Magnetic sense

Magnetoreception (also magnetoception) is a sense which allows an organism to detect a magnetic field to perceive direction, altitude or location. This sensory modality is used by a range of animals for orientation and navigation, and as a method for animals to develop regional maps. For the purpose of navigation, magnetoreception deals with the detection of the Earth's magnetic field.

Magnetoreception is present in bacteria, arthropods, molluscs and members of all major taxonomic groups of vertebrates. Humans are not thought to have a magnetic sense, but there is a protein (a ) in the eye which could serve this function.

An unequivocal demonstration of the use of magnetic fields for orientation within an organism has been in a class of bacteria known as magnetotactic bacteria. These bacteria demonstrate a behavioural phenomenon known as magnetotaxis, in which the bacterium orients itself and migrates in the direction along the Earth's magnetic field lines. The bacteria contain magnetosomes, which are nanometer-sized particles of magnetite or iron sulfide enclosed within the bacterial cells. The magnetosomes are surrounded by a membrane composed of phospholipids and fatty acids and contain at least 20 different proteins They form in chains where the moments of each magnetosome align in parallel, causing each bacterium cell to essentially act as a magnetic dipole, giving the bacteria their permanent-magnet characteristics.

For animals the mechanism for magnetoreception is unknown, but there exist two main hypotheses to explain the phenomenon. According to one model, magnetoreception is possible via the radical pair mechanism. The radical-pair mechanism is well-established in spin chemistry, and was speculated to apply to magnetoreception in 1978 by Schulten et al.. In 2000, was proposed as the "magnetic molecule", so to speak, that could harbor magnetically sensitive radical-pairs. Cryptochrome, a flavoprotein found in the eyes of European robins and other animal species, is the only protein known to form photoinduced radical-pairs in animals. The function of cryptochrome is diverse across species, however, the photoinduction of radical-pairs occurs by exposure to blue light, which excites an electron in a chromophore. The Earth's magnetic field is only 0.5 Gauss and so it is difficult to conceive of a mechanism, other than phase shift, by which such a field could lead to any chemical changes other than those affecting the weak magnetic fields between radical pairs. Cryptochromes are therefore thought to be essential for the light-dependent ability of the fruit fly Drosophila melanogaster to sense magnetic fields.

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