The ability of animals to perceive the earth's magnetic field and use it to determine its location is referred to as the magnetic sense or orientation to the earth's magnetic field . The ability to orientate oneself to the earth's magnetic field was only proven experimentally in animals and bacteria since the mid-1960s . The so-called “magnetic compass” of migratory birds is best studied today , but the magnetic sense is still considered a largely unexplored sensory performance of animals.
Already Charles Darwin had in his work On the Origin of Species wrote, in 1859 there but not asked pasted text, like a bird may know North and South and "doing his course white adhere so well, as if he were a compass with him led "His answer:". we do not know "in the early 1930s confirmed biologists in one study, already mentioned by Darwin random observations, according to which migratory birds in cages held in autumn comparable to their wild counterparts. Zugunruhe have. At the same time it was proven that the migratory birds kept in cages prefer to fly or hop in a certain direction, whereby this direction roughly corresponds to the direction of migration of free-living conspecifics when they take off. In connection with the homing behavior of many animal species, so called by ethologists , the geomagnetic field was discussed as a possible cause as early as 1941. An early assumption that migratory birds orientate themselves on the earth's magnetic field was also expressed in 1855 by the zoologist Alexander Theodor von Middendorff after studies in Russia.
At the beginning of the 1960s, the ornithologist , an expert on bird migration and then professor of zoology at the Johann Wolfgang Goethe University in Frankfurt am Main , suggested that Friedrich Wilhelm Merkel investigate the biological basis of bird migration experimentally. Merkel had previously researched the energy balance of migratory birds in particular, but at the same time also looked at the orientation ability of migratory birds. First, a round cage was built on the roof of the Frankfurt Zoological Institute, around which a strong, artificial, static magnetic field could be generated. By superimposing the weak natural geomagnetic field in the cage, magnetic south should point in a different, "wrong" direction, on the assumption that the test animals would then hop or try to fly in a "wrong" preferred direction. However, this experimental set-up did not produce any clear results at first, which is why Wolfgang Wiltschko set up a test cage and magnetic coils in 1963 for his doctoral thesis in an empty basement room of the institute, which had been used as a vacuum chamber for earlier experiments by another working group and was completely clad with steel plates. The steel casing of the room completely shielded the earth's magnetic field, which is why Wiltschko was able to work with a weaker artificial magnetic field than before on the roof. The bottom of the cage and the edges of the cage were covered with dusted paper, which left traces of any scratches made by the birds' claws . To check whether his “model animals” - robins caught in the Frankfurt Botanical Garden - showed migratory unrest under the selected conditions, he put one of his seven robins into the cage as a test. The next day, on October 12, 1963, he discovered "a fantastic preference for direction south" based on the tracks. In the following days, Wiltschko succeeded in reproducing the behavior of his first test animal with the other six robins. Even after turning the artificial magnetic field by 90 degrees, the preferred direction to the south - but actually to the east or west in relation to the shielded terrestrial magnetic field - was retained. This is how he achieved the first experimental proof that animals perceive a static magnetic field and can adapt their behavior to this magnetic field. His publication of these findings marked the beginning of a new branch of research in behavioral ecology . He later backed up his findings by studying black warblers and domestic pigeons .
Initially, the publications of the Frankfurt ornithologists met with great skepticism from their specialist colleagues, as several other working groups were unable to reproduce and thus confirm Wiltschko's findings. The main obstacle to the repeatability elsewhere was, as it turned out in retrospect, that on the one hand the earth's magnetic field had to be shielded, but at the same time an artificial static magnetic field had to be built up and whose field strength could not deviate too strongly from that of the earth's magnetic field. It was not until 1972 that the Frankfurt research results were recognized internationally, as it were, when they were published in the journal Science ; this publication is often quoted today by other specialist authors: as the first description of a newly discovered sensory organ in the animal world.
Animal species with a proven magnetic sense (selection)
The ability to sense the earth's magnetic field is widespread in nature. After robins, warblers and house pigeons, the magnetic sense has also been demonstrated in several dozen other species , such as termites and ants , wasps and honey bees , field cockchafer , Drosophila melanogaster and the house mother ; in molluscs , crustaceans , amphibians and reptiles ; with European eels and various salmon ; in wood mice , golden hamsters , domestic horses and other mammals .
In Caenorhabditis elegans , neurons were identified that respond to magnetic fields and influence the animals' burial behavior.
The magnetic sense had already been demonstrated in around 20 migratory bird species in 2007, i.e. as a basis for orientation on long flight routes over land or - in the case of seabirds - over the Atlantic, for example. The fact that the magnetic sense could also play a role at close range initially seemed implausible - apart from the home pigeons' homing behavior. In the case of chicks and zebra finches , however, this could be demonstrated in experiments.
