Scientists have long known that migrating birds and homing pigeons navigate in part by sensing the Earth's magnetic fields, especially at night or in overcast conditions when visual landmarks or sunshine are in short supply. But exactly where this magneto-sensing occurs in the body—and the mechanism that enables it—remains a matter of intense debate. A new paper published in the journal Science suggests that homing pigeons have iron-rich immune cells in their livers that help them detect magnetic fields and transmit that information to the brain.
There are three primary hypotheses for how birds might sense Earth's geomagnetic field. One is a compass-like mechanism, whereby the Earth exerts a pull on magnetic particles in a bird's upper beak that relays directional information via a large nerve in the cranium. A second is that it happens biologically via cellular ion channels sensitive to voltage, enabling birds to sense changes in the magnetic field. And a third suggests that physical effects on retinal pigments enable birds to detect photons and send signals to the brain, although this mechanism is really only viable in the light.
None fully explain how animals can sense magnetic fields. However, “We had some clues that the liver and spleen have magnetic properties, because they break down red blood cells and so store much iron in the body,” said co-author Clivia Lisowski of the University of Bonn and the University Hospital Bonn. This refers to a 2015 paper suggesting that red pulp macrophages in the spleens of mice and humans are intrinsically superparamagnetic and hence more sensitive to magnetic fields. But it wasn't clear if those properties were involved in any kind of magnetoreception.