BY THE OPTIMIST DAILY EDITORIAL TEAM
For decades, researchers looked for the seat of magnetoreception in all the obvious places: the eyes, the inner ear, the beak. A study just published in Science points somewhere none of them expected. The organ doing the magnetic navigation work in homing pigeons appears to be the liver.
Iron accumulation with a purpose
The cells involved are macrophages. Their usual job is to break down old red blood cells, accumulating iron as they go. Researchers at the University of Bonn found dense concentrations of these iron-laden macrophages in pigeon liver tissue, pressed against nerve fibers. “This actually makes it very likely that [the nerve cells and macrophages] are communicating,” says Clivia Lisowski, the immunologist who led the study.
The navigation angle came from a conference coffee break. Immunologist Christian Kurts had been studying the iron buildup in macrophages when he ran into animal behaviorist Martin Wikelski. “We had this eureka moment,” Kurts says. “Maybe we test whether these cells are involved.”
So they did. The team trained 34 pigeons on a 12-mile route through the German Alps, then experimentally disabled the liver macrophages in some of the birds. On overcast days, those pigeons couldn’t get home. In clear weather, they were fine, navigating by the sun instead. The liver appears to cover the magnetic side of things. The sky covers the rest.
Two compasses, not one
“The sense of magnetism has been a mystery for a century, and nobody could solve where that sits and how that works,” says Wikelski, a director at the Max Planck Institute of Animal Behavior. “Now, we think we have found, really, a workable solution.”
Not everyone is ready to call it solved. Thorsten Ritz, a biophysicist at UC Irvine, has spent years building the case that songbirds detect Earth’s magnetic field using light-sensitive molecules in the eye, a different mechanism with its own supporting evidence. He thinks the field should stay open. “There are almost always multiple solutions to how an animal can get an evolutionary advantage,” he says. “I am in favor of keeping an open mind rather than trying to find winners and losers.”
Researchers Simon Spiro and Hal Drakesmith, writing alongside the study in Science, think both camps may be right about different things. “Perhaps one process dominates for long-distance navigation, whereas another is used for more specific destination-finding,” they write.
What comes next
Whether sea turtles, gray whales, and spiny lobsters use the same iron-macrophage system is now an open question. The team still needs to trace which nerve pathways carry the signal from the liver to the brain, and which parts of the brain do something with it. “Every little bit of it that we can resolve in the future will of course substantiate the validity of the findings,” Kurts says.
Evolution tends to find the same solution independently more often than seems reasonable. Whether this is one of those cases or not is still unclear.
Source study: Science– Homing pigeon navigation relies on superparamagnetic macrophages under overcast conditions
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