Biophysicists have found that cuticulosomes (small magnetic organelles in the inner ear of birds) are most likely not involved in the mechanism of magnetic reception. With the help of quantum magnetic microscopy, scientists have shown that the magnetic susceptibility of these particles is insufficient to transmit a signal through electromechanical channels. This means that cuticulosomes have a different physiological function in the body of birds, and other structures in the inner ear or on the retina are responsible for the susceptibility to the magnetic field, researchers write in the Proceedings of the National Academy of Sciences.
Many animals, such as birds, when traveling long distances, sense the earth’s magnetic field and use it for orientation in space. The presence of magnetoreception in birds is confirmed by numerous observations and behavioral studies, however, all manifestations of magnetoreception are rather weak, therefore, how exactly magnetic receptors work is still unclear.
Moreover, scientists not only do not fully understand the mechanism of the receptor, but do not even know exactly where it is located. Scientists believe that the main candidates for magnetoreceptors are cryptochromes in the retina (they contain pairs of correlated radicals with an unpaired electron), small inorganic magnetic particles in the inner ear (they create a torque in a magnetic field) and electroreceptors (they react to changes in the magnetic field due to the electromagnetic induction). Perhaps, in some cases, these systems can work in concert.
According to biologists, cuticulosomes, small spherical organelles several hundred nanometers in size, located in the hair cells of the vestibular apparatus, should be responsible for magnetoreception in the inner ear. These organelles mainly consist of particles of magnetic ferrihydrite, a crystalline form of iron oxide with inclusions of water molecules.
To investigate whether cuticolosomes could actually be part of the magnetic receptors in pigeons, biophysicists led by David A. Simpson of the University of Melbourne carefully studied their chemical composition, structure and magnetic properties using quantum magnetic microscopy. The researchers took tissue samples from two sections of the pigeon’s inner ear and looked at how the cuticulosomes in them respond to an external magnetic field ranging from 200 to 1400 gauss. For this, biophysicists used a magnetic sensor based on color centers in diamond. Having measured the energy difference between the ground and excited quantum states of the color center, the scientists calculated the value of the Zeeman splitting for each point of the analyzed slice, and based on this data, they built a map of the stray magnetic field in the cuticolosome.
It turned out that there are indeed magnetic areas in the iron-containing areas of the cuticulosomes. According to the measurement data, these are superparamagnetic and ferrimagnetic ferrihydrite inclusions. The obtained values of the magnetic susceptibility of all these areas were within the interval between 0.029 and 0.22. This is an order of magnitude less than, for example, magnetite – iron oxide Fe3O4.
According to the calculations of scientists, the maximum mechanical force that a magnetic element with such a susceptibility can create during rotation is only 10-18 Newtons. This is about five orders of magnitude less than in any receptor that works on the mechanoelectric principle of signal transmission. Thus, cuticolosomes cannot react strongly enough to the Earth’s magnetic field and, accordingly, cannot be part of the magnetosensitive receptor.
The main function of cuticulosomes, according to scientists, is not magnetoreception, but storage of excess iron and stabilization of stereocilia – mechanosensitive organelles in the vestibular apparatus. The authors of the work note that the method they developed specifically for this study is quite simple and will make it possible to study in a similar way other magnetic structures-candidates for the role of magnetic receptors.
The participation of cryptochrome receptors on the retina of the bird eye in the mechanism of magnetic reception is considered by many scientists to be more probable. For example, in 2019, chemists were able to show that molecular analogs of these receptors respond to fairly weak magnetic fields. A change in the direction of the field vector of only 50 microtesla already noticeably affects the response of such a chemical compass. And before that, Swedish biologists noticed that the magnetic sensitivity of birds is associated with the polarization of light: if the source of polarized light is directed perpendicular to the magnetic field, the birds completely lose their ability to navigate.
