Pigeons Navigate Using Magnetic Sensors In Their Livers

Immune cells packed with iron could help birds detect Earth's magnetic field

To the Point

• Previously unknown navigation mechanism: pigeons may sense the Earth's magnetic field using iron-rich immune cells in their livers, a quantum effect.
• Navigation experiments: removing iron-containing immune cells in the liver disrupted pigeons' sense of direction under overcast skies when the sun was not visible.
• Sensing via the immune system: findings suggest a new link between immunity and sensory perception in birds, and potentially other animal species.

How pigeons fly hundreds of kilometers and still find their way home has long fascinated people. Now, researchers say a surprising answer may be hidden, not in the brain or eyes of birds, but in the liver.

A study published in Science suggests that special cells in the liver of pigeons can sense the Earth's magnetic field, giving the birds an internal compass.

The special cells, known as "macrophages," are immune cells that break down old red blood cells. As part of this process, they accumulate iron, giving them quantum properties that may allow them to respond to magnetic fields. Without these cells intact, pigeons could not navigate home, the study shows.

Previously unknown mechanism

"We didn't expect immune cells to act like sensors for magnetic fields at all. Our results reveal a previously unknown mechanism for magnetic perception in animals," says Christian Kurts, Director at the Institute of Molecular Medicine and Experimental Immunology at the University Hospital Bonn, and one of the study's co-senior authors.

"What looks like a 'gut feeling' in bird navigation may actually have a physical basis," adds Martin Wikelski, Director at the Max Planck Institute of Animal Behavior and the other co-senior author of the study.

The source of magnetic sensing

For decades, scientists have known that migratory birds and homing pigeons rely in part on the Earth's magnetic field to navigate. But exactly how they detect it remains one of biology's unsolved mysteries. Competing theories have suggested that birds might "see" magnetic fields through light-sensitive molecules in the eye, or detect them using tiny magnetic particles in the beak. None has come up with convincing experimental support.

The new study proposes a different mechanism for magnetic sensing, supported by a combination of lab tests and behavioral experiments. The team included immunologists from the University of Bonn and the University Hospital Bonn, physicists from the University of Duisburg-Essen, and ornithologists at the Max Planck Institute of Animal Behavior.

To identify where magnetic cells are found in pigeons, the researchers used techniques known as "vibrating sample magnetometry" and "magnetic cell separation" to screen organs thought to be involved in magnetic sensing, including the eyes, beak, and brain. They also examined the liver and spleen.

"We had some clues that the liver and spleen have magnetic properties, because they break down red blood cells and so store much iron in the body," says first author Clivia Lisowski, from the University of Bonn and the University Hospital Bonn, who led the immunological work.

The results supported that idea. Of all the tissues examined, the liver showed the highest concentration of iron. "Iron is crystallized in oxide nanoparticles making the cells superparamagnetic and reactive to magnetic fields. We found by far the strongest magnetic response in liver tissue," adds Ulf Wiedwald, from the University of Duisburg-Essen.

Further analysis identified macrophages in the liver as the cells responsible.

From sensing to navigating

To test if liver macrophages played a role in navigation, the ornithological team conducted experiments on pigeons that were trained to return from distances over twenty kilometers back to their aviary at the Max Planck Institute of Animal Behavior in Konstanz, Germany. After the macrophages were removed, pigeons lost their sense of direction on overcast days when the sun was obscured. When the sun was visible, however, the pigeons successfully navigated home, likely using solar cues. Together, these results illustrate the mechanism behind how birds use magnetic sensing, in addition to the sun's orientation, for navigation.

With evidence that these cells influence navigation, the researchers then looked for how signals from the liver might be relayed. Electron microscopy showed that the iron-rich macrophages sit close to nerve fibers, suggesting a pathway for magnetic information to reach the brain.

First evidence

Lisowski says: "These findings provide the first concrete evidence of how the Earth's magnetic field can be perceived within the body and passed on to the brain to guide movement."

The study brings together known biological processes, including iron metabolism and how the immune and nervous systems communicate, into a clear answer to the fundamental question of how animals navigate.

"Animal navigation is one of the most fascinating phenomena in nature," says Wikelski. "If immune cells are part of how birds sense direction, it would fundamentally change how we understand navigation."

Many questions remain, particularly how signals from these cells are processed in the brain. Beyond birds, these findings could have implications for animals such as sharks that navigate without light. It's possible that other animals, and perhaps even humans, may respond to magnetic fields in ways not yet understood.