Magnetoception, the fantastic ability to perceive magnetic fields. A skill though impossible for long. It was difficult to assume that a 0.5 Gauss Earth’s magnetic field (your fridge has one with 100 Gauss) could have some effect on living things. However, the magnetic field perception was supported since the very beginning by experimental observation and evidence: some animals are affected in their orientation or navigational behaviour by magnetic field variations. Some of those animals have an “internal compass”, helping them to keep their North-South orientation. Few of them are able of even more: to use Earth´s magnetic field as a GPS. They get positional information by previously learned destination field´s features and surrounding magnetic topography. It is said they have “magnetic maps”.
It is known that many animals use the stars, the sun positions, light polarization and color gradients to navigate, but seem to be obvious that to use magnetic-field sensing is far more robust and reliable if you are unable to build some kind of artificial map. Otherwise, how the migratory birds would be able to find their way, accurately, year after year even in cloudy dark days and nights. However, let´s to think for a second, we have the consequence but, what is the cause? Until not long ago, to find it was a challenge just for one thing: the living matter is transparent to magnetism. How to perceive a sort of field which is able to pass though a row of elephants unfazed? What kind of receptor might be able to sense and amplify something as weak as Earth’s magnetic field? The answer to those questions begins with a microscopic bacteria and it has still no ending but lots of direct or indirect quantum mechanics behind.
Salvatore Belline discovered the bacterial magnetotaxis in the early 60s, in the middle 70s Richard Blakemore 1 describes “novel structured particles, rich in iron, within intracytoplasmic membrane vesicles” (called latter ferromagnetic crystals within magnetosomes). He himself proposes the mechanism: these particles confer to the bacteria a magnetic moment placing them in the direction of the magnetic field. In many cases the advantage for them is not to know were is the north but to know were is up (oxygenated water) or down (water poor in oxygen and muddy bottoms). Then, they will choose what they like. Thus, magnetosome chains or arrays were like a passive system for those bacteria, exactly like the needle of a compass. In fact, bacteria were able to feel the weak Earth´s magnetic-field using this ferromagnetic structures even being dead.
Afterwards, magnetite crystals were found gradually in other microscopic species, fungi and big animals that were previously assumed to have magnetoception capability. But in the latter cases the sole presence of a microscopic magnetite particles (F3O4) or other magnetic minerals in few cells of the body can not explain the orientation of the whole organism by that system. The torque generated by few picograms of magnetite can not alienate over a meter long and two hundred kilos loggerhead sea turtle with the magnetic field. The simplest scenario to explain how those magnetic structures work animals would be that the crystals may transmit its oscillation under the magnetic field by a sort of molecular mechanisms: Opening ion channels, generating conformational changes in different types of receptors, etc. The real scenario, though, is usually more complicated, and the molecular mechanism is still unknown, but the discovery of iron