How do pigeons sense Earth's magnetic field? Scientists say they’ve solved the mystery.

Pigeons and other birds can do it. So can sea turtles and spiny lobsters, moths and mole rats, gray whales and big brown bats.

Many members of the animal kingdom can detect the subtle undulations of Earth’s magnetic field. Relying on a hidden biological compass, these animals use invisible magnetic cues from deep inside the Earth to orient themselves on the surface and make epic migrations across both sky and ocean.

But exactly how these animals perceive the planet’s magnetic pull has remained one of biology’s biggest mysteries for decades.


Now, a group of scientists in Germany say they have finally found where homing pigeons house their internal compass—and it wasn’t in it in their eyes, ears, or beaks. Surprisingly, their internal compass appears to be embedded in immune cells in their livers, according to a new study in the journal Science. 

“The sense of magnetism has been a mystery for a century, and nobody could solve where that sits and how that works,” says Martin Wikelski, a director at the Max Planck Institute of Animal Behavior in Germany and a National Geographic Explorer who co-wrote the study. “Now, we think we have found, really, a workable solution.”

Earth’s invisible magnetic blanket 


Deep under our planet’s surface churns a tempestuous sea of liquid iron and nickel. The molten metal’s movement transforms Earth into a giant magnet whose field stretches into outer space, protecting everything that flies or swims below it from dangerous cosmic radiation. 


Zoologists since at least the 19th century have suspected that birds rely in part on Earth’s magnetism for navigation. Experiments in the 1960s showed captive robins altered their movements in response to artificial magnetic fields. 

While the roster of magnet-sensitive animals has grown since to include sharks, salmon, and many other creatures, the mechanism behind the mysterious sense—dubbed magnetoreception—remained unsolved. 

Unlike eyes or ears, which sit near the surface of bodies to pick up light and sound, the organ responsible for detecting the planet’s magnetic tug could theoretically be anywhere since Earth’s magnetic field “completely penetrates an animal's body,” says Thorsten Ritz, a biophysicist at the University of California at Irvine who was not involved in the latest study. For years, scientists have hotly debated evidence that birds detect Earth’s magnetic field with compounds in their eyes, particles in their beaks, or fluid in their inner ears.

Wikelski’s hunt began more than a decade ago after a chance meeting with immunologist Christian Kurts during a conference coffee break. Wikelski was studying avian migration while Kurts was doing research involving immune cells called macrophages that become sensitive to magnets after consuming old red blood cells and accumulating iron. “We had this eureka moment,” Kurts says. “Maybe we test whether these cells are involved.” 

Finding a way home


The researchers screened the eyes, beak, brain, spleen, and liver of pigeons, finding large concentrations of iron-laden macrophages in the tissue of the liver. The immune cells were nestled up to nerve fibers.

“This actually makes it very likely that [the nerve cells and macrophages] are communicating,” says Clivia Lisowski, an immunologist at the University of Bonn who led the study. 

To see if liver macrophages aid in avian navigation, the team conducted a series of experiments on 34 pigeons trained to fly a 12-mile route through the German countryside at the edge of the Alps, dodging hawks and falcons on the way.

“Once they are trained, they are perfect,” Wikelski says. “They evade everybody. They fly home as if the devil is behind them.”

But when the macrophages were knocked out, the pigeons were unable to fly home when it was overcast. When the clouds retreated and sun reemerged, birds without working macrophages flew home fine.

The results suggest the birds use both visual and magnetic cues—that is, both the sun shining above and the magnetic field pulsing below—to find their way. When the sun is obscured, the pigeons appear to rely heavily on their internal compass to navigate.

The researchers acknowledge there is still a lot that needs to be nailed down about their proposed magnetoreception mechanism. What exactly goes on inside macrophages that attunes them to the planet’s magnetic field? Which nerves carry the signal to the birds’ brain? And which parts of the brain process that information?

“There are many gaps in the chain of arguments, and every little bit of it that we can resolve in the future will of course substantiate the validity of the findings,” Kurts says. 

They also want to determine if other animals also use their immune system as a compass—or if life has evolved more than one way of deciphering Earth’s magnetism. 

Ritz, who was not involved in the study and whose own research suggests songbirds use magnetically sensitive molecules in their eyes to “see” Earth’s otherwise invisible magnetic sheath, says the proposed macrophage mechanism is plausible. 

“There are almost always multiple solutions to how an animal can get an evolutionary advantage,” Ritz says. “I am in favor of keeping an open mind rather than trying to find winners and losers. So, for now, I am fully in the ‘both can be true’ camp.”

Indeed, the new study adds to the long-simmering debate of what is responsible for magentoreception. Writing in a companion Perspective piece in Science, researchers Simon Spiro and Hal Drakesmith note the new work does not rule out the possibility that other organs beyond the liver may play a role, especially when the sun isn't shining.

"Perhaps one process dominates for long-distance navigation, whereas another is used for more specific destination-finding, with both operating with different degrees of precision," they write. "Indeed, it could be prudent to have more than one way of getting home in the dark."