In 2005, Wolfgang Wiltschko's working group succeeded in demonstrating that chicks of the domestic fowl a few days old can find their “mother” again with the help of the magnetic field if she is hidden behind a screen. Before the tests, the chicks were imprinted on a red table tennis ball so that it acted as a “mother” without causing noises or other attracting signals behind the screen. The chicks were then placed in the middle of a white, rectangular and closed box that did not offer any orientation points, but had a screen in front of every corner. The chicks were now conditioned to look for their "mother" behind the screen that was to the north of this experimental set-up. After successful conditioning, the box with the chicks sitting inside was exposed to an artificial magnetic field which, however, was rotated 90 degrees in relation to the earth's magnetic field - "artificial north" now, for example, indicated "earthly east". Even under these conditions, the chicks continued to look for their “mother” in the “north”, i.e. in the east of the earth's magnetic field: The researchers assessed this behavior as evidence that the chicks sought the hiding place of the red tennis ball solely with the help of their “magnetic sensor”.
In an extension of this experimental set-up, the embossed and conditioned chicks were tested under monochromatic red light (wavelength: 645 nm) or blue light (465 nm), with other chicks the upper beak - which was described as a potential location for magnetoreceptors - was anesthetized. Result: The “mother” was specifically sought out under blue light, but not under red light. And the numbness of the upper beak did not affect the quick location of the "mother". Both findings are similar to those known from robins and were interpreted in 2007 to the effect that the magnetic sense of chickens also seems to be linked to visual perception .
Behavioral researchers from Bielefeld trained zebra finches to look for food in an apparatus - similar to the experiment with chicken chicks - that was hidden in one of four places in the cage. As with the chicks, the feed was laid out in a specific direction in relation to the artificial magnetic field. When the horizontal component of the magnetic field was rotated, the birds looked for food in the corresponding other location. This ability to orientate was lost in an oscillating magnetic field. Another study reported in 2017 that the zebra finch's magnetic compass, similar to that of migratory birds, is light-dependent.
The Australian termite Amitermes meridionalis built extremely narrow, reminiscent of grave stones termite hill , the walls can be high around three meters long and up to four meters. This type of termite is also known as a compass termite because its burrows are always oriented in a north-south direction; In 1978, two Swiss zoologists even proved that the burrows are not oriented towards geographic north, but rather towards magnetic north. Because the mounds took several years to build, it was difficult to test the magnetic sense of these termites experimentally. For example, four young burrows were abandoned shortly after a magnet had been integrated into each, while four control burrows, into which an iron piece of the same size had been inserted, remained inhabited even after seven years. Experiments by Peter M. Jacklyn, a researcher at Charles Darwin University, provided more convincing evidence of a magnetic sense . For his doctoral thesis at the end of the 1980s, he had already removed the top centimeters of the termite mounds during the dry season and observed their repair - initially under the influence of a magnet, later under a variable artificial magnetic field and in a control group of mounds without the influence of artificial magnetism . The structures were each repaired according to the existing orientation of their base. However, there were deviations in the reconstruction of the chambers inside the mound: In the magnetically undisturbed mounds they are predominantly applied parallel or perpendicular to the main axis; this order is less strictly adhered to under a disruptive magnetic field.
An optimization of the temperature inside the termite mounds is assumed to be a biological benefit of the north-south orientation.
Various species of ants have been shown to be able to orient themselves in the earth's magnetic field. The first evidence in ants was published in 1993 after observations on the red fire ant . The antennae of the ants are discussed as a possible “seat” of the magnetic sense.
The species of the so-called desert ants from the genus Cataglyphis live in the arid areas along the Mediterranean as well as in the Middle East and Central Asia and cover distances of more than 100 meters away from their nests in search of food. Investigations of their orientation behavior showed that they use both visual features (landmarks) and smells to find their way and are also able to count the number of steps they take. The observers noticed that the ants walk here and there ("meandering") while searching for food, but return straight to the nest after finding prey. Field studies in Greece on ants of the species Cataglyphis noda , which were published in 2018, indicated that this ability can also be attributed to their magnetic sense .
Young Cataglyphis noda (= C. nodus ) spend the first four weeks exclusively in their subterranean nest. Only then are they active outside - until their death - as forage-seeking workers. “Before an ant goes looking for food, however, it has to calibrate its navigation system. To this end, it shows an extremely special behavior pattern for two to three days: In so-called learning runs, the animals explore the immediate surroundings around the nest entrance and repeatedly turn pirouettes around their own body axis. Recordings with high-speed cameras show that the ants keep stopping during these rotations. The special thing about the longest of these interruptions: At this moment the animals always look exactly back in the direction of the nest entrance, although they cannot see it - a tiny hole in the ground. "Researchers at the Julius Maximilians University of Würzburg generated using a mobile Helmholtz coil pair created an artificial magnetic field in the ants' natural habitat , which they could align differently in relation to the natural terrestrial magnetic field: with the result that the ants no longer looked in a predictable way in the direction of the actual nest entrance, but in the direction given by the coil pair Direction.
