Magnetoreception

Magnetoreception (also magnetoception) is a sense which allows an organism to detect a magnetic field to perceive direction, altitude or location. This sensory modality is used by a range of animals for orientation and navigation, and as a method for animals to develop regional maps. For the purpose of navigation, magnetoreception deals with the detection of the Earth's magnetic field.Magnetotactic bacteria is a class of bacteria known to use magnetic fields for orientation. These bacteria demonstrate a behavioral phenomenon known as magnetotaxis which is how the bacterium orients itself and migrates in the direction along the Earth's magnetic field lines. The bacteria contain magnetosomes, which are nanometer-sized particles of magnetite or iron sulfide enclosed within the bacterial cells. The magnetosomes are surrounded by a membrane composed of phospholipids and fatty acids and contain at least 20 different proteins. Magnetosomes form in chains where the magnetic moments of each magnetosome align in parallel, causing each bacterium cell to essentially act as a magnetic dipole, giving the bacteria their permanent-magnet characteristics.The nematode Caenorhabditis elegans was proposed to orient to the magnetic field of the Earth using the first described set of magnetosensory neurons. Worms appear to use the magnetic field to orient during vertical soil migrations that change in sign depending on their satiation state (with hungry worms burrowing down, and satiated worms burrowing up). However, recent evidence challenges these findings.A number of amphibians and reptiles including salamanders, toads and turtles exhibit alignment behaviours with respect to the Earth's magnetic field.Homing pigeons can use magnetic fields as part of their complex navigation system. William Keeton showed that time-shifted homing pigeons are unable to orient themselves correctly on a clear, sunny day which 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. This led to the hypothesis that under particular conditions, homing pigeons rely on magnetic fields to orient themselves. Further experiments with magnets attached to the backs of homing pigeons demonstrated that disruption of the bird's ability to sense the Earth's magnetic field leads to a loss of proper orientation behavior under overcast conditions. There have been two mechanisms implicated in homing pigeon magnetoreception: the visually mediated free-radical pair mechanism and a magnetite based directional compass or inclination compass. More recent behavioral tests have shown that pigeons are able to detect magnetic anomalies of 186 microtesla (1.86 Gauss).Domestic hens have iron mineral deposits in the sensory dendrites in the upper beak and are capable of magnetoreception. Because hens use directional information from the magnetic field of the Earth to orient in relatively small areas, this raises the possibility that beak-trimming (removal of part of the beak, to reduce injurious pecking, frequently performed on egg-laying hens) impairs the ability of hens to orient in extensive systems, or to move in and out of buildings in free-range systems.Work with mice, mole-rats and bats has shown that some mammals are capable of magnetoreception. When woodmice are removed from their home area and deprived of visual and olfactory cues, they orient themselves towards their homes until an inverted magnetic field is applied to their cage. However, 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 when woodmice are displaced, they use magnetic fields to orient themselves if there are no other cues available. However, early studies of these subjects were criticized because of the difficulty of completely removing sensory cues, and in some because the tests were performed out of the natural environment. In others, the results of these experiments do not conclusively show a response to magnetic fields when deprived of other cues, because the magnetic field was artificially changed before the test rather than during it.Despite more than 50 years of research, a sensory receptor in animals has yet to be identified for magnetoreception. It is possible that the entire receptor system could fit in a one-millimeter cube and have a magnetic content of less than one ppm. As such, even discerning the parts of the brain where the information is processed presents a challenge.