Magnetoreception is asense which allows anorganism to detect theEarth's magnetic field. Animals with this sense include somearthropods,molluscs, andvertebrates (fish, amphibians, reptiles, birds, and mammals). The sense is mainly used for orientation andnavigation, but it may help some animals to form regional maps. Experiments onmigratory birds provide evidence that they make use of acryptochrome protein in the eye, relying on the quantumradical 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.
Birds have iron-containing materials in their upper beaks. There is some evidence that this provides a magnetic sense, mediated by thetrigeminal nerve, but the mechanism is unknown.
Biologists have long wondered whethermigrating animals such asbirds andsea turtles have an inbuilt magnetic compass, enabling them tonavigate using theEarth's magnetic field. Until late in the 20th century, evidence for this was essentially onlybehavioural: 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 andinclination (dip) of the magnetic field. In 1977, M. M. Walker and colleagues identified iron-based (magnetite) magnetoreceptors in the snouts ofrainbow trout. In 2003, G. Fleissner and colleagues found iron-based receptors in the upper beaks ofhoming pigeons, both seemingly connected to the animal'strigeminal nerve. Research took a different direction in 2000, however, when Thorsten Ritz and colleagues suggested that aphotoreceptor protein in the eye,cryptochrome, was a magnetoreceptor, working at a molecular scale byquantum entanglement.[1]
In animals, the mechanism for magnetoreception is still under investigation. Two main hypotheses are currently being discussed: one proposing a quantum compass based on aradical pair mechanism,[2] the other postulating a more conventional iron-based magnetic compass withmagnetite particles.[3]
According to the first model, magnetoreception is possible via theradical pair mechanism,[5] which is well-established inspin chemistry. The mechanism requires two molecules, each with unpaired electrons, at a suitable distance from each other. When these can exist in states either with theirspin axes in the same direction, or in opposite directions, the molecules oscillate rapidly between the two states. That oscillation is extremely sensitive to magnetic fields.[6][7][8][9] Because the Earth's magnetic field is extremely weak, at 0.5gauss, the radical pair mechanism is currently the only credible way that the Earth's magnetic field could cause chemical changes (as opposed to the mechanical forces which would be detected via magnetic crystals acting like a compass needle).[9]
In 1978, Schulten and colleagues proposed that this was the mechanism of magnetoreception.[10] In 2000, scientists proposed thatcryptochrome – aflavoprotein in therod cells in the eyes of birds – was the "magnetic molecule" behind this effect.[11] It is the only protein known to form photoinduced radical-pairs in animals.[5] The function of cryptochrome varies by species, but its mechanism is always the same: exposure to blue light excites an electron in achromophore, which causes the formation of a radical-pair whose electrons arequantum entangled, enabling the precision needed for magnetoreception.[12][13]
Many lines of evidence point to cryptochrome and radical pairs as the mechanism of magnetoreception in birds:[4]
Despite 20 years of searching, no biomolecule other than cryptochrome has been identified capable of supporting radical pairs.[4]
In cryptochrome, a yellow moleculeflavin adenine dinucleotide (FAD) can absorb a photon of blue light, putting the cryptochrome into an activated state: an electron is transferred from a tryptophan amino acid to the FAD molecule, forming a radical pair.[4]
Of the six types of cryptochrome in birds, cryptochrome-4a (Cry4a) binds FAD much more tightly than the rest.[4]
Cry4a levels inmigratory birds, which rely on navigation for their survival, are highest during the spring and autumn migration periods, when navigation is most critical.[4]
The Cry4a protein from theEuropean robin, a migratory bird, is much more sensitive to magnetic fields than similar but not identical Cry4a from pigeons and chickens, which are non-migratory.[4]
These findings together suggest that the Cry4a of migratory birds has beenselected for its magnetic sensitivity.[4]
Behavioral experiments on migratory birds support this theory. Caged migratory birds such as robins display migratory restlessness, known byethologists asZugunruhe, in spring and autumn: they often orient themselves in the direction in which they would migrate. 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. The field would not have interfered with an iron-based compass. Further, birds are unable to detect a 180 degree reversal of the magnetic field, something they would straightforwardly detect with an iron-based compass.[4]