Leaf cutter ants of the species Atta colombica named after Christopher Columbus carry their prey into their nests on relatively narrow paths. In a field experiment, researchers at the Smithsonian Tropical Research Institute lifted returning ants off their path, turned them back and forth, and then set them down in the field not far from their path. Most ants did not choose the shortest route back to their path, but strived to return to the nest on a direct route - off the path of their conspecifics. However, when the researchers superimposed the earth's magnetic field with strong electromagnetic pulses, the ants no longer ran specifically in the direction of the nest, but in purely random directions from their starting point. But if an artificial magnetic field was generated in such a way that it was aligned exactly opposite to the terrestrial one, then the ants would move straight away from their nest. In 2008, the researchers suspected that the magnetic sense might be vital if, for example, heavy rain washed away the scent marks on the path and no other orientation aids were available.
Red wood ants
Red wood ants were conditioned in a laboratory experiment in 1995 to seek out a feeding site accessible after crossing a selection chamber mounted inside a coil. This selection chamber had entrances in four directions, the feeding place prepared with honey was in the direction magnetic north. If the magnetic field of the coil was rotated by 90 degrees, the behavior of the foraging ants also changed in a predictable way, that is, they continued to run preferably in the "north" direction, which was now given by the experimenter.
Monarch butterflies ( Danaus plexippus ) are known to fly in large flocks from the northeast of the USA and Canada to their wintering areas in Mexico in autumn. For the first time in 1999 it was proven that these butterflies can use the earth's magnetic field to keep the north-south direction in addition to a sun compass . For this purpose, around 300 monarch butterflies were caught at one of their well-known "resting places" in the US state of Kansas and a few dozen of them were exposed to a strong magnetic field. While the animals of the untreated control group flew in a statistically significant direction south after being released, there was no preferred direction for the animals exposed to an artificial magnetic field.
In the following years there were several contradicting publications, in some of which it was doubted that butterflies had a magnetic sense. In 2014, however, a laboratory study was published in which, after experiments in a flight simulator, it was demonstrated that monarch butterflies can orient themselves using the inclination of the magnetic field, especially when the sky is heavily overcast, but only if they can also use the UV light penetrating through the clouds. A light (between 380 and 420 nm) can perceive. At the same time it was reported that the antennas are likely to be used as the "seat" of the light-dependent inclination compass. According to one of the authors of the study, the complexity of this orientation system corresponds to that of birds and sea turtles. In a review article in 2018, it was noted that the monarch butterflies probably also have a "geomagnetic map".
In 1975 it was proven that the Caribbean lobster ( Panulirus argus ) finds its way back into its range of action if it is removed from it in the experiment and released again several kilometers away. However, it remained unclear by which internal or external influences this homing behavior is supported. Evidence for a magnetic sense was finally obtained in a laboratory experiment that was published in 1985: In a round container with six exits, a total of five lobsters were conditioned to swim through the northernmost exit. Two of the animals learned this task - and when the magnetic field of these two lobsters was rotated, their preferred direction also changed accordingly. The author of the study pointed out that the most common wrongly chosen direction in these animals was the southern exit.
The green sea turtle ( Chelonia mydas ), which can be found in all tropical and subtropical seas, orients itself on the earth's magnetic field in order to return to laying eggs on the same beach for the first time years after hatching. It is assumed that the inclination (= the angle of inclination) of the field lines of the magnetic field at the place of birth is learned permanently through imprinting . In an experiment, 20 green sea turtles were caught shortly before laying their eggs on a beach in Mayotte , equipped with a transmitter and released on the high seas at four locations around 120 kilometers away. Some of them also had a magnet attached to their heads. Result: 19 of the 20 turtles found their way back to Mayotte and continued to lay their eggs, with the animals equipped with magnets traveling significantly longer than the other animals.
Observations at the egg-laying sites of the loggerhead sea turtle ( Caretta caretta ) on the eastern Atlantic coast of Florida indicate that these turtles also use the earth's magnetic field. In a study, the exact position of the egg-laying sites between 1993 and 2011 was related to the slight changes in the earth's magnetic field that occurred at the same time, which had led to a change in the local geomagnetic features along the coastline. Indeed, there was a clear correlation between the two events; this was assessed as independent confirmation of the attribution of magnetic sense based on the home-finding behavior of turtles as documented by the transmitter.
For the Mississippi alligator , references to a magnetic sense were published as early as 1984, and in 2010 it was also experimentally proven that the gecko Cyrtodactylus philippinicus can orient itself on the earth's magnetic field. This was the first evidence of a magnetic sense in scale creeps .
The hypothesis of a “geomagnetic imprint” in salmon was first presented in 2008 to explain the migration of various Pacific salmon species ( red salmon , king salmon , ketal salmon ) in analogy to the sea turtles that have already been researched . In order to find your way back to the place of birth after sexual maturity, in addition to a magnetic sense, you also need a memory for the strength and orientation of the magnetic field on the way to the place of birth, which you can follow like on a track. In fact, a few years later it could be proven that salmon also have an "innate magnetic map" and orient themselves using magnetic field strength and inclination. For the rainbow trout , which also belongs to the Pacific salmon , reactions to magnetic fields have been demonstrated in laboratory experiments .
