Pigeon livers contain cells that sense Earth's magnetic field

Scientists have long known that Earth's magnetic field helps migratory birds and homing pigeons find their way. But the exact mechanism behind this biological navigation has remained a puzzle. A new study published in the journal Science reveals that the answer lies inside an unexpected place: the pigeon liver.

The key players are immune cells called macrophages. These cells normally destroy old red blood cells, causing iron to build up inside them. This iron makes the macrophages superparamagnetic, meaning they can be magnetized when exposed to a magnetic field. "When pigeons fly, the nanoparticles align with the magnetic field and become 'magnetized,'" explains Clivia Lisowski, a post-doctoral researcher in immunology at the University of Bonn and co-author of the study. "Like that, pigeons can sense Earth's magnetic field."

Researchers identified the liver as the key organ by testing different tissues. Study co-author Ulf Wiedwald, a nanoscience expert at the University of Duisburg-Essen in Germany, found that the liver showed a significantly stronger magnetic response than any other tissue examined. The team then focused on the macrophages within the liver to understand their role in navigation.

To test how these cells help pigeons navigate, researchers studied birds trained to fly back to their aviary in Konstanz, Germany, from more than 12.4 miles away. Pigeons with removed macrophages became lost on overcast days but still found their way home when the sun was visible. This shows that pigeons rely on both magnetic sensing and solar cues to navigate. The findings suggest that iron-rich macrophages function as an internal compass the birds can use when other visual references are unavailable.

The team also discovered that these magnetic cells sit close to nerve fibers in the liver. This proximity allows magnetic information to travel directly to the pigeon brain. "The iron-rich macrophages are close to nerve fibers, indicating that magnetic information can get to the brain via this route," the researchers found. This physical connection explains how the birds process and use magnetic signals during flight.

Lisowski says the discovery has broad implications beyond bird navigation. "We think that this ferrimagnetic mechanism can actually explain how birds migrating at night, or sharks or bats or other animals migrating in dark environments can perceive Earth's magnetic field." The research also opens new questions about the immune system itself. Lisowski notes that immune cells must sense their environment to perform functions like defending against pathogens and healing wounds. "Our finding that the immune system can also sense the Earth's magnetic field is a complete new layer in this concept of 'immuno-sensation' and opens the door to new research," she explains.