How do animals navigate?

Birds tested in controlled magnetic fields shift their preferred direction when the field is shifted. The sense responds to the inclination of the field rather than its polarity, so it distinguishes poleward from equatorward rather than north from south.

It depends on light, and on specific wavelengths: under yellow or red light, or in darkness, the normal compass orientation fails. These are the properties that led Ritz and colleagues to propose a chemical mechanism. Proponents of magnetic particles reply that an inclination compass does not tell the two apart, and offer other explanations for the light dependence.

Absorbing a photon in a molecule called cryptochrome, present in the retina, can produce a pair of radicals inside the protein, with their electron spins either opposed or aligned. The two electron spins are linked, and a magnetic field can influence how that linkage evolves, which changes the proportions of the products of the reaction.

The effect is tiny and the Earth’s field is weak, but a model radical pair has been shown to respond to the direction of an Earth-strength field, and the mechanism accounts for the inclination compass and the need for light. Critics point to what it has not answered: how a signal a million times smaller than the thermal energy around it is read in a living cell, and how birds orient under green light, outside the range absorbed by the form of the molecule that starts the reaction.

In 2021 cryptochrome 4 from the European robin was shown to be magnetically sensitive in a test tube, and more so than the same protein from chicken and pigeon, in a field of 30 millitesla, several hundred times the Earth’s. That is not the same as demonstrating it in a bird, and its authors say so.

Several candidate receptor cells have been reported and then reinterpreted. The best known, magnetite-bearing nerve endings in the pigeon’s upper beak, had been widely accepted in the field until a 2012 study identified the iron-rich cells there as macrophages, immune cells rather than neurons. Supporters of receptors in the beak still dispute parts of that reading.

A sense whose receptors cannot be located after this long is unusual. Part of the reason may be that magnetic fields pass through tissue: nothing like a lens or an ear is needed to gather the stimulus, so the receptor cells could lie almost anywhere in the body, with no organ to point to them.

Each account’s case is made mostly by the people who work on it, and the two camps have become entrenched, which is worth knowing when they are weighed against each other. The magnetite account’s proponents argue that biogenic magnetite is demonstrably made by organisms, that crystals of the right size and arrangement would transduce a field directly, and that the absence of a confirmed receptor is a statement about how hard these cells are to find. Its critics reply that the search has been led astray by immune cells and by iron contamination from the laboratory itself.

A compass gives direction. Getting to a specific place requires knowing where you are, and displaced birds can correct for it, which implies a map as well.

Candidate map cues include magnetic intensity and inclination, smell and landmarks, and more speculatively infrasound. Controlled experiments support the magnetic and smell cues in some species, the evidence for infrasound is still thin, and no general account covers the range of animals that do this.