When an American battalion was trapped behind enemy lines in World War I, pigeons saved the soldiers by delivering coordinates when human messengers could not. Pigeons then carried financial news and stock prices across a 76-mile gap in Europe’s telegraph networks. During the Cold War, the CIA attached small cameras to pigeons to take aerial reconnaissance photos.
But how did these birds travel? Scientists have discovered a new mechanism with incredible precision, especially in cloudy conditions. After all, they are literally following their intuition.
Through a series of flight and laboratory experiments, researchers discovered that pigeons can use special cells in their livers as internal compasses. These iron-rich cells exhibited interesting quantum properties that allow the pigeons to sense the direction of the Earth’s magnetic field, according to a study published Thursday in the journal Science. Without the cells, the pigeons would go missing under certain weather conditions.
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“This is a big mystery in the field of how birds use magnetic fields to find their direction,” said Christian Kurz, senior co-author and immunologist at the University Hospital Bonn in Germany. “Magnetic fields, no one would have guessed that immune cells could also sense magnetic fields. This is a new feature of the immune system.”
Pigeons, like many birds, use several tactics to navigate terrain. This includes tracking the sun’s position, smells, landmarks, and most curiously, magnetic fields. In particular, it relies on tracking magnetic fields when no other clues are available. One of the main ideas about how birds sense magnetic fields is that their retinas contain light-sensitive particles that allow them to orient themselves by literally “seeing” magnetic fields. But what if there is no sunlight?
“Maintaining orientation is very important for birds migrating at night, but it’s also very important for pigeons in adverse conditions,” said Martin Wikelski, senior co-author and director of the Max Planck Institute for Animal Behavior. Some birds become disoriented and disoriented during magnetic storms, where solar particles distort the Earth’s magnetic field.
Scientists flocked to find a new explanation. During a coffee break at a conference 10 years ago, Kurz met Wikelski, an ecologist who was trying to solve the pigeon puzzle. Kurt told him about a recent breakthrough in immunology in which his team was able to isolate magnetic cells from rodent spleens. They looked at each other and said, “Eureka, that’s it!” At that moment, Kurtz, director of the Institute for Molecular Medicine and Experimental Immunology, recalled: They came up with a theory to remove the magnetic cells and see if the birds lose their ability to navigate in cloudy conditions.
Back in the lab, Kurz and his immunology team examined magnetic cells in pigeon organs and found that the liver contained the highest concentration of iron. Immune cells here break down old, damaged red blood cells and accumulate iron from hemoglobin for a short period of time.
Cells are not inherently magnetic, but when placed in a magnetic field they can exhibit magnet-like properties at the tiny quantum level of nanoparticles. This is a type of magnetism known as “superparamagnetism.”
“When the pigeons pass through the Earth’s magnetic field, the electrons[in the liver’s immune cells]all line up in the same direction and become superparamagnetic,” said Klivia Lisowski, a biologist at the University of Bonn and co-author of the study. Cells can transmit information to the brain via nerve connections through the liver. This allows the pigeons to sense the magnetic field and “decide whether to fly left or right.”
To test whether this idea would work, Wikelski and his fellow ecologists trained 34 pigeons to navigate a 12-mile route in southern Germany, flying them under clear skies and completely cloudy conditions, but some of the birds were depleted of iron-containing immune cells. They expected to see the biggest changes during flights in cloudy skies, when the birds have to rely on the Earth’s magnetic field.
And like a bird staring at a fallen sandwich, they saw exactly what they wanted. All pigeons with iron-rich cells successfully completed the route in 70 to 90 minutes under sunny and cloudy conditions. However, the iron-depleted pigeons got lost under cloudy skies and ended up going in the wrong direction or blowing past their destination. When the clouds cleared and the sun appeared, they returned home.
“Without a compass, you get lost and end up going in circles,” Wikelski says.
In a follow-up flight, the research team found that iron naturally re-accumulated in the modified pigeons’ immune cells, which could be reorientated by a magnetic field.
Some scientists not involved in the study told CNN that the study suggests a possible new mechanism for magnetic perception. But other researchers aren’t completely sold on the new idea.
Joseph Kirschvink, a geophysicist at the California Institute of Technology, said the study requires more direct evidence that these superparamagnetic materials are sensing magnetic fields.
This substance is also present in other animals, such as bees, and in the brains of Alzheimer’s patients. But Kirschvink said scientists have not been able to show how these particles can reliably detect fields. He thinks they have very different functions and could be a “dead end” in understanding how birds sense magnetic fields.
Nevertheless, the research team’s next steps include figuring out how immune cells communicate with nerves to send messages to the brain. The team also established a satellite system to track pigeons around the world and learn more about their navigation skills.
“It’s becoming clear that the immune system, our entire body filled with immune cells, senses the environment,” Wikelski says. “This is really a revolution in our understanding of how the body works in general.”
