Magnetoreception

Experiments on migratory birds provide evidence that they make use of a cryptochrome protein in the eye, relying on the quantum radical pair mechanism to perceive magnetic fields.

This effect is extremely sensitive to weak magnetic fields, and readily disturbed by radio-frequency interference, unlike a conventional iron compass.

Cartilaginous fish including sharks and stingrays can detect small variations in electric potential with their electroreceptive organs, the ampullae of Lorenzini.

Until late in the 20th century, evidence for this was essentially only behavioural: many experiments demonstrated that animals could indeed derive information from the magnetic field around them, but gave no indication of the mechanism.

In 1972, Roswitha and Wolfgang Wiltschko showed that migratory birds responded to the direction and inclination (dip) of the magnetic field.

In 2003, G. Fleissner and colleagues found iron-based receptors in the upper beaks of homing pigeons, both seemingly connected to the animal's trigeminal nerve.

Research took a different direction in 2000, however, when Thorsten Ritz and colleagues suggested that a photoreceptor protein in the eye, cryptochrome, was a magnetoreceptor, working at a molecular scale by quantum entanglement.

[10] In 2000, scientists proposed that cryptochrome – a flavoprotein in the rod cells in the eyes of birds – was the "magnetic molecule" behind this effect.

In 2004, Thorsten Ritz showed that a weak radio-frequency electromagnetic field, chosen to be at the same frequency as the singlet-triplet oscillation of cryptochrome radical pairs, effectively interfered with the birds' orientation.

[14] The second proposed model for magnetoreception relies on clusters composed of iron, a natural mineral with strong magnetism, used by magnetotactic bacteria.

[9] Both the exact location and ultrastructure of birds' iron-containing magnetoreceptors remain unknown; they are believed to be in the upper beak, and to be connected to the brain by the trigeminal nerve.

The organs are mucus-filled and consist of canals that connect pores in the skin of the mouth and nose to small sacs within the animal's flesh.

[21][22][23] Sandbar sharks, Carcharinus plumbeus, have been shown to be able to detect magnetic fields; the experiments provided non-definitive evidence that the animals had a magnetoreceptor, rather than relying on induction and electroreceptors.

[9] The yellow stingray, Urobatis jamaicensis, is able to distinguish between the intensity and inclination angle of a magnetic field in the laboratory.

[30] The ability to detect and respond to magnetic fields may exist in plants, possibly as in animals mediated by cryptochrome.

Experiments by different scientists have identified multiple effects, including changes to growth rate, seed germination, mitochondrial structure, and responses to gravity (geotropism).

[31] Fishes inc. sockeye salmon Amphibians inc. cave salamander Mammals inc. big brown bat Reptiles inc. box turtle Birds inc. homing pigeon Diptera inc. fruit fly Hymenoptera inc. honey bee inc. giant sea slug Plants inc. pea The giant sea slug Tochuina gigantea (formerly T. tetraquetra), a mollusc, orients its body between north and east prior to a full moon.

[38] In particular, the Brazilian stingless bee Schwarziana quadripunctata is able to detect magnetic fields using the thousands of hair-like sensilla on its antennae.

When randomly displaced 150 metres (490 ft) from their breeding sites, these toads can navigate their way back,[46] but this ability can be disrupted by fitting them with small magnets.

After teaching a group of these box turtles to swim to either the east or west end of an experimental tank, a strong magnet disrupted the learned routes.

In loggerhead and leatherback turtles, breeding takes place on beaches, and, after hatching, offspring crawl rapidly to the sea.

Although differences in light density seem to drive this behaviour, magnetic alignment appears to play a part.

For instance, the natural directional preferences held by these hatchlings (which lead them from beaches to the sea) reverse upon experimental inversion of the magnetic poles.

[18][57] Second, pigeon magnetic field detection is impaired by sectioning the trigeminal nerve and by application of lidocaine, an anaesthetic, to the olfactory mucosa.

In the search for magnetite receptors, a large iron-containing organelle (the cuticulosome) of unknown function was found in the inner ear of pigeons.

[59][60] Areas of the pigeon brain that respond with increased activity to magnetic fields are the posterior vestibular nuclei, dorsal thalamus, hippocampus, and visual hyperpallium.

[64] When the same mice are allowed access to visual cues, they are able to orient themselves towards home despite the presence of inverted magnetic fields.

Experimental exposure to magnetic fields leads to an increase in neural activity within the superior colliculus, as measured by immediate gene expression.

The more time the mole rats were exposed to a magnetic field, the greater the immediate early gene expression within the InGi.

[70] In migratory soprano pipistrelles (Pipistrellus pygmaeus), experiments using mirrors and Helmholtz coils show that they calibrate the magnetic field using the position of the solar disk at sunset.