How pigeons are able to find their way home while traveling hundreds of miles has puzzled scientists for decades. A new study suggests the answer may lie in an unexpected place: the liver.
According to a study published in sciencepigeons use specialized immune cells in their livers to detect the Earth’s magnetic field, which may provide an internal navigation system.
Researchers discovered that these cells, called macrophages, accumulate iron while breaking down old red blood cells. Iron may give cells a unique magnetic property, allowing them to respond to the planet’s magnetic field. Once the cells were removed, the pigeons struggled to find their way home, suggesting a previously unknown role in navigation.
“We never expected that immune cells could act like sensors for magnetic fields. These results reveal a previously unknown mechanism of magnetic perception in animals,” says Professor Christian Kurz, director of the Institute of Molecular Medicine and Experimental Immunology at the University Hospital Bonn and co-senior author of the study.
“What appears to be a ‘gut feeling’ in bird navigation may actually have a physical basis,” added Professor Martin Wikelski, director of the Max Planck Institute for Animal Behavior and another co-senior author of the study.
The long exploration of birds’ magnetic senses
Scientists have long known that homing pigeons and migratory birds use the Earth’s magnetic field as one of several tools for navigation. But how exactly animals detect the scene remains one of biology’s greatest mysteries.
Over the years, researchers have proposed several possibilities. Some theories suggest that birds may be able to sense magnetic fields through light-sensitive molecules in their eyes. Some pointed to tiny magnetic particles in its beak. Despite years of research, neither idea has received strong experimental support.
New research combines expertise in immunology, physics and animal behavior to offer a different explanation. The research team included scientists from the University of Bonn, the University Hospital Bonn, the University of Duisburg-Essen, and the Max Planck Institute for Animal Behavior (MPI-AB).
Iron-rich liver cells exhibit strong magnetic properties
To determine where magnetic sensing occurs, the researchers examined several organs previously associated with magnetoreception, including the eyes, beaks, and brain. They also analyzed the liver and spleen using techniques known as “vibrating sample magnetometry” and “magnetic cell separation.”
“We have a clue that the liver and spleen are magnetic because they break down red blood cells and store a lot of iron in the body,” said lead author Dr. Klivia Lisowski of the University of Bonn and University Hospital Bonn, who led the immunology study.
The results were amazing. Of all the tissues studied, the liver contained the highest concentration of iron and caused the strongest magnetic response.
“Iron is crystallized in oxide nanoparticles, making cells superparamagnetic and highly responsive to magnetic fields. We found the strongest magnetic response in liver tissue,” added Professor Wolf Wiedwald of the University of Duisburg-Essen.
Further studies revealed that liver macrophages are responsible for these magnetic properties.
Navigation experiment reveals important role
The researchers then tested whether macrophages actually affected navigation.
At MPI-AB in Konstanz, Germany, pigeons were trained to return to their aviaries from more than 20 kilometers away. Scientists removed macrophages in the liver and observed how the birds behaved.
The results were dependent on the weather. On cloudy days when the sun was hidden, pigeons without macrophages became disoriented and had difficulty finding their way home. However, on clear days, it probably relied on the sun instead of the Earth’s magnetic field as a navigational cue to successfully return home.
These findings suggest that birds use magnetic information, along with solar cues, to orient themselves during flight.
How magnetic signals reach the brain
After establishing the link between liver cells and navigation, the researchers looked for ways to get information to the brain.
Using electron microscopy, they found that iron-rich macrophages were located near nerve fibers. This arrangement suggests a possible pathway for magnetic information to be transmitted from the liver to the nervous system and ultimately the brain.
Professor Lisofsky said: “These findings provide the first concrete evidence of how the Earth’s magnetic field is perceived within the body and communicated to the brain to guide movement.”
The study brings together several well-established biological processes, including iron metabolism and communication between the immune and nervous systems, to help explain how animals sense magnetic fields.
“Animal navigation is one of the most fascinating phenomena in nature,” Wikelski says. “If immune cells are part of direction sensing in birds, it would fundamentally change the way we understand navigation.”
Impact beyond birds
Although the findings answer important questions, many questions remain. Researchers need to figure out exactly how the brain processes signals from these cells.
This discovery could have far-reaching implications beyond pigeons. Animals such as sharks are known to effectively navigate without relying on light, raising the possibility that similar mechanisms exist in other species.
Researchers suggest that many animals, and perhaps even humans, may respond to magnetic fields in ways that are not yet fully understood.
