Magnetoception

Magnetoception (or magnetoreception) is the ability to detect a magnetic field to perceive direction, altitude or location. This sense plays a role in the navigational abilities of several animal species and has been postulated as a method for animals to develop regional maps.

Magnetoception is most commonly observed in birds, where sensing of the Earth's magnetic field is important to the navigational abilities during migration; it has also been observed in many other animals including fruit flies, honeybees and turtles, bacteria and fungi, as well as lobsters, sharks and stingrays.

Mechanisms
The phenomenon is poorly understood, and there exist two main hypotheses to explain magnetoception.

In pigeons and other birds, researchers have identified a small heavily innervated region of the upper beak which contains biological magnetite and is believed to be involved in magnetoception.

Evidence has also been found that the light-sensitive molecule cryptochrome in the photoreceptor cells of the eyes is involved in magnetoception. According to one model, cryptochrome when exposed to blue light gets activated and forms a pair of two radicals (molecules with a single unpaired electron) where the spins of the two unpaired electrons are correlated. The surrounding magnetic field affects the kind of this correlation (parallel or anti-parallel), and this in turn affects the length of time cryptochrome stays in its activated state. Activation of cryptochrome may affect the light-sensitivity of retinal neurons, with the overall result that the bird can "see" the magnetic field. Cryptochromes are also essential for the light-dependent ability of the fruit fly Drosophila melanogaster to sense magnetic fields.

It is believed that birds use both the magnetite-based and the radical pair-based approach, "with the radical pair mechanism in the right eye providing directional information and a magnetite-based mechanism in the upper beak providing information on position as component of the 'map'".

There are, however, two types of magnetic sensing mechanisms that have been discovered. The first is the inductive sensing methods used by sharks, stingrays and chimaeras (cartilaginous fish). These species possess a unique electroreceptive organ known as ampullae of Lorenzini which can detect a slight variation in electric potential. These organs are made up of mucus-filled canals that connect from the skin's pores to small sacks within the animal's flesh that are also filled with mucus. The sensing method of these organs is based on Faraday's law; a time-varying magnetic field moving through a conductor induces electric potential across the ends of the inductor. In this case the conductor is the moving through a magnetic field, and the potential induced depends on the time varying rate of flux through the conductor according to $$V_{ind}=-\frac{d\phi}{dt}$$. These organs detect very small fluctuations in the potential difference between the pore and the base of the electroreceptor sack. An increase in potential results in a decrease in the rate of nerve activity, and a decrease in potential results in an increase in the rate of nerve activity. This is analogous to the behavior of a current carrying conductor; with a fixed channel resistance, an increase in potential would decrease the amount of current detected, and vice verse. These receptors are located along the mouth and nose of sharks and stingrays.

The second known method of magnetic sensing, or magnetoception, is found in a class of bacteria known as magnetotactic bacteria. These bacteria demonstrate a phenomenal behaviorism known as magnetotaxis, in which the bacteria orients itself and migrates in the direction along the earth's magnetic field lines. The bacteria is made up of magnetosomes, which are individual minerals of magnetite enclosed within the bacteria cells. Each bacteria cell essentially acts as a magnetic dipole. They form in chains where the moments of each magnetosome align in parallel, giving the bacteria its permanent magnet characteristics. These chains are formed symmetrically to preserve the crystalline structure of the cells. These bacteria are said to have permanent magnetic sensitivity.

In humans
Humans have magnetite deposits in the bones of the nose, specifically the sphenoidal/ethmoid sinuses. Beginning in the late 1970s the group of Robin Baker at the University of Manchester began to conduct experiments that purported to exhibit magnetoception in humans: people were disoriented and then asked about certain directions; their answers were more accurate if there was no magnet attached to their head. These results could not be reproduced by other groups and the evidence remains ambiguous. Recently some other evidence for human magnetoception has been put forward: low-frequency magnetic fields can produce an evoked response in the brains of human subjects.

Other organisms
In bees, it has been observed that magnetite is embedded across the cellular membrane of a small group of neurons; it is thought that when the magnetite aligns with the Earth's magnetic field, induction causes a current to cross the membrane which depolarizes the cell.

Crocodiles are believed to have magnetoception, which allows them to find their native area even after being moved hundreds of miles away. Some have been strapped with magnets to disorient them and keep them out of residential areas.

In 2008, a researcher team led by Hynek Burda using Google Earth accidentally discovered that magnetic fields affect the body orientation of cows and deer during grazing or resting. In a followup study in 2009, Burda and Sabine Begall observed that magnetic fields generated by power lines disrupted the orientation of cows from the Earth's magnetic field.

Certain types of bacteria (magnetotactic bacteria) and fungi are also known to sense the magnetic flux direction; they have organelles known as magnetosomes containing magnetic crystals for this purpose.

Some migratory bird species, specifically European robins, have shown behavioral evidence of having a magnetic inclination compass. This was first realized by the unusual behavior of birds in captivity during their natural migratory seasons. The birds tended to position themselves at the location within their cage that corresponded with the direction of their instinctive migration path. Experiments were conducted in which the orientation of the earth's field was distorted with an applied external field. The applied field was controlled such that only the horizontal or vertical component of the earth's field was reversed (by applying a field twice as strong and opposite in direction). The intensity and inclination angle of the applied field was kept equal to the earth's natural field as measured at the location of the experiment. It was found that Robins are sensitive to both the horizontal and vertical components of the earth's field; reversing either component (individually) resulted in disorientation of the birds. However, when both components were reversed simultaneously, the equivalent of changing the magnetic polarity of the earth, had no effect on their orientation. Thus, the Robins can determine whether they are flying poleward or equator-ward based on the inclination angle of the earth's field with respect to their normal [to the ground], or vertical direction. However, they cannot determine the difference in the magnetic polarity of the present field. This explains the basis for naming their sensing method a inclination compass as opposed to the standard compass we are used to.

Spiny lobsters have shown evidence of having a magnetic polarity compass. They are sensitive only to the horizontal component of the magnetic field, and the vertical component has no effect on their behavior. By sensing the horizontal component only, they can sense the polarity of the magnetic field. This is opposite the effect of the inclination compass found in birds, where reversing the vertical or horizontal component was equally effective in disorientating them.

Some species of sea turtles have utilized the earth's magnetic field for directional orientation. Loggerhead and leatherback sea turtles have been studied and show orientating abilities based on both lighting clues and the surrounding magnetic field.