From 2007 onwards, Henrik Mouritsen attempted to replicate this experiment. Instead, he found that robins were unable to orient themselves in the wooden huts he used. Suspecting extremely weak radio-frequency interference from other electrical equipment on the campus, he tried shielding the huts with aluminium sheeting, which blocks electrical noise but not magnetic fields. When he earthed the sheeting, the robins oriented correctly; when the earthing was removed, the robins oriented at random. Finally, when the robins were tested in a hut far from electrical equipment, the birds oriented correctly. These effects imply a radical-pair compass, not an iron one.[4]
In 2016, Wiltschko and colleagues showed that European robins were unaffected bylocal anaesthesia of the upper beak, showing that in these test conditions orientation was not from iron-based receptors in the beak. In their view, cryptochrome and its radical pairs provide the only model that can explain the avian magnetic compass.[12] A scheme with three radicals rather than two has been proposed as more resistant to spin relaxation and explaining the observed behaviour better.[14]
The second proposed model for magnetoreception relies on clusters composed ofiron, a natural mineral with strong magnetism, used by magnetotactic bacteria. Iron clusters have been observed in the upper beak of homing pigeons,[15] and other taxa.[16][5][17][18] Iron-based systems could form a magnetoreceptive basis for many species including turtles.[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 thetrigeminal nerve. This system is in addition to the cryptochrome system in the retina of birds. Iron-based systems of unknown function might also exist in other vertebrates.[19]
Another possible mechanism of magnetoreception in animals is electromagnetic induction incartilaginous fish, namelysharks,stingrays, andchimaeras. These fish haveelectroreceptive organs, theampullae of Lorenzini, which can detect small variations inelectric potential. 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. They are used to sense the weak electric fields of prey and predators. These organs have been predicted to sense magnetic fields, by means ofFaraday's law of induction: as a conductor moves through a magnetic field an electric potential is generated. In this case the conductor is the animal moving through a magnetic field, and the potential induced (Vind) depends on the time (t)-varying rate of magnetic flux (Φ) through the conductor according to
The ampullae of Lorenzini detect very small fluctuations in the potential difference between the pore and the base of the electroreceptor sac. An increase in potential results in a decrease in the rate of nerve activity. This is analogous to the behavior of a current-carrying conductor.[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.[23] Electromagnetic induction has not been studied in non-aquatic animals.[9]
Theyellow stingray,Urobatis jamaicensis, is able to distinguish between the intensity and inclination angle of a magnetic field in the laboratory. This suggests that cartilaginous fishes may use the Earth's magnetic field for navigation.[20]
Magnetite magnetosomes inGammaproteobacteria strain SS-5. (A) Chain of highly elongated magnetosomes. (B) Part of a chain. (C) The magnetosome in the lower right in (B), viewed along the direction, with itsFourier transform in the lower right.[24]
Magnetotactic bacteria of multiple taxa contain sufficient magnetic material in the form ofmagnetosomes, nanometer-sized particles ofmagnetite,[25] that the Earth's magnetic field passively aligns them, just as it does with a compass needle. The bacteria are thus not actually sensing the magnetic field.[26][27]
A possible but unexplored mechanism of magnetoreception in animals is throughendosymbiosis with magnetotactic bacteria, whose DNA is widespread in animals. This would involve having these bacteria living inside an animal, and their magnetic alignment being used as part of a magnetoreceptive system.[28]
It remains likely that two or more complementary mechanisms play a role in magnetic field detection in animals. Of course, this potential dual mechanism theory raises the questions of to what degree each method is responsible for the stimulus, and how they produce a signal in response to the weak magnetic field of the Earth.[9]
In addition, it is possible that magnetic senses may be different for different species. Some species may only be able to detect north and south, while others may only be able to differentiate between the equator and the poles. Although the ability to sense direction is important in migratory navigation, many animals have the ability to sense small fluctuations in earth's magnetic field to map their position to within a few kilometers.[9][29]
