Migration, magnetic fields and balls of iron

Balls of iron in the inner ear cells of migratory birds may help explain how these animals detect magnetic fields to navigate around the planet.

Every year millions of birds make incredible journeys across the oceans and continents guided by the Earth’s magnetic fields. How these animals detect the magnetic fields has puzzled scientists for decades.

In a collaborative project between researchers at the University of Western Australia  and the Research Institute of Molecular Pathology (IMP) in Vienna, iron balls were discovered in hair cells of the inner ear of migratory birds.

“It’s very exciting. We find these iron balls in every bird, whether it’s a pigeon or an ostrich, but not in humans,” said Mattias Lauwers, the IMP researcher who discovered the balls.

The hair cells of the inner ear are sensory neurons and are responsible for detecting sound and gravity. The researchers found that each cell had a single iron ball, and the balls were in the same place in every cell.

UWA research associate Dr Jeremy Shaw, who has studied iron in a range of animals from molluscs to humans, said it was an astonishing finding.

“Despite decades of research, these conspicuous balls of iron had never been observed previously,” said Shaw, who specialises in the use of analytical electron microscopy and helped analyse the iron balls. “Nature keeps surprising us with the various ways iron can be utilised by animals.”

The finding builds on previous work by the IMP and UWA, which showed that iron-rich cells in the beak of pigeons - previously believed to be the magnetic sensors - were really just blood cells.

The cells in the inner ear are much better candidates for being the elusive magnetoreceptors because they’re neurons.

“But we’re a long way off understanding how magnetic sensing works,” said IMP researcher and group leader Dr David Keays. “We still don’t know what these mysterious iron balls are doing.”

The work was published in April in Current Biology.