In 1997, the influence of magnetic fields on the behavior of European eels ( Anguilla anguilla ) was published for trout , in 2013 and 2017 . In 2018, neuroethological laboratory studies demonstrated a magnetic sense for zebrafish and medaka .
The fact that mammals can also orient themselves in the earth's magnetic field has so far only been experimentally proven for relatively few species - in contrast to birds and insects in particular. Above all, it turned out to be difficult to identify mammals with a natural, spontaneous and expansive locomotion pattern that could also be exposed to an artificial magnetic field with reasonable effort. As a substitute, for example, wild boars ( Sus scrofa ) and warthogs ( Phacochoerus africanus ) have tried to analyze whether they align their body axis in a certain direction when resting.
In addition to the homing behavior of birds, the comparable behavior of some mammal species has also been researched since the 1930s. One of the first publications dealt with the wood mouse in 1936, in 1981 this animal species was the first mammal to demonstrate a magnetic sense , which, according to a study published in 2015, can be disturbed by electrosmog , which suggests that wood mice have a magnetic receptor of the radical pair model .
African gray mull
The African gray mull lives in its natural environment in large colonies and very extensive duct systems. The species became one of the test animals researched by several working groups after it had been established in the field and in the laboratory that these exclusively underground animals create their nests mainly in the southeast of their duct system. Therefore, at the end of the 1980s, some gray mulls caught in Zambia were kept at Frankfurt University . Among other things, a round cage was built for them and it was observed exactly where the nest was made - as expected, this was preferably done in the southeast. If an artificial magnetic field was switched on over the round cage, which was turned by 120 degrees to the earth's north ( counter-clockwise ), then the preferred direction for nest building changed accordingly to earth west-southwest. Additional tests have shown that the directional behavior observed - both underground and artificial magnetic fields - is not influenced by light. In contrast to birds, the angle of inclination of the field lines does not play a role in the gray mulls.
Bats hunt at night and can orient themselves in three-dimensional space with the help of echolocation , among other things . However, some species cover long distances every year between their breeding grounds and their winter quarters: the rough-skin bat, for example, twice a year around 2000 kilometers between Estonia and southern France . It is unlikely that bats can orientate themselves over long distances in the starry sky , since they can only see clearly at close range. The first indications of a magnetic sense in bats were published in Nature at the end of 2006 after field experiments on Eptesicus fuscus , a species that occurs between northern South America and southern Canada . The test animals were caught from their roost in a barn in the US state of New Jersey and released again around 20 kilometers away. Thanks to a 0.5 gram radio transmitter attached to her body, her flight path could be documented by a Cessna 152 or 170 chasing her. Bats in the control group flew purposefully and almost directly to their roost. If, on the other hand, the bats were exposed to an artificial magnetic field for an additional 90 minutes before they were released - 45 minutes before to 45 minutes after sunset - which was either 90 degrees to the right or 90 degrees to the left compared to "earthly north", then when these test animals initially flew for a while in the direction given by the artificial magnetic field, which was 90 degrees “wrong”. According to the researchers, the bats' “inner compass” is calibrated at sunset. How the initially misguided test animals finally found their way to their hometown was not clarified in this study.
In 2014, another research group reported after field experiments on the European great mouse-eared mouse ( Myotis myotis ) that these bats calibrate their “inner compass” with the help of polarized light at dusk . The test animals were brought in boxes around 25 kilometers away from their home and then exposed to the fading daylight in different ways: some animals were exposed to the natural vibration level of the light from the sky, a second group was simulated a shift in the direction of vibration by 90 degrees, and a control group saw the sky without any polarization. After the animals had been released in the darkest night, the direction of flight of all animals was recorded using small transmitters. The result of the experimental set-up showed that the polarized light at twilight had a clear influence on the home-finding behavior of the bats: Animals with a view of the natural direction of oscillation and animals without any view of the direction of oscillation flew to their home location; those animals, however, which were exposed to a twisted direction of oscillation, initially flew away in the wrong direction, with greater scatter at the same time. An accompanying information from the Max Planck Institute for Ornithology stated: “With this simple experiment, the researchers were able to show for the first time that bats use the polarization pattern in the evening sky to calibrate their internal magnetic compass for orientation. How exactly this works is still unclear. "
In 2015, it was demonstrated under laboratory conditions that bats of the species Nyctalus plancyi , which occurs in southern China, can still perceive magnetic fields that have only a fifth of the natural field strength.
