Pigeons can fly hundreds of miles from an unfamiliar location and still find their way home. Scientists have puzzled over how they do it for decades. New research points to an answer no one expected: the liver.
According to a study published in Science, pigeons appear to use specialized immune cells in their livers to sense Earth's magnetic field. Those cells, called macrophages, pick up iron as they break down old red blood cells. That iron gives them magnetic properties that researchers believe allow the birds to orient themselves using the planet's magnetic field.
When the researchers removed those cells from pigeons, the birds struggled to navigate home, particularly under overcast skies when other cues like the sun were not available. That result pointed directly to the liver cells as a key part of the birds' internal compass.
The findings come from a team that included scientists from the University of Bonn, the University Hospital Bonn, the University of Duisburg-Essen, and the Max Planck Institute of Animal Behavior. The study combined expertise from immunology, physics, and animal behavior, three fields that do not often work together on the same problem.
"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," said Prof. 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.
The other co-senior author, Prof. Martin Wikelski, Director at the Max Planck Institute of Animal Behavior, put it a different way. "What looks like a 'gut feeling' in bird navigation may actually have a physical basis," he said.
Scientists have known for years that homing pigeons and migratory birds use Earth's magnetic field as one of several navigation tools. The harder question has always been how, exactly, they detect it. Two competing theories have driven most of the research. One proposed that light-sensitive molecules in the birds' eyes could respond to magnetic fields. The other focused on tiny magnetic particles found in the beak. Neither theory gathered strong experimental support over years of investigation.
The new liver-based explanation is a significant departure from both. The macrophages at the center of this study are a type of immune cell, not a sensory cell. Their primary job is to clean up old red blood cells. The fact that this process loads them with iron, and that the iron appears to give them a secondary function as magnetic detectors, was not something researchers had considered before.
The discovery was reported by Science Daily and traces back to the Max Planck Institute of Animal Behavior. It raises questions that go well beyond pigeons. If immune cells can serve as environmental sensors in birds, similar mechanisms could exist in other animals, including migratory species that travel thousands of miles each year. Researchers have not yet confirmed whether that is the case, but the pigeon findings open a new line of inquiry into how animals across many species might be detecting forces that human senses cannot register at all.
The study also points to an unexpected overlap between the immune system and sensory biology, two areas of research that have largely developed along separate tracks. Whether that connection turns out to be narrow or broad is a question the field will likely be working on for some time.
