In winter, lakes and rivers across the country are filled with migratory birds flying in from the north. How do birds keep such precise bearings while flying thousands of kilometers? German scientists have newly found a compass in birds' ears. It had been known that substances in the retina or beak respond to magnetic fields, but this means they also "hear" direction through the ear.
David Keays, a professor at LMU, and his team in Germany said in Science on the 21st that "experiments confirmed that pigeons detect magnetic fields in the inner ear and send electrical signals to the brain." The team validated the pigeon experiment results through single-cell RNA decoding and tracing of brain signal pathways.
It has been known that various animals such as migratory birds, turtles, and trout sense the direction and intensity of Earth's magnetic field to orient themselves, but the exact mechanism has remained unclear. Eric Warrant, a professor at Lund University in Sweden, said, "The ultimate holy grail of sensory biology is to understand magnetoreception," adding, "This study is the most definitive demonstration of a neural pathway involved in magnetic field processing in animals."
◇ Hair cells in the inner ear act as a compass
The inner ear contains the vestibular organ, which allows animals to maintain balance while moving. Composed of three semicircular canals arranged at right angles, the utricle, and the saccule, the vestibular organ is filled with a gelatinous lymph fluid. When the body moves, hair cells sway with the lymph fluid, generating electrical signals that are transmitted to the brain.
In 1882, French zoologist Camille Viguier proposed that a magnetic field would induce a minute current in the liquid of the inner ear, indicating direction like a compass needle. At the time, no animal was known to sense magnetic fields. The German team has validated Viguier's theory after 143 years.
The team experimented with racing pigeons that can find their way home accurately from hundreds of kilometers away. First, they exposed six pigeons to a magnetic field stronger than Earth's for more than an hour. While the birds' heads were fixed, they continually changed the direction of the magnetic field.
Next, they made the brains of euthanized pigeons transparent to check how magnetic signals were transmitted. When fat is removed from the brain and a hydrogel, a jelly-like substance composed mostly of water, is infused, it becomes transparent, allowing genetic material and proteins to be seen.
The team injected antibodies that react with neurons that recently had gene activity in the brain. Microscopic observations showed that the brain regions receiving information from the inner ear were activated by the magnetic field. This means the inner ear sends magnetic field information to the brain.
The team analyzed hair cells in the inner ear of three pigeons. RNA decoding showed that hair cells express many proteins sensitive to electromagnetic changes. Genetic information in DNA is transcribed into RNA and used for protein synthesis. By decoding RNA, one can see how much protein is produced.
◇ The eye's retina and the beak may also serve as compasses
A compass needle points to the North and South Poles under the influence of Earth's magnetic field. One can navigate by reading this direction. So far, two candidates have vied as biological compasses. First, scientists found iron-containing cells in the upper part of pigeons' beaks. The idea is that iron responds to Earth's magnetic field like a compass needle.
The second candidate is the cryptochrome protein found in the retinas of migratory birds. Migratory birds gauge Earth's magnetic field at dusk to set their course, and this protein primarily responds to blue light, which is abundant in sunlight at that time.
Studies have also suggested the two work together. In 2016, a team at Peking University in China reported in Nature Materials that they had found a protein in fruit flies called MagR that binds to both iron and cryptochrome. This protein has been found not only in long-distance migrants such as pigeons and turtles but also in humans. In the pigeon retina, MagR binds with cryptochrome to form a cylindrical structure, and like a compass needle, the cylinder aligns with the magnetic field.
In 2018, a team at the University of California, San Francisco (UCSF) identified proteins that sense magnetic fields in sharks and rays. Neurons express proteins that are sensitive to changes in current, the signal of neural activity, and in some cases, the addition of 10 amino acids to the protein allowed it to detect currents induced by changes in magnetic fields.
The Keays team found the same type of variant in pigeon genes the following year. This time, they also confirmed that such variant proteins are abundant in the inner ear and mapped the pathway by which electrical signals induced by changes in the magnetic field travel to the brain. Through experiments exposing pigeons to magnetic fields in the dark, Keays also showed that the brain does not require light to receive magnetic field information.
Of course, these results do not definitively rule out compasses in the retina or beak. Keays also said animals may have more than one magnetoreceptive organ. Ulrich Müller, a neurobiologist at Johns Hopkins University in the United States, said, "The findings are highly compelling," but added, "It is necessary to remove the key genes of the Keays model using CRISPR-Cas9 and see whether magnetosensory ability disappears."
References
Science (2025), DOI: https://doi.org/10.1126/science.aea6425
Current Biology (2019), DOI: https://doi.org/10.1016/j.cub.2019.09.048
Nature Materials (2016), DOI: https://doi.org/10.1038/nmat4484