As the hatchlings emerged from their eggs on a Florida beach, Ken Lohmann lay in wait. He scooped up 32 of the tiny turtles and whisked them off to a dark room and a water tank surrounded by electromagnetic coils. One by one, he fitted his captives with miniature Lycra harnesses so he could track where they were swimming – .
Lohmann’s seminal 1991 experiment confirmed what many had long suspected: the turtles could sense the magnetic field, switching their swimming direction in response to it. How else, unless they could sense Earth’s field, would they be able to navigate in the open ocean?
And it’s not just turtles – species as varied as mice, and fruit flies can navigate as surely as if equipped with a compass. But no one can find the “magnetoreceptor” – the biological machinery that enables this feat.
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We do know that there are bundles of neurons in some animals’ brains that deal with magnetic information. In 2009, neuroscientists David Dickman and Le-Qing Wu of the Baylor College of Medicine in Houston, Texas, imaged pigeons’ brains while varying the angle of a magnetic field. They observed 53 pairs of neurons changing their firing as the angle changed.
But where were they getting the signals from? “There is no obvious organ, like an ear or a nose,” says of the University of Oxford. “The sensor could be anywhere inside the animal’s body.” Different animals need not necessarily have the sensors in the same place, either.
One suspect is cryptochrome, a protein found in the eyes of many animals including birds and trout. It is known to produce chemical fragments called radicals in a way that depends on magnetic fields, and fruit flies engineered to lack the gene for cryptochrome .
But that can’t be the full story. For a start, we humans have cryptochrome in our eyes, too – and we can’t sense magnetism. And it is not clear how the radicals get translated into a signal the brain can interpret.
Perhaps we’ve taken a wrong turn. Last year, Xie Can of Peking University found a magnetically sensitive protein that can control muscle and nerve cells. Could this be a candidate? “I doubt it is completely wrong,” says Hore, who says he saw the protein particles in the lab spinning around as a magnet was waved nearby.
Other researchers are more sceptical about the protein, whose magnetic properties supposedly stem from several atoms of iron caught in the grip of a cysteine amino acid running along its backbone. These iron particles are far enough apart that they shouldn’t create a magnetic response strong enough to provoke the spinning Hore saw.
He and others are now digging more deeply into the biochemistry of the different candidate proteins to understand how they might produce signals that ultimately get to the brain. Until we can work that out, this is one area where we’re all finding it a little hard to navigate.
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