How does the mysterious sixth sense of migratory birds work? During their long-distance journeys, they use sensors in their eyes to orient themselves to the geomagnetic field, suggesting biophysical test results. These are special proteins in the retina that react to magnetic fields. This property of the so-called cryptochrome 4 is only particularly pronounced in migrating bird species such as the robin, according to the results.
Seeing, hearing, smelling, tasting and touching are the standard – but some animals still have a sixth sense: some birds, fish, turtles and even representatives of mammals show a sense of orientation on their long-distance journeys, which is apparently based on the perception of magnetic fields is based. For some time now, an interdisciplinary team of researchers from the Universities of Oldenburg and Oxford has been on the trail of the secret of this amazing sensory perception in migratory birds. Various indications have already suggested that the biological compass is located in the bird’s eye. Certain light-sensitive proteins in the animals’ retina were targeted as possible “compass needles”.
A potential magnetic sensor in sight
The confirmation of this presumption is now based on the decoding of the genetic code of a candidate protein from the robin retina. This enabled the researchers to produce this so-called cryptochrome 4 in large quantities in bacterial cultures. They were then able to investigate to what extent the biomolecule is sensitive to magnetic fields. Various methods were used, including magnetic resonance measurements and new spectroscopic methods.
The researchers were initially able to confirm: Cryptochrome 4 actually has physical properties that are required to function as “biological compass needles”. “Now it is no longer a guess that these biomolecules are magnetically sensitive, but we can see it,” says co-author Henrik Mouritsen from the University of Oldenburg. The team was also able to demonstrate the mechanism that underlies the reactions to magnetic fields. The protein shows a light-driven chemical change, which in turn mediates quantum effects with magnetic significance. “Electrons, which can move within the molecule after the system has been activated by light, play a decisive role here,” explains co-author Mouritsen.
Biophysical compass needles
The arrangement of the tryptophan building blocks of the protein is decisive for this, further studies have shown. The electrons hop from one tryptophan to the next, creating so-called radical pairs, which are magnetically sensitive. If the protein structure of the robin cryptochrome is changed only slightly, the movement of the electrons is blocked and the magnetic sensitivity is lost, the experiments showed. A further indication that the special versions of the cryptochrome 4 actually represent the long sought-after magnetic sensors for migration behavior, resulted in comparisons with the counterparts of species that do not belong to the migratory birds. For example, the studies of cryptochrome 4 in chickens showed a reaction to light, but these protein versions were found to be comparatively less sensitive to magnetic fields.
However, so far there has been no definitive proof that cryptochrome 4 is the magnetic sensor we are looking for, the scientists admit. “To do this, we would have to prove that this process also takes place in the eyes of birds,” emphasizes Mouritsen. In addition, the magnetic fields used in the experiments were stronger than the earth’s magnetic field. The authors assume, however, that the molecules react much more sensitively in their natural environment than under laboratory conditions. This is because the proteins in the cells of the retina are probably fixed and aligned in the same direction, which presumably increases their sensitivity to the direction of the magnetic field.
The scientists suspect that the sensory stimuli are further intensified by the interaction with other proteins in the retinal cells. The team is currently looking for these previously unknown partner molecules – so the researchers want to stay on the ball: “If we succeed in further evidence, we would show that this quantum physical mechanism makes animals sensitive to environmental stimuli that are six orders of magnitude below the threshold, which was previously considered perceptible, ”concludes Mouritsen.
Source: University of Oldenburg, specialist article: Nature, doi: 10.1038 / s41586-021-03618-9