Magnetoreception and Navigation in Migratory Animals: Mechanisms and Evidence

Animals as different as songbirds, sea turtles, salmon and butterflies reach destinations they have never seen, and part of the information they use comes from the geomagnetic field. This review separates the two capacities involved, a compass that reads direction and a map that estimates position, and examines the evidence for each. Behavioural work in orientation cages established that the avian compass responds to the inclination of field lines rather than to their polarity, that it requires light of short wavelength, and that it is disrupted by weak radio-frequency fields. These properties point to a quantum mechanism in which a light-induced radical pair interconverts between singlet and triplet states at a rate that depends on the orientation of the molecule in the field, and cryptochrome proteins from the retina of a migratory songbird show magnetic sensitivity in vitro. A second mechanism based on biogenic magnetite is supported by conditioning experiments and by trigeminal recordings, though the cells once identified as receptors in the pigeon beak proved to be macrophages. Displacement experiments in reed warblers, sea turtles and salmon indicate a magnetic map based on inclination and declination. No receptor has been confirmed in any species.