Magnetoreception
Magnetoreception is a
Birds have iron-containing materials in their upper beaks. There is some evidence that this provides a magnetic sense, mediated by the trigeminal nerve, but the mechanism is unknown.
History
Biologists have long wondered whether migrating animals such as birds and sea turtles have an inbuilt magnetic compass, enabling them to navigate using the Earth's magnetic field. 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 1977, M. M. Walker and colleagues identified iron-based (magnetite) magnetoreceptors in the snouts of rainbow trout. 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.[1]
Proposed mechanisms
In animals
In animals, the mechanism for magnetoreception is still under investigation. Two main hypotheses are currently being discussed: one proposing a quantum compass based on a
Cryptochrome
According to the first model, magnetoreception is possible via the
In 1978, Schulten and colleagues proposed that this was the mechanism of magnetoreception.[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.[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 a chromophore, which causes the formation of a radical-pair whose electrons are quantum 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 molecule flavin 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 in migratory 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 the European 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 been selected for its magnetic sensitivity.[4]
Behavioral experiments on migratory birds also support this theory. Caged migratory birds such as robins display migratory restlessness, known by
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 cryptochrome can be activated in the dark, removing the objection that the blue light-activated mechanism would not work when birds were migrating at night. A different radical pair is formed by re-oxidation of fully-reduced FADH−. Experiments with European robins, using flickering light and a magnetic field switched off when the light was on, showed that the birds detected the field without light. The birds were unaffected by local 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]
Iron-based
The second proposed model for magnetoreception relies on clusters composed of iron, 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 the trigeminal 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]
Electromagnetic induction
Another possible mechanism of magnetoreception in animals is electromagnetic induction in
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]
The yellow 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]
Passive alignment in bacteria
Magnetotactic bacteria of multiple taxa contain sufficient magnetic material in the form of magnetosomes, nanometer-sized particles of magnetite,[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 through
Unanswered questions
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]
Taxonomic range
Magnetoreception is widely distributed taxonomically. It is present in many of the animals so far investigated. These include
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
Eukaryotes |
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cryptochrome |
In molluscs
The giant sea slug
In insects
The fruit fly
Magnetoreception has been studied in detail in insects including honey bees, ants and termites.[37] Ants and bees navigate using their magnetic sense both locally (near their nests) and when migrating.[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.[39][40]
In vertebrates
In fish
Studies of magnetoreception in
In amphibians
Some of the earliest studies of amphibian magnetoreception were conducted with cave 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]
Both European toads (Bufo bufo) and natterjack 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]
In reptiles
The majority of study on magnetoreception in reptiles involves turtles. Early support for magnetoreception in turtles was provided in a 1991 study on hatchling loggerhead 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 on Terrapene carolina, a box 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. 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.[52]
In birds
For a long time the
In mammals
Some mammals are capable of magnetoreception. When woodmice 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.[67] 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.[68]
The Zambian mole-rat, a subterranean mammal, uses magnetic fields to aid in nest orientation.[70] 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 the superior colliculus, as measured by immediate gene 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.[69]
Bats may use magnetic fields to orient themselves. They use
Red foxes (Vulpes vulpes) may use magnetoreception when predating small rodents like mice and voles. They attack this kind of prey using a specific high-jump, preferring a north-eastern compass direction. Successful attacks are tightly clustered to the north.[73]
It is unknown whether humans can sense magnetic fields.
See also
- Electroreception
- Magnetobiology
- Quantum biology
- Salmon run
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