Quirks and Quarks9:25Pigeons use their livers to find their way home
During the First World War, an American major anxiously sent up a homing pigeon into the smoky sky while he and his unit remained trapped behind enemy lines.
He and his men had held their position for several days when American artillery began shelling the area, unknowingly striking their own troops in the chaos of battle.
The pigeon, named Cher Ami, flew straight through heavy fire and successfully delivered the message tied to its leg calling off the bombardment, a transmission that ultimately saved the surviving soldiers.
For centuries, pigeons like Cher Ami have been trusted to fly long distances with important messages — from announcing Olympic winners in Ancient Greece to carrying breaking news for early news agencies.
Clivia Lisowski, an immunologist at the University of Bonn in Germany, says scientists have long known that homing pigeons use a combination of cues such as the sun, smells and visual landmarks to navigate.
When skies are overcast or flights stretch into the night and those cues disappear, she says, pigeons appear to also rely on the Earth’s magnetic field.
But what’s really stumped scientists for decades is how pigeons actually tap into that magnetism. They’ve known the birds can do it, but they’ve never been able to figure out exactly how this internal “compass” works.
“There has been a theory that special light-sensitive molecules in the eye might transfer this magnetic information, and other people thought maybe there are small magnetic particles in the beak of pigeons, but none of these theories really could answer the story completely,” Lisowski told Quirks & Quarks host Bob McDonald.
Now, new research led by Lisowski and her team points to a different possibility: iron-rich immune cells in the liver may be acting like sensors that pick up Earth’s magnetic field and help guide the birds’ navigation.
‘They had this Eureka moment’
The idea unexpectedly came out of earlier research in mice, says Lisowski. Her co-author, immunologist Christian Kurts at the University of Bonn, found that immune cells in the spleen break down old red blood cells and end up accumulating iron, which makes them unusually sensitive to magnetic fields.
When Kurts later talked with animal behaviour scientist Martin Wikelski at Germany’s Max Planck Institute of Animal Behavior, they started wondering if the same kind of cells exists in birds too.
“They had this Eureka moment,” said Lisowski.
This is when Lisowski stepped in. Interested in how cells sense and communicate with their environment, she says she was “captivated by the idea” and joined Kurts’ lab.
After screening several organs, the researchers found that the liver, rather than the spleen, contained the highest concentration of iron.
Found throughout the body, a kind of immune cell called a macrophage acts like a recycling system — engulfing pathogens, damaged cells and other debris, says Lisowski. In the process, these break down old red blood cells and pull out iron from haemoglobin, the molecule that gives blood its red colour.
The researchers also discovered that these macrophages sit close to nerve fibres, hinting at a possible pathway to send magnetic information to the brain.
To test their hypothesis, the team temporarily depleted macrophages in pigeons using a drug called clodronate. After waiting a day for the drug to take effect, they released the birds under “completely overcast conditions where no solar cues or any other information were available.”
The difference was evident — the untreated pigeons were able to orient themselves, but the ones without working macrophages struggled.
“They were completely lost, they were confused, they couldn’t find their way … so they flew in all kinds of directions,” said Lisowski.
David Bird, an emeritus professor of wildlife biology at Montreal’s McGill University who was not involved in the study, called the work “an amazing finding,” and says it could lead to discoveries about other creatures navigating this way.
“It’ll be all migratory birds and of course other animals as well … like the sharks and the dogs and I’ll bet even those cats that get lost … [and] make those miraculous trips across the continent to find a way home,” he said.
A new way of experiencing the world
Scott MacDougall-Shackleton, the director of the advanced facility for avian research at Western University in London, Ont., says the “super intriguing and very novel” research could also potentially contribute to learning more about how our own bodies work.
“Normally we think about our eyes, our nasal [lining], our taste buds in our tongue, our inner ear giving us a sense of balance, our touch receptors in our skin,” he said.
“We never think about the liver as this organ that would somehow detect the outside world.”
Still, MacDougall-Shackleton, who’s also a professor in psychology and biology, says more research is needed to confirm the mechanism and rule out other explanations, including whether the drugs may have affected more than just immune cells.
He says he’s also curious to know how exactly the magnetic information gets passed to the brain and “gets used by the pigeon to go in the appropriate direction.”
Lisowski says the research has already changed how she views the immune system.
“For a long time, people just thought it’s there to defend us against pathogens and to degrade bacteria or viruses, but the immune system has so many components,” she said.
“Here in Bonn, we have a big hub of immunologists and we are all investigating how the immune system is actually a sensory organ.”
