How Do Animals Sense Magnetic Fields? The Sense Nobody Can Locate

The Earth's magnetic field provides two distinct kinds of information, and animals appear to use both. Direction comes from the field lines, which run roughly north to south, giving a compass. Position comes from the fact that the field is not uniform: its intensity and its inclination, the angle at which the field lines enter the ground, vary systematically with latitude, so a measurement of both amounts to an approximate coordinate. Birds and turtles have been shown in laboratory experiments to respond to changes in intensity and inclination as though they were being moved geographically, which indicates a map sense rather than merely a compass. The signal is extremely weak, roughly fifty microtesla, which is a small fraction of a fridge magnet, and it is this weakness that makes the detection mechanism so difficult to explain.

Three proposals have serious support and they are not exclusive:

• •Magnetite, crystals of iron oxide found in bacteria, fish and the beaks of some birds, which physically align with the field and could pull on nerve endings. The mechanism is proven in magnetotactic bacteria, which simply orient passively, and the vertebrate evidence has been damaged by a 2012 study showing that iron-rich cells in pigeon beaks are immune cells rather than neurons
• •Radical pairs in the eye, a quantum mechanism in which a photon striking a cryptochrome protein in the retina creates a pair of molecules with entangled electron spins whose recombination rate depends on the magnetic field's orientation, altering the visual signal so that the bird may literally see the field as a pattern overlaid on its vision
• •Electromagnetic induction, in which an animal moving through the field generates a small voltage, a mechanism that works well in seawater because it conducts, and which is the likely explanation in sharks and rays through their electroreceptive ampullae
• •The radical pair proposal has the most supporting evidence in birds, including that the compass is light-dependent, works only in certain wavelengths, and is disrupted by weak radio-frequency fields of exactly the kind that should interfere with electron spins

The field was opened by Wolfgang and Roswitha Wiltschko, who showed in 1972 that European robins in orientation cages responded to artificially rotated magnetic fields, and, unexpectedly, that the birds use an inclination compass rather than a polarity compass: they distinguish poleward from equatorward by the angle of the field lines rather than by which end is north, which means reversing the field's polarity does not confuse them while inverting the inclination does. Ken Lohmann's work on loggerhead turtles demonstrated that hatchlings respond to combinations of field intensity and inclination corresponding to specific parts of the Atlantic by swimming in the direction appropriate to that location, which is a map sense present without any experience. Mole rats, bats, salmon, spiny lobsters, honeybees and fruit flies have all shown magnetic responses. A 2013 study reported that cattle and deer tend to align their bodies north to south when grazing, found from satellite imagery, a result that has been both replicated and disputed.

The receptor has resisted identification for reasons that are structural rather than accidental. Magnetic fields pass through tissue unimpeded, so unlike light or sound there is no organ that must be positioned at a surface and no opening to look for, and the receptor could be anywhere in the body and could consist of a small number of cells. The signal energy is far below the thermal noise of a cell, which rules out most simple mechanisms and is why the quantum proposal is taken seriously despite sounding exotic. Experiments are hard to control, since the laboratory environment contains electrical equipment producing fields that can swamp the one being tested, and several early results failed to replicate for that reason. The field also went through a damaging episode when a series of high-profile papers on magnetoreception in a fruit fly and in other systems came from a laboratory later found to have manipulated data, which set back confidence in the area considerably.

Beyond the puzzle, the sense has consequences. If birds navigate using a magnetic compass in the eye that operates through electron spins, then radio-frequency noise from human infrastructure could interfere with it, and a 2014 study found that urban electromagnetic noise disrupted the orientation of robins in a city, an effect that vanished when the huts were screened. Light pollution matters for the same reason if the mechanism is light-dependent. Strandings of whales have been correlated with geomagnetic disturbances in several studies, on the hypothesis that solar activity distorts the field they navigate by. And the possibility that a biological system exploits quantum coherence at body temperature, which physicists long assumed impossible because such states collapse almost instantly in a warm wet environment, has made the bird compass one of the central examples in the emerging field of quantum biology, alongside photosynthesis and enzyme catalysis.

The Earth's field supplies both a compass, from the direction of the field lines, and an approximate map, from the way intensity and inclination vary with latitude, and animals from robins to turtles have been shown experimentally to use both. Three mechanisms compete: iron oxide crystals that align physically, electromagnetic induction which works in seawater, and a light-dependent quantum process in retinal proteins whose electron spins respond to field orientation, which has the strongest evidence in birds. No vertebrate receptor has yet been identified.