iron-rich cells – A new study in Science reports that iron-rich immune cells in homing pigeons’ livers act like an internal compass when clouds block sunlight. Researchers killed macrophages in half of 34 birds, drove them 19 kilometers, and tracked their flights: birds with in
Clouds can erase a sky’s directions in minutes. For homing pigeons, that matters.
Researchers reported May 28 in Science that the birds’ navigation may hinge not on their eyes. but on an organ they don’t normally get credit for: the liver. In overcast conditions—when sunlight can’t be used—white blood cells that accumulate iron in the pigeon’s liver appear to function as an internal compass. guiding the direction birds take when magnetic cues are needed.
For decades, scientists argued about whether and how animals sense Earth’s magnetic field. One prominent theory focused on proteins in birds’ eyes that could react in magnetic fields. sometimes described as a “quantum effect.” But no one had proved how that mechanism works. and the debate often stalled because other animals that use magnetism to orient—such as bats and sharks—lack those eye proteins.
More than a decade ago. immunologist Christian Kurts of the University of Bonn in Germany sparked an alternate line of thought during a conference coffee break. He was frustrated that macrophages in mouse spleens would stick to magnetic columns used to separate different cell types. ruining his experiments. The reason. he discovered. was that the cells accumulated and recycled damaged red blood cells’ iron atoms—iron that aligned in magnetic fields.
Ornithologist Martin Wikelski of the Max Planck Institute of Animal Behavior in Radolfzell, Germany, had not accepted existing eye-based theories. He remembers thinking. “That’s the solution of how a magnetic system could work in birds.” Kurts. who had not previously focused on bird navigation. and Wikelski decided to test whether homing pigeons (Columba livia) carried similar immune cells that could interact with magnetism.
Cell biologist Clivia Lisowski of the University of Bonn checked what types of pigeon cells might be magnetic. Cells taken from the birds’ beak and eyes. which had been implicated in magnetic sensing before. did not show the same behavior. Only macrophages in the pigeon’s liver attached to magnetic columns.
Inside the liver, the team found millions of iron-filled white blood cells positioned near the organ’s nerve network. The researchers suggest these cells could provide the brain with directional information about the Earth’s magnetic field.
The experiments were designed around a simple reality: pigeons typically rely on sunlight first. To make sure the birds weren’t “cheating” with skylight cues. the researchers watched the weather for overcast days—because pigeons prefer to use sunlight 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.
Roughly 24 hours before a cloudy day, the researchers treated half of 34 homing pigeons in a way that kills macrophages. They then drove all 34 birds 19 kilometers away and released them with GPS trackers.
The difference was immediate. Birds with intact macrophages made it home in about 70 minutes. Birds with depleted macrophages 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.
Susanne Åkesson. an animal ecologist at the University of Lund in Sweden who was not involved with the work. said the next step is figuring out how the information moves. “Next we need to know how the [cells] transfer information to the nervous system and what brain areas are affected. ” she said. She also warned that open questions remain beyond pigeons: it’s unclear whether songbirds. bats. sharks. or other magnetic-sensing animals have these white blood cells in their livers.
Skepticism didn’t disappear, either. “There are certainly going to be nonbelievers. ” said neuroethologist John Phillips of Virginia Tech in Blacksburg. Virginia. who was not involved with the study. But he added that the work is hard to dismiss: the science was “so well done” that even nonbelievers “can’t ignore this.”.
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