Dogs orientate themselves at close range mainly on the basis of the smells of their surroundings. However, a research team from the University of Duisburg-Essen and the Czech Agricultural University in Prague headed by Hynek Burda believes that dogs can also perceive the earth's magnetic field. In any case, this is what the researchers derived from a statistical analysis of the dogs' very special behaviors: their posture when they empty their bladder or bowel in the wild without a leash. In 70 dogs from 37 breeds more than 7000 excretions were documented and in particular the direction in which the longitudinal axis of the dogs was aligned was noted. The collected data did not initially reveal a preferred direction, but did so after the documented excretions had been related to the slight fluctuations - changes in intensity and direction of the field lines - of the terrestrial magnetic field: only in phases in which the magnetic field was stable did they align the dogs in a north-south direction. These statistical analyzes, published in 2013, were the first empirical evidence that dogs also have a magnetic sense. A study published in 2020 by the same research group revealed that dogs that run after hunting prey in open terrain, for example, use the magnetic field to take a shortcut back to the starting point instead of on their trail. It is possible that their magnetic sense - similar to that of birds - is linked to the visual system.
European red foxes also seem to have a magnetic sense. At least this is suggested by observations of natural, undisturbed hunting behavior, which were published in 2011. The fox's “hunting strategy” in relation to a mouse, for example, is based on a surprise effect: the fox sneaks up, jumps up and over the potential prey and grabs it from above when it falls. In the snow and in high vegetation, the fox locates its prey primarily thanks to its sensitive hearing. Thanks to the help of 23 experienced biologists and hunters, between April 2008 and September 2010, almost 600 hunting jumps in 84 wild foxes in snow and high vegetation were recorded and measured in the Czech Republic . It turned out that - regardless of the time of day, the season and the weather - more than 80 percent of the jumps in the north direction (mostly in the north-northeast direction) and around 60 percent of the jumps in the south direction were successful; the success rate of jumps in other directions was less than 15 percent. The underlying biological mechanisms are unknown.
A number of indications suggest that whales also feel magnetism. For example, stranding of living whales was analyzed in order to draw conclusions about normal orientation behavior from the incorrect orientation behavior. For the British main island , the Natural History Museum in London has a data collection going back decades on such strandings, so that in 1985 a total of 137 landings of live whales could be evaluated, including 29 mass strandings with more than three animals. There were also nearly a thousand finds of dead whales. No geographical similarities were discovered for the landing sites of still living whales - some were rocky coastal areas, others sandy ones; some of them were flat coasts, some of them rapidly deepening. The only thing they had in common was that all landing sites had anomalies of the geomagnetic field, namely a below-average strong geomagnetic field in which the field lines point landward. The whales, driven to dead, were randomly distributed over the coast. In 1986, from the analysis of 212 whale strandings on the coasts of the USA by a second group of researchers, the conclusion was also drawn that the only common feature of these strandings were local deviations in the terrestrial magnetic field (local minima). Local geomagnetic minima are also correlated with sightings of fin whales off the northeast coast of the USA in autumn and winter.
In 2018, a publication made two solar storms responsible for the stranding of 29 male sperm whales in January 2016 in the North Sea area. This disturbance of the magnetosphere has disturbed the orientation of the whales and led them into shallow water, where they are stranded at low tide.
The first experimental evidence of magnetosensitivity in whales was published in autumn 2014; The test animals were six bottlenose dolphins in the Planète Sauvage , a zoo on the French Atlantic coast. In a basin of the bottlenose dolphins one of two identical and equally heavy barrels was alternately placed, which differed only in one detail: One of the two barrels contained a strong magnet. It was observed that the bottlenose dolphins swam a little faster in the direction of the barrel if it contained the magnet (after an average of 5.7 minutes instead of 6.2 minutes), otherwise the interactions did not differ between the two barrels. According to this experimental approach, the bottlenose dolphins were able to differentiate between objects based on their magnetic properties. In the journal Science , however, it was noted that the subject of the test arrangement was a very strong magnetic field and that the interpretation of the findings should be verified with an earth-like magnetic field strength.
Hypotheses on the “seat” of the magnetic sense
The "seat" of the magnetic sense has not yet been proven with absolute certainty in any of the animal species examined so far. Although there are experimental findings that refer to the eyes and upper bills of migratory birds, for example, it has not yet been possible to trace and document the conduction of excitation between the suspected receptor cells and the brain in these species, which have been researched intensively for decades .
With migratory birds and domestic pigeons
The north-south direction is indicated by a technical magnetic compass because its needle aligns itself in the direction of the earth's magnetic field and its field lines run roughly in the geographic north-south direction over large areas of the earth. Since 1972, however, based on the research of Wolfgang and Roswitha Wiltschko, it has been generally accepted that migratory birds do not orient themselves in the same way to the polarity of the terrestrial magnetic field, but to the inclination , i.e. the angle of inclination of the field lines relative to the earth's surface: pictorially formulated, the field lines penetrate the North at a steep angle into the earth, but at the equator they run parallel to the earth's surface. The birds therefore differentiate between “ poleward ” and “ equatorial ”. In addition, in 1978 Wiltschko & Wiltschko had demonstrated identical orientation achievements in domestic pigeons and in 1981 also confirmed earlier assumptions that the magnetic sense in these birds is light-dependent in an experiment: domestic pigeons that were brought from their home to a distant place in complete darkness flew to the First let them go around completely disoriented in a similar way as if they had been exposed to a disruptive magnetic field during transport. Studies on reed warblers in 2017 provided indications that the "inclination compass" can also help enable a reliable east-west flight direction.
