By releasing homing pigeons with disabled magnetic immune cells, researchers tested whether the birds needed these cells to find their way back to the roost.
Christian Ziegler/Max Planck Institute of Animal Behavior
Homing pigeons don’t rely on gut instinct to return to the roost. But a nearby organ — the liver — might point the way.
White blood cells in the birds’ livers accumulate iron and act as an internal compass when clouds block the sun that normally helps them navigate, researchers report May 28 in Science. While scientists generally agree that some animals use Earth’s magnetic field to guide migrations, they had not pinned down how, and the new work offers a surprising explanation.
For decades, researchers have fiercely debated first if and then how birds sense magnetic fields and use them for navigation. One prominent idea involves proteins in their eyes undergoing a reaction in magnetic fields. No one has been able to prove exactly how this so-called “quantum effect” is in play. Other animals that orient using Earth’s magnetism, such as bats and sharks, lack the proteins, so the debate languished unresolved.
Ornithologist Martin Wikelski of the Max Planck Institute of Animal Behavior in Radolfzell, Germany, and immunologist Christian Kurts of the University of Bonn in Germany stumbled on another idea more than a decade ago at a conference coffee break. Kurts mentioned how frustrated he was that immune system cells called macrophages in mouse spleens would stick to magnetic columns in instruments used to separate different types of cells, ruining his experiments.
The reason the macrophages were sticking, he discovered, was that they accumulated and recycled damaged red blood cells’ iron atoms, which aligned in magnetic fields. Wikelski had never really accepted current theories about how birds sense magnetic fields through eye proteins and recalls thinking: “That’s the solution of how a magnetic system could work in birds.” Kurts had never given bird navigation much thought, but the duo decided to work together to see if homing pigeons (Columba livia), too, had these kinds of immune cells.
Cell biologist Clivia Lisowski of the University of Bonn checked to see whether cells from the birds’ beak and eyes, which were previously implicated in magnetic sensing, and from the spleen and liver, which process red blood cells, were magnetic. Only macrophages in the pigeon’s liver attached to magnetic columns, she found. Within the liver, the scientists found millions of iron-filled white blood cells near the organ’s nerve network, suggesting these cells could tell the brain which way to head based on the Earth’s magnetic field.
To tease out the macrophages’ role, the team watched the weather for overcast days, as pigeons prefer to use sunlight to guide their journeys and use the magnetic field only as a last resort. “It’s very important that the birds don’t have a clue where the sun is,” Kurts says.
About 24 hours before a cloudy day, the researchers gave half of 34 homing pigeons a treatment that kills macrophages. They drove the pigeons 19 kilometers away and released them with GPS trackers attached. Those with intact macrophages reached home in about 70 minutes. Those with a depleted supply flew every which way and didn’t make it back home until the sun came out the next day. On sunny days, pigeons given the treatment flew directly home.
“Next we need to know how the [cells] transfer information to the nervous system and what brain areas are affected,” says Susanne Åkesson, an animal ecologist at the University of Lund in Sweden not involved with the work. She says it’s also unclear whether songbirds, bats, sharks or other magnetic-sensing animals have these white blood cells in their livers.
When it comes to the findings, “there are certainly going to be nonbelievers,” says neuroethologist John Phillips of Virginia Tech in Blacksburg, who was not involved with the study. But the science was so well done, he says, that even nonbelievers “can’t ignore this.”
