Magnetoception

As a magnetic sense or orientation on Earth's magnetic field, the ability of animals is referred to perceive the Earth's magnetic field and use it for the determination of position. The ability to orient on the Earth's magnetic field has been demonstrated experimentally in animals and also in bacteria only since the mid 1960s. The best studied is now the so-called " magnetic compass " of migratory birds, yet the magnetic sense is still considered a largely unexplored sense of animal performance.

Historical

1965 suggested the ornithologist, an expert on bird migration and former Professor of Zoology at the Johann Wolfgang Goethe- University, Friedrich Wilhelm Merkel to investigate the biological basis of bird migration and experimentally. This suggestion follows, Wolfgang Wiltschko designed for his doctoral thesis in the basement of the Zoological Institute in Frankfurt am Main a special cage, which could be a hand strong enough shielded from Earth's magnetic field, but around which he could produce a weak static magnetic field artificially. In his " animal model " robin he was the first researcher of the experimental evidence that animals can perceive a static magnetic field and change their behavior in response to this magnetic field; his publication of these findings marked the beginning of a new branch of research in behavioral ecology. Later he secured his findings from studies through to Whitethroats and pigeons.

Initially met with the publications of the Frankfurt ornithologists great skepticism in their professional colleagues, as it failed several other working groups to reproduce Wiltschkos findings and to confirm so. Main obstacle to the repeatability was elsewhere, as was shown, in retrospect, one hand shielding the Earth's magnetic field, but at the same time an artificial static magnetic field had to be rebuilt and its intensity was not allowed to deviate too much from that of the Earth's magnetic field. Only in 1972, the Frankfurt research results have been recognized to speak internationally through its publication in the journal Science; This publication is now to some extent as the first description of a newly discovered sense organ in the animal world.

After Robin, Whitethroat and pigeons the magnetic sense has meanwhile been demonstrated in approximately 50 species: for example, in termites and ants, wasps and honey bees in field May beetles, Drosophila melanogaster and the matron; in molluscs, crustaceans, amphibians and reptiles, in European eels and various salmon in wood mice, hamsters, domestic horses and other mammals.

2005 succeeded in proving that chicks of domestic fowl can find their "mother" with the help of the magnetic field, if it has been hidden behind a modesty panel.

Magnetic sense of birds

A technical magnetic compass shows the direction based on the orientation of the field lines of the magnetic field in the horizontal plane, this case run in a north-south direction. The magnetic sense of birds, however, is based on the detection of the inclination of the geomagnetic field: birds register the inclination angle of the magnetic field lines relative to the earth's surface. So do you differentiate between ' poleward ' and ' equatorward ', because at the pole, the magnetic field lines perpendicular to the top while they run at the equator exactly parallel to the earth's surface.

Physiological mechanisms

The magnetic sense in the eye is likely to work with the help of the so-called radical - pair formation, which was described in 1976 by ​​the German biophysicist Klaus Schulten. According to this model is the magnetic receptor of a pair of molecules that can be activated by light, then, is a result of the transfer of an electron, a very short-lived, so-called radical pair. This pair alternates constantly between two quantum mechanically possible states. After its collapse, molecules can form with different properties, depending on the state in which this radical pair was last located. This final state is, however, according to the model, depending on the inclination: meet when the magnetic field lines perpendicular to the pronounced radical pair is formed to each other, a different ratio of the two chemical end products, as if the magnetic field lines converge relatively flat on the radical pair. As a result, a physical condition is ( the local magnetic field) in a chemical givenness "translated " and so come an essential step for the performance with the help of a specialized sense organ according to this model.