A possible explanation for the interplay of magnetic sense and visual perception was presented in 2000 by the German biophysicist Thorsten Ritz from Klaus Schulten's group , called the “ radical pair model ”, in which Ritz focused on theoretical considerations and experiments Doctoral supervisor Schulten resorted. According to this model, the magnetic receptor consists of a molecule in which, under the influence of the earth's magnetic field - excited by photons - an electron is transferred from one part of the molecule to another, creating a very short-lived, so-called radical pair with two free electrons arises. Depending on the angular momentum of these free electrons, the excited molecules constantly change between two quantum mechanically possible states (singlet or triplet state). After the radical pair disintegrates, molecules with different properties can form, depending on the state in which this pair was last. According to the model, however, this final state depends on the angle of inclination of the field lines: If the magnetic field lines hit the radical pair in a pronounced perpendicular manner, the relationship between the two chemical end products is different than if the magnetic field lines hit the radical pair relatively flat. As a result, according to this model, a physical condition (the local magnetic field) is “translated” into a chemical condition, thus taking an essential step towards perception with the help of a specialized sensory organ.
As early as 2000, the author of the radical pair hypothesis referred to the cryptochromes as presumably responsible molecules for the interplay of magnetic sense and visual perception. Four years later, Henrik Mouritsen, using the example of garden warblers, published that migratory birds have a variant of cryptochrome 1 - cryptochrome 1a - in their retina , which occurs in the cytosol of special ganglia cells; In addition, these cells are highly active in garden warblers when the birds orient themselves in the earth's magnetic field at night. It has also been shown that no cryptochrome 1 is detectable in the retina of non-migrating zebra finches at night. In-depth experiments in 2009 were interpreted to the effect that the cryptochrome-based dynamics of the singlet-triplet change in migratory birds is relatively long-lived and that their cryptochrome also has other properties that make it appear particularly suitable for finding directions in the earth's magnetic field. This also includes that cryptochrome 1 responds to blue-green light and that the robins tested only orient themselves in the earth's magnetic field in light between around 400 (blue) and 550 nanometers (green). Supplementary investigations also found cryptochrome 1a in UV-sensitive cones of the retina. These publications and the findings of other studies refer to the eyes - more precisely: to the retina - as the "seat" of the magnetic sense of migratory birds and could mean that the animals "see" the course of the earth's magnetic field. In 2018, the Mouritsen working group also presented Cryptochrome 4 as a presumably responsible molecule for the interplay between magnetic sense and visual perception.
For a while, it was irritating that, according to experiments by Wiltschko & Wiltschko from the years before 2002 - and also afterwards - robins can only orient themselves to the magnetic field if their right eye is not taped off, while no disability was detectable with taped left eye ; this seemed to indicate a pronounced lateralization of magnetoreception. In 2011, researchers from Oldenburg contradicted these findings and ascribed the ability to magnetoreceive in both eyes. A year later, the contradiction was resolved by additional experiments by the Wiltschko working group: young birds on their first journey south can still orientate themselves with both eyes on the earth's magnetic field. “In the following spring, the ability is already shifted to the right eye, but still flexible. After covering the right eye for six hours, the compass in the left eye was active again. On the other hand, the next time the birds migrate in autumn, the lateralization is more focused on the right eye and thus the left hemisphere. The researchers interpret this as a maturation process “: The areas of the right hemisphere would be free for other tasks, whereby the lateralization in young animals can still be influenced by environmental influences. It has also been proven that the right eye only enables orientation in the earth's magnetic field if it is able to recognize contours. If 70 percent of the potentially eye-catching light was filtered out, the magnetic sense of the robins tested was no longer detectable. If the incidence of light in the left eye was reduced, this had no effect on the magnetic sense.
Iron mineral-based magnetoreception
In 1975 the orientation of a living being - a bacterium - in the earth's magnetic field was described for the first time and this behavior was traced back to magnetic crystals in its interior. These observations immediately led to speculations that the mineral magnetite (Fe 3 O 4 ) could also play a role in the orientation of animals. In fact, magnetite was detected in numerous animal species from numerous phyla and in different areas of the body in the following years , in birds especially in the eye sockets and nasal passages . In 2000 and 2001 studies on domestic pigeons attracted particular attention, in which accumulations of very small crystals were described in the skin of the upper beak, which with the help of crystallographic methods have been shown to be superparamagnetic and are innervated by the trigeminal nerve . A second biologist couple - like Wiltschko & Wiltschko - based in Frankfurt am Main, Gerta and Günther Fleissner , in cooperation with geophysicists from the University of Munich, after they discovered the coupling of magnetite and trigeminal nerve, came to the conclusion that the magnetite-containing structures of the The upper beak of domestic pigeons could act as a magnetic receptor. In cooperation with the experimental physicist Gerald Falkenberg from the Hamburg Synchrotron Radiation Laboratory (HASYLAB) at the German Electron Synchrotron , Fleissner's beak organ was also detected in migratory birds - robins and warblers - as well as in domestic chickens, and its function was interpreted in the sense of a biological magnetometer : the structures in the upper beak of the Birds are therefore able to measure the locally different field strength of the earth's magnetic field and thus contribute to the creation of a “geomagnetic map”, while the light-dependent cells in the eye detect the orientation of the earth's magnetic field. In the same year, researchers from the Neurosensory Research Group at the University of Oldenburg confirmed the findings with the help of neurobiological measurements and reported that crystal structures containing iron minerals in the upper beak are connected to the brain stem via nerve tracts . When these nerve tracts were blocked in the robin experiment, under the influence of light this had no effect on the functionality of the “magnetic compass in the eye”, but the animals are disoriented in complete darkness if the upper beak is also anesthetized.
