Magnetoreception
By Nicholas Chen
Magnetoreception is the ability of certain organisms to detect magnetic fields in order to get a sense of direction or location. Many animals have shown the ability to use the Earth's magnetic field to orient themselves and gain information about their surroundings. These include salmon, sea turtles, lobsters, fruit flies, and most notably birds which use this sensory function to migrate.
Background
The basis of magnetoreception comes from the Earth's magnetic field, which presents itself as a large dipole with a magnetic north and a magnetic south. Because of its polarized nature, magnetic fields radiate outwards from the earth in a looping pattern. These magnetic "fields lines" are what organisms use to orient themselves. However, the details behind the physiological mechanisms for magnetic field detection are still elusive. The exact sensory organ or body part in homing pigeons for example has not been determined, but neurons have been shown to be sensitive to geomagnetic fields and take into account aspects like direction, polarity, and intensity.
Mechanisms
Although the existence of magnetoreception was plagued by suspicion for many years, a clear example of an organism making use of the Earth's magnetic fields is found in the bacterium Magnetobacterium bavaricum. After investigation, the bacterium was found to have crystals of [math]\displaystyle{ Fe_{3}O_{4} }[/math] or magnetite inside its cells, which all polarize in the direction of present magnetic fields, forming a chain. This gives the bacterium magnetic characteristics and allows the cell to consistently travel along these magnetic field lines. The necessary length of magnetite to become magnetized and sensitive to effects of magnetic fields is extremely small- 50 nm.
Other organisms possess electroreceptive organs which can detect electric fields. However, these animals could potentially also detect magnetic fields through Faraday's Law. The time-varying change in flux could generate an electric potential.
A Mathematical Model
[math]\displaystyle{ V_{system}= -{\frac{dϕ}{dt}} }[/math]
Using this equation, the electric potential can be found by treating the animal moving through the Earth's magnetic field as a conductor over a period of time.
Difficulties with progress
There are many difficulties detecting the exact sensory receptor in charge of magnetoreception. Multiple factors including the use of other animal senses like sight, the physical size of such a receptor, and combined mechanisms make it hard to pinpoint the various effects. Furthermore, the purpose of magnetoreception is unclear as well, whether animals use it to navigate or create a familiarity "map".
External links
References
S.H.K. Eder et al., “Magnetic characterization of isolated candidate vertebrate magnetoreceptor cells,” PNAS, 109:12022-27, 2012
L.Q. Wu, J.D. Dickman, “Neural correlates of a magnetic sense,” Science, 336:1054-57, 2012.
C.D. Treiber et al., “Clusters of iron-rich cells in the upper beak of pigeons are macrophages not magnetosensitive neurons,” Nature, 484:367-70, 2012.