The " seat " of the magnetic sense in birds has not been demonstrated to date with absolute certainty. Suitable molecules in particular so-called by the researchers cryptochromes are called, among other things, in high concentration were detected in the ganglion cell layer and the photoreceptors of garden warblers. Accordingly, the retina would be the seat of the magnetic sense. By Wolfgang Wiltschko comes the observation that the orientation of migratory birds on the magnetic field only succeed if her right eye is not masked, while monocular left sighted animals seem then to be helpless, indicating a strong lateralization of magnetoreception. Other research results indicate a magnetoreception in both eyes, they are currently being debated ( 2011). Another Frankfurt research group also came to the conclusion that even in the area of the beak magnetically sensitive structures are in place.

Alternative hypotheses

Another molecule that is used as magnetic receptor in question is the magnetite, which was initially discovered in magnetotactic bacteria, which also targeted toward the earth's magnetic field. It is iron oxide crystals ( Fe3O4), depending on the size have different magnetic properties. The two types that are suspected to be magnetic receptors in animals, it is even so-called single-domain particles, which have a permanent magnetic moment and therefore can orient the surrounding magnetic field. Below a certain size, the magnetite crystals are, however, thermally unstable so that their magnetic moment fluctuates and adapts only when ambient magnetic field at the field line direction. This property is known as superparamagnetic. It is believed that the magnetite are associated with the membrane and which, through their interaction with the earth's magnetic field changes in activity, for example, by any mechanosensitive ion channels can be opened or closed.

In pigeons and trout high concentrations already been localized in neurons of magnetite in their beaks or in the nasal pit. In addition, it was shown at Reisstärling by electrophysiological studies have shown that parts of the Drilling nerve ( trigeminal nerve ), which is innervated by these neurons show activity changes with changing intensity of the ambient magnetic field.

The magnetite -based receptors are now also suspected in other birds and a few other animal groups, such as some insects and sea turtles.

Magnetic sense in reptiles

Many sea turtles, such as the Atlantic green turtles, are based on the magnetic field of the earth to return for the first time years after hatching egg laying to the same beach. It is believed that the inclination of the field lines of the magnetic field is learned permanently by embossing on the place of birth.

Recently, it was also confirms experimentally that the Gecko Cyrtodactylus philippinicus can orient the magnetic field of the earth. This is the first record of a geomagnetic sense at Schuppenkriechtieren.

Magnetic sense in cattle and deer

In 2008, researchers came to the University of Duisburg -Essen with the view that cattle and deer have a magnetic sense. They evaluated images from Google Earth, which showed cattle herds. It became clear that two-thirds of the animals were or were grazing or resting in a north-south direction. Inspired by this analysis the exact measurements Czech researchers in the national park Sumava the body axis orientation of roe deer and red deer during sleep; also in these animals evidence was found on a preference for the north-south orientation. However, a 2011 published work with larger data base could not detect any dependence of the orientation of the animals of the earth's magnetic field. The researchers therefore assume that cattle do not have a magnetic sense and interpret the results of the previous work as errors in the data analysis.

Magnetic sense in humans

Whether or not the person perceive the Earth's magnetic field and can use to determine the direction for local changes, has been poorly understood. The validity of the published studies that affirm a magnetic sense in humans, is also controversial because they are derived from a single workgroup.

End of the 1970s, Robin Baker had Manchester with experiments started at the University in which subjects first drove around in a car blindfolded crisscross and were finally asked to indicate the starting point of the random walk. According to his publication, the subjects were able to significantly correct the direction point than controls, which had secured a bar magnet at the back. This test arrangement was repeated immediately by several groups, but the results could not be replicated elsewhere. Robin Baker, however, varied his approach, lead by example, subjects blindfolded and on winding roads through forests and then the direction was facing north. Furthermore, subjects were placed blindfolded on swivel chairs, irregularly to the left and turned right and asked for an abrupt stop, the direction in which her ​​face looking. While Baker While claiming to prove quite inaccurate, but nevertheless significantly correct direction instructions, these results from other research groups could not be confirmed. Whether the group is inferior in Manchester self-deception, or whether the person actually has at least a weak ability to use the Earth's magnetic field for their orientation behavior, according to expert opinion is a question that can be answered only after further experiments.