Electromagnetic induction in the inner ear
An Austrian working group reported in 2019 on possible indications that magnetic fields by means of electromagnetic induction in the inner ear of pigeons change the permeability of certain calcium channels in the hair cells and that this could help to perceive the earth's magnetic field.
In other animal species
In the majority of the more than 50 animal species for which behavior-relevant reactions to changes in the natural or an artificial magnetic field have been proven, it is still unknown how the magnetic field is perceived. While magnetite has been detected in numerous animal species, and there is evidence of cryptochrome 1 even in numerous mammals, the connection between potential receptors and the brain is largely unexplored in the individual species. In addition, the detection of these physiological markers is at most a necessary, but not a sufficient requirement for the animals in question to actually have a magnetic sense.
An exception is the American cockroach , whose orientation in the magnetic field was interpreted in a laboratory experiment as the “radical pair model” and comparable to that of migratory birds. In the case of salmon fish, on the other hand, there is evidence that they have magnetite-based magnetoreception in the area of their noses.
Cattle and deer
In 2008, researchers at the University of Duisburg-Essen working with Hynek Burda came to the conclusion that cattle and deer also have a magnetic sense. They evaluated images from Google Earth which showed herds of cattle - 8510 animals in 308 herds. It was determined that two thirds of the animals were standing or lying in a north-south direction while grazing or resting. Inspired by this evaluation, Czech researchers measured the body axis alignment of roe deer and red deer during sleep in the Bohemian Forest National Park ; In these animals too - 2974 individuals in 241 locations - indications of a preference for north-south orientation were found. The evaluation of the data also showed that this directional preference was not detectable in the vicinity of high-voltage lines. A study published in 2011 with a significantly smaller database (3412 cattle in 232 herds), on the other hand, could not determine any dependence of the orientation of the animals on the earth's magnetic field. The researchers around Lukas Jelinek from the Technical University in Prague interpreted the results of the previous work as an error in the data evaluation. In response to these allegations, the researchers of the study, published in 2008, massively criticized the statistical analysis by Jelinek et al.: In half of those published by Jelinek et al. The orientation of the bodies cannot be recognized with sufficient accuracy because the images are too blurred or come from steep slopes or because they were taken near high-voltage lines. If the cattle are excluded from the analysis on these unsuitable images, a reanalysis of the data by Jelinek et al. a confirmation of the findings published in 2008 - a criticism made by Jelinek et al. was again rejected.
In a commented classification of the dispute, the journal Nature quoted the biologist Sönke Johnsen from Duke University , who is researching the geomagnetic orientation of sea turtles, as saying that some of the images should not have been evaluated and that the findings reported in 2008, "Although puzzling, still exist."
Effects of "electrosmog"
The detection of their magnetic sense was successful in numerous animal species because their orientation in the earth's magnetic field was disturbed by an artificial magnetic field. The obvious question was therefore whether human-made electromagnetic fields - " electrosmog " - from high-voltage power lines, for example, can irritate migratory birds on their routes between winter and summer roosts or have any other influence on the behavior of animals. However, a review article published in 2005 could not report any reliable findings for harmful effects. On the contrary, the reproductive success of ospreys had improved both in the USA and in Germany, for example, because a significant proportion of the breeding pairs - three out of four in Germany - brooded on high-voltage pylons.
According to the observations in cattle and deer published in 2008, however, power lines seem to influence the behavior of at least these animals; the study at that time has not yet been verified by any other group of experts on the basis of more recent data.
Behavioral studies by Oldenburg biologists from Henrik Mouritsen's working group on robins showed in 2014 that the magnetic sense of these animals can apparently be disturbed by radio waves in the frequency range between 50 kHz and 5 MHz. It remained open, however, why this effect was observed in the city of Oldenburg, but not also in the city of Frankfurt am Main, where - in the densely populated, centrally located Westend - robins and domestic pigeons had been tested in the magnetic field decades before.
Experiments on humans
Little research has been done so far as to whether humans can perceive the earth's magnetic field and use it to determine direction when changing location. The validity of the published studies that affirm a magnetic sense in humans is also controversial, since they come from a single working group.