Magnetoreception is widely distributed taxonomically. It is present in many of the animals so far investigated. These includearthropods,molluscs, and amongvertebrates in fish, amphibians, reptiles, birds, and mammals. Its status in other groups remains unknown.[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, seedgermination,mitochondrial structure, and responses to gravity (geotropism). The results have sometimes been controversial, and no mechanism has been definitely identified. The ability may be widely distributed, but its taxonomic range in plants is unknown.[31]
The giant sea slugTochuina gigantea (formerlyT. tetraquetra), amollusc, orients its body between north and east prior to a full moon.[32] A 1991 experiment offered a right turn to geomagnetic south and a left turn to geomagnetic east (aY-shaped maze). 80% ofTochuina made a turn to magnetic east. When the field was reversed, the animals displayed no preference for either turn.[33][34]Tochuina's nervous system is composed of individually identifiableneurons, four of which are stimulated by changes in the applied magnetic field, and two which are inhibited by such changes.[34] The tracks of the similar speciesTritonia exsulans become more variable in direction when close to strongrare-earth magnets placed in their natural habitat, suggesting that the animal uses its magnetic sense continuously to help it travel in a straight line.[35]
The fruit flyDrosophila melanogaster may be able to orient to magnetic fields. In onechoice test, flies were loaded into an apparatus with two arms that were surrounded by electric coils. Current was run through each of the coils, but only one was configured to produce a 5-Gauss magnetic field (about ten times stronger than the Earth's magnetic field) at a time. The flies were trained to associate the magnetic field with a sucrose reward. Flies with an altered cryptochrome, such as with an antisense mutation, were not sensitive to magnetic fields.[36]
Magnetoreception has been studied in detail in insects includinghoney bees,ants andtermites.[37] Ants and bees navigate using their magnetic sense both locally (near their nests) and when migrating.[38] In particular, the Brazilian stingless beeSchwarziana quadripunctata is able to detect magnetic fields using the thousands of hair-likesensilla on its antennae.[39][40]
Studies of magnetoreception inbony fish have been conducted mainly with salmon. Bothsockeye salmon (Oncorhynchus nerka) andChinook salmon (Oncorhynchus tschawytscha) have a compass sense. This was demonstrated in experiments in the 1980s by changing the axis of a magnetic field around a circular tank of young fish; they reoriented themselves in line with the field.[41][42]
Some of the earliest studies of amphibian magnetoreception were conducted withcave salamanders (Eurycea lucifuga). Researchers housed groups of cave salamanders in corridors aligned with either magnetic north–south, or magnetic east–west. In tests, the magnetic field was experimentally rotated by 90°, and salamanders were placed in cross-shaped structures (one corridor along the new north–south axis, one along the new east–west axis). The salamanders responded to the field's rotation.[43]
Red-spotted newts (Notophthalmus viridescens) respond to drastic increases in water temperature by heading for land. The behaviour is disrupted if the magnetic field is experimentally altered, showing that the newts use the field for orientation.[44][45]
BothEuropean toads (Bufo bufo) andnatterjack toads (Epidalea calamita) toads rely on vision and olfaction when migrating to breeding sites, but magnetic fields may also play a role. 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.[47]
Magnetoreception plays a part in guidingloggerhead hatchlings to the sea[48]
The majority of study on magnetoreception in reptiles involves turtles. Early support for magnetoreception in turtles was provided in a 1991 study on hatchlingloggerhead turtles which demonstrated that loggerheads can use the magnetic field as a compass to determine direction.[49] Subsequent studies have demonstrated that loggerhead and green turtles can also use the magnetic field of the earth as a map, because different parameters of the Earth's magnetic field vary with geographic location. The map in sea turtles was the first ever described though similar abilities have now been reported in lobsters, fish, and birds.[48] Magnetoreception by land turtles was shown in a 2010 experiment onTerrapene carolina, abox turtle. 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.[50][51]
Orientation toward the sea, as seen in turtle hatchlings, may rely partly on magnetoreception. Inloggerhead andleatherback 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.[52]
