Homing pigeons owe their remarkable ability to travel long distances not to their eyes, brains, or beak, but to special cells in their liver, a new study reveals.
Pigeons’ ability to travel hundreds of miles and still find their way home has long puzzled scientists.
Previous theories suggested the birds could detect the Earth’s magnetic fields through light-sensitive molecules in their eyes, or tiny magnetic particles in their beaks.
But neither idea has received strong experimental confirmation despite decades of research.
Now a new study reveals the answer may lie in an unexpected place – the liver.
Scientists found that homing pigeons may use specialised immune cells in their liver and spleen to detect Earth's magnetic field, providing them with an internal navigation system.
These immune cells, called macrophages, were found to accumulate iron while breaking down old red blood cells.
The accumulated iron provides these immune cells unique magnetic properties, including the ability to respond to the planet's magnetic field, according to a new study published in the journal Science.
Researchers found that when the macrophages were removed, pigeons struggled to find their way home, pointing to their previously unknown role in navigation.
“What looks like a 'gut feeling' in bird navigation may actually have a physical basis,” said Martin Wikelski, director at the Max Planck Institute of Animal Behaviour in Germany.
"If immune cells are part of how birds sense direction, it would fundamentally change how we understand navigation,” Dr Wikelski said.
While scientists have long known that migratory birds and homing pigeons use the Earth's magnetic field as a tool for navigation, exactly how animals detect that field has remained one of biology's biggest mysteries.
This new study found that the liver and spleen could play a key role in navigation by combining expertise from immunology, physics, and animal behaviour.
In the study, scientists examined multiple organs of homing pigeons that were previously linked to the ability to sense magnetic fields, including the eyes, beak, and brain.
"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," said Clivia Lisowski, another author of the study.
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Among all the tissues and organs analysed, the liver had the highest concentration of iron and produced the strongest magnetic response.
Further tests confirmed that liver macrophages were responsible for these magnetic properties.
Scientists then tested whether the macrophages actually influence navigation.
For this, they removed liver macrophages from pigeons that had been trained to return to their aviary from locations more than 20km away in Germany.
Researchers found that on overcast days, when the sun was hidden, pigeons that lacked the macrophages lost their sense of direction and had difficulty navigating home.
On sunny days, however, they could still successfully return, indicating that the birds use magnetic information alongside solar cues to orient themselves during flight.
Using advanced microscopy, scientists then assessed how information from the pigeons’ liver travel to the brain.
They found that the iron-rich macrophages sit close to nerve fibres, hinting at a possible pathway through which magnetic information is transmitted from the liver to the nervous system and ultimately to the brain.
"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,” Dr Lisowski said.
