Has birds’ mysterious ‘compass’ organ been found at last?

Pigeons can sense Earth’s magnetic field by detecting tiny electrical currents in their inner ears, researchers suggest. Such an inner compass could help to explain how certain animals can achieve astonishing feats of long-distance navigation.

The team performed advanced brain mapping as well single-cell RNA sequencing of pigeon inner-ear cells. Both lines of evidence point to the inner ear as the birds’ ‘magnetoreception’ organ. The results appeared in the Science on 20 November ¹.

“This is probably the clearest demonstration of the neural pathways responsible for magnetic processing in any animal,” says Eric Warrant, a sensory biology researcher at the University of Lund in Sweden. Studies have suggested that various animals, including turtles, trout and robins, can sense the direction and strength of magnetic fields, although the evidence has sometimes been contested — and the mechanisms have remained controversial.

Bird-brained navigation

Two leading hypotheses have led the research into how birds sense magnetic fields. One is a quantum-physics effect in retina cells where birds ‘see’ magnetic fields. Another is that microscopic iron oxide particles in the beak could act as tiny compass needles. However, it’s largely unknown where magnetic information is sensed in animals’ brains and how sensory neurons confer sensitivity to electromagnetic changes.

In 2011, researchers found hints that magnetic fields triggered pigeons’ vestibular system, the organ that enables vertebrates to sense accelerations (including gravity) and helps them to stay balanced². The structure is made of three fluid-filled loops which are mutually perpendicular, so they can communicate to the brain the direction of an acceleration by breaking it down into three ‘x, y, z’ components.

David Keays, a neuroscientist at Ludwig-Maximilian University of Munich in Germany, designed an experiment that could reveal how pigeons’ brains respond to magnetic fields.

Keays’ team exposed six pigeons to a magnetic field slightly stronger than Earth’s for just over an hour. The birds’ heads were immobilized and the magnetic field was continually rotated to simulate the heads’ motions with respect to Earth’s geomagnetic field.

Next, the team used a method for measuring activation patterns of neurons across the brain — by measuring a genetic marker of cell activity in pigeon brains made transparent by a technique called clearing. Maps of brain activity in birds that had been exposed to magnetic fields were compared to a control group that had not been exposed to magnetic fields.

The results showed neuronal activity related to magnetic fields in the brain region that receives input from the vestibular system, as well as in regions that help to integrate various sensory stimuli. This result narrowed down the list of potential compasses to one — the vestibular system — although it did not explain how pigeon neurons can physically sense magnetic fields.

A magnetic sense organ

In principle, a conducting material in an organism could produce electric currents in response to magnetic fields, giving an animal a ‘magnetic sense’ — a mechanism that has been proposed as early as 1882 by French zoologist Camille Viguier³.

In previous research, Keays had looked for a molecular mechanism of this magnetoreception by following inspiration from the biophysics of sharks and skates, which have organs that sense minute electric fields to help them find prey⁴. These animals express a protein sensitive to changes in neurons’ electrical activity, but modified with a 10-amino-acid-long insertion that allows them to sense electrical currents generated from magnetic fields.