Homing pigeons use magnetic fields as part of their complexnavigation system.[53]William Keeton showed that time-shifted homing pigeons (acclimatised in the laboratory to a different time-zone) are unable to orient themselves correctly on a clear, sunny day; this is attributed to time-shifted pigeons being unable to compensate accurately for the movement of the sun during the day. Conversely, time-shifted pigeons released on overcast days navigate correctly, suggesting that pigeons can use magnetic fields to orient themselves; this ability can be disrupted with magnets attached to the birds' backs.[54][55] Pigeons can detect magnetic anomalies as weak as 1.86gauss.[56]
For a long time thetrigeminal system was the suggested location for a magnetite-based magnetoreceptor in the pigeon. This was based on two findings: First, magnetite-containing cells were reported in specific locations in the upper beak.[15] However, the cells proved to be immune systemmacrophages, notneurons able to detect magnetic fields.[18][57] Second, pigeon magnetic field detection is impaired by sectioning the trigeminal nerve and by application oflidocaine, an anaesthetic, to the olfactory mucosa.[58] However, lidocaine treatment might lead to unspecific effects and not represent a direct interference with potential magnetoreceptors.[57] As a result, an involvement of the trigeminal system is still debated. In the search for magnetite receptors, a large iron-containing organelle (thecuticulosome) 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 posteriorvestibular nuclei,dorsal thalamus,hippocampus, andvisual hyperpallium.[61]
Domestic hens have iron mineral deposits in the sensorydendrites in the upper beak and are capable of magnetoreception.[16][62] Beak trimming causes loss of the magnetic sense.[63]
Some mammals are capable of magnetoreception. Whenwoodmice are removed from their home area and deprived of visual and olfactory cues, they orient towards their homes until an inverted magnetic field is applied to their cage.[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. This indicates that woodmice use magnetic fields to orient themselves when no other cues are available. The magnetic sense of woodmice is likely based on a radical-pair mechanism.[65]
TheZambian mole-rat is one of several mammals that use magnetic fields, in their case for nest orientation.[66]
TheZambian mole-rat, a subterranean mammal, uses magnetic fields to aid in nest orientation.[67] In contrast to woodmice, Zambian mole-rats do not rely on radical-pair based magnetoreception, perhaps due to their subterranean lifestyle. Experimental exposure to magnetic fields leads to an increase in neural activity within thesuperior colliculus, as measured by immediategene expression. The activity level of neurons within two levels of the superior colliculus, the outer sublayer of the intermediate gray layer and the deep gray layer, were elevated in a non-specific manner when exposed to various magnetic fields. However, within the inner sublayer of the intermediate gray layer (InGi) there were two or three clusters of cells that respond in a more specific manner. The more time the mole rats were exposed to a magnetic field, the greater the immediate early gene expression within the InGi.[66]
Magnetic fields appear to play a role inbat orientation. They useecholocation to orient themselves over short distances, typically ranging from a few centimetres up to 50 metres.[68] When non-migratory big brown bats (Eptesicus fuscus) are taken from their home roosts and exposed to magnetic fields rotated 90 degrees from magnetic north, they become disoriented; it is unclear whether they use the magnetic sense as a map, a compass, or a compass calibrator.[69] Another bat species, the greater mouse-eared bat (Myotis myotis), appears to use the Earth's magnetic field in its home range as a compass, but needs to calibrate this at sunset or dusk.[70] In migratory soprano pipistrelles (Pipistrellus pygmaeus), experiments using mirrors andHelmholtz coils show that they calibrate the magnetic field using the position of the solar disk at sunset.[71][72]
Red foxes (Vulpes vulpes) may be influenced by the Earth's magnetic field whenpredating small rodents like mice and voles. They attack these prey using a specific high-jump, preferring a north-eastern compass direction. Successful attacks are tightly clustered to the north.[73]
There is not yet a consensus on whether humans can sense magnetic fields or not, but it is being studied and some researchers have found evidence suggesting it.[74][75] Theethmoid bone in the nose contains magnetic materials.[76][75] Magnetosensitive cryptochrome 2 (cry2) is present in the human retina.[77] Human alphabrain waves are affected by magnetic fields, but it is not known whether behaviour is affected.[74][77]
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