional sensors and development of capable techniques to measure the activity of cryptochromes in photoreceptor cells in vivo “.
“If we can prove that cryptochrome 4 is the magnetic sensor, we will have demonstrated a fundamentally quantum mechanism that makes animals sensitive to environmental stimuli a million times weaker than previously thought possible,” concludes the Oxford chemist.
Jingjing Xu et al. “Magnetic sensitivity of cryptochrome 4 from a migratory songbird”. Nature, 2021.
Rights: Creative Commons.