To travel far on cloudy days, pigeons listen to their gut

(CNN) — When an American battalion was trapped behind enemy lines in World War I, a pigeon delivered the coordinates that helped save the soldiers when no human messenger could. Later, pigeons carried financial news and stock prices across a 76-mile gap in Europe’s telegraph network. During the Cold War, the CIA strapped tiny cameras to pigeons to snap aerial reconnaissance photographs.

But how did these birds navigate their journeys? Scientists have uncovered a new mechanism of their uncanny precision, especially in overcast conditions. It turns out, they follow their gut instincts — literally.

Through a series of flight and lab experiments, researchers found pigeons can use special cells in their liver as an internal compass. These iron-rich cells displayed intriguing quantum properties that allowed the pigeons to sense Earth’s magnetic field for direction, according to a study published Thursday in Science. Without the cells, the pigeons became lost under certain weather conditions.

“It is a big riddle in the field of how birds use the magnetic field to find directions,” said Christian Kurts, a senior co-author and immunologist at the University Hospital Bonn in Germany. “Magnetic fields — no one would ever have estimated that immune cells can also sense that. This is a new function of the immune system.”

Pigeons, as well as many birds, use several tactics to traverse terrain, including tracking the position of the sun, smells, landmarks and, most mysteriously, the magnetic field — which they especially rely on when the other cues aren’t available. One leading idea for how birds sense magnetic fields proposed that their retinas contained light-sensitive particles that allowed them to literally “see” the field for directions. But what about when sunlight isn’t around?

“To keep your direction, that’s very important for birds at night that migrate, but also for pigeons in bad conditions,” said Martin Wikelski, a senior co-author and director at the Max Planck Institute of Animal Behavior. Some birds also become disoriented and lose their sense of direction during geomagnetic storms, when solar particles distort Earth’s magnetic field.

The scientists looked for a new explanation to flock to. During a coffee break at a conference 10 years ago, Kurts met Wikelski, an ecologist, who was trying to solve this pigeon puzzle. Kurts told him about a recent immunology breakthrough where his team could isolate magnetic cells from rodent spleens. They looked at each other and had a “Eureka, oh, that’s it!” moment, recalled Kurts, director of the Institute of Molecular Medicine and Experimental Immunology. They came up with a theory to test: take away the magnetic cells and see if birds lost their navigation abilities in cloudy conditions.

Back in the lab, Kurts and his immunology team screened the pigeon organs for magnetic cells and found the liver contained the highest concentration of iron. Immune cells here degraded old and damaged red blood cells, accumulating iron from the hemoglobin for a short period.

While the cells are not inherently magnetic, they can display magnet-like properties at the tiny, quantum level of nanoparticles when placed in a magnetic field — a type of magnetism known as “superparamagnetism.”

“Once the pigeon passes through the Earth’s magnetic field, (the electrons in their liver immune cells) all arrange in the same direction, making them superparamagnetic,” said Clivia Lisowski, a biologist at the University of Bonn and co-author of the study. The cells can transmit the information to the brain through nerve connections running through the liver. This allows the pigeon to sense the magnetic field, and then it “decides to fly left or right.”

To test if this idea would take flight, Wikelski and his fellow ecologists trained 34 pigeons to navigate a 12-mile route in southern Germany and flew them under sunny and completely overcast conditions, but depleted the iron-containing immune cells in some birds. They expected to see the biggest change during flights under overcast conditions, where the birds would need to rely on Earth’s magnetic field.

And like a bird eyeing a dropped sandwich, they saw exactly what they’d hoped. All the pigeons with the iron-rich cells successfully completed the route in 70 to 90 minutes under sunny and overcast conditions. However, the iron-depleted pigeons got lost under overcast skies — traveling in the opposite direction or blowing past their destination. Once the clouds cleared and the sun was visible, they returned home.

“If you don’t have a compass, you lose your direction, you go in circles,” said Wikelski.

In follow-up flights, the team found the iron naturally re-accumulated in the immune cells of the altered pigeons, who were then able to properly orient with the magnetic field again.

Some scientists not affiliated with the research told CNN the study suggested a new possible mechanism for magnetic perception. But other researchers aren’t completely convinced of this new idea.

The research needs more direct evidence that these superparamagnetic materials are sensing the magnetic field, said Joseph Kirschvink, a geophysicist at the California Institute of Technology.

The materials are found in other animals too, like honeybees, and in the brains of Alzheimer’s patients. But Kirschvink said scientists haven’t been able to show how these particles can reliably detect the field. He thinks they might have a different function altogether and end up being a “dead end” for understanding how birds sense the magnetic field.

Nonetheless, the team’s next steps include uncovering any mechanisms of how immune cells communicate with nerves to send messages to the brain. The team also established a satellite system to track pigeons globally and learn more about their navigation skills.

“It’s becoming clear that the immune system, so basically our entire body, which is full of immune cells, is sensing the environment,” said Wikelski. “It really is a transformation in our understanding how a body works in general.”

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