At the end of the 1970s, Robin Baker had started experiments at the University of Manchester in which test subjects were first driven back and forth in a car blindfolded and then asked to point to the starting point of the odyssey. According to his publication, the test subjects were able to point the direction significantly more correctly than control subjects who had a bar magnet attached to the back of their head. This test arrangement was immediately repeated by several working groups, but the results could not be reproduced elsewhere. Robin Baker, on the other hand, varied his approach by, for example, leading test subjects blindfolded and on winding paths through forests and then having them point the direction north. In addition, blindfolded test persons were placed on swivel chairs, turned irregularly to the left and right and, after an abrupt stop, asked which direction their face was facing. While Baker claimed to be able to demonstrate rather imprecise but nevertheless significantly correct directions, these results could not be confirmed by other research groups either. According to experts, whether humans have at least a weak ability to use the earth's magnetic field for orientation behavior is a question that can only be answered after further experiments.
Researchers at the California Institute of Technology reported in 2019 that the brains of some test subjects exhibited altered alpha waves of lower amplitude when exposed to a rotating magnetic field whose intensity was equal to the natural one. These observations may be related to findings published in 2018 by researchers at the Ludwig Maximilians University in Munich . They examined 822 samples from a total of seven brains of deceased people and found magnetite crystals particularly frequently in the cerebellum and brain stem . The two research groups did not discuss whether the findings are related to a possible human magnetic sense.
Many bacteria can also use the earth's magnetic field to find direction. This behavior was first described in 1975 by Richard P. Blakemore in Science and named as magnetotaxis . Previously, the Italian doctor Salvatore Bellini had discovered what he called the magnetosensitivity of bacteria as early as 1958 at the Institute for Microbiology at the University of Padua and described it in two drafts for specialist publications in 1963, but his faculty did not allow them to be published.
Magnetosensitive bacteria are a heterogeneous group of gram-negative protozoa that can align and move along the geomagnetic field lines. They owe this ability to special organelles in their interior, the magnetosomes - mostly nanometer-sized crystals of the ferromagnetic minerals magnetite (Fe 3 O 4 ) or greigerite (Fe 3 S 4 , rarely), which are arranged in chains and bound to cell membranes . Their magnetic moment causes the bacteria - both living and dead - to rotate and arrange themselves parallel to the field lines and in the northern hemisphere with the front end facing north. Most magnetosensitive bacteria live anaerobically or microaerobically on the bottom of water. They were discovered in water samples in the USA in 1975, which makes the evolutionary benefit of their “northward orientation” immediately obvious: If these bacteria drift from the bottom into higher, more oxygen-rich water layers, then the downward inclination in the northern hemisphere will reliably guide them downwards towards the sediment . Orientation in a magnetic field is therefore an alternative for very small organisms to orientation to the gravitational field that is not possible for them . The first bacteria with reversed polarity - the front facing south - were described from the southern hemisphere in 1980.
The magnetotaxis of the bacteria therefore represents a borderline case in connection with a "magnetic sense", because the orientation in the magnetic field in these bacteria is not an active navigation, but a passive process, caused by the geomagnetism acting on the magnetic particles; only the movement along the field lines is an active process. Nonetheless, the result ultimately corresponds to the animals' orientation and homing behavior. Analyzes of marine sediments have also shown that the very early ancestors of the bacteria living today had magnetosomes and the ability to biomineralize . 700 million year old finds from South Africa are considered certain, but it is assumed that the magnetotaxis of anaerobic bacteria originated much earlier in the history of the earth - possibly as early as the Archean Era more than two billion years ago.
In 2014 it became known that the transfer of 30 genes of the magnetotaxic bacterium Magnetospirillum gryphiswaldense to the photosynthetic, non-magnetotaxic bacterium Rhodospirillum rubrum resulted in Rhodospirillum rubrum forming chains of magnetic crystals which correspond to those of Magnetospirillum gryphiswaldense and are like this align in the earth's magnetic field.
The first experiments on the growth of plants under the influence of static magnetic force fields were carried out as early as the 1920s, and influences on the growth of the primary leaves of germinated wheat seeds were reported. The observed phenomena were traced back to magnetically influenceable rotational movements of the cell plasma . In the 1960s, these early experiments were taken up again and evidence was found that “the seeds of some types of grass germinated better when they were aligned parallel to the earth's magnetic field. Something similar was found in the root growth of wheat, but not that of rye. ”A systematic review in December 2005 came to the conclusion that the findings were contradicting and that it remained unexplained what the use of magnetic fields in plants could be for. A second review in 2014 came to a similar conclusion. It is assumed that cryptochromes are also involved in the reception of magnetic fields in plants: Cryptochromes absorb the blue component of sunlight and, among other things, affect the growth of plants. In experiments in mouse-ear cress ( Arabidopsis thaliana for example, demonstrated that her) hypocotyl can be reduced by a strong magnetic field -Growth, but not in Arabidopsis mutants lacking good working Chryptochrom.
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