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Last year, he published a paper in Nature Materials describing a protein complex that he dubbed MagR, which consists of iron crystals enveloped in a double helix of cryptochrome—the two main theories rolled into one.

Xie  hopes that others will strengthen his case with further experiments, such as inactivating the gene for MagR in certain fruit-fly tissues to see whether it affects the animals’ sense of direction.

Xie’s team has shown that MagR and Cry are produced in the same cells in pigeon retinas — the birds’ proposed magnetoreception centre — but MagR and Cry are found in many cells, says Keays.

“With such a small amount of iron, one has to ask whether in vivo, at physiological temperatures, MagR is capable of possessing magnetic properties at all,” he says.

MagR’s biocompass activity might simply be the result of experimental contamination, says Michael Winklhofer, a magnetism specialist and Earth scientist at Ludwig Maximilian University of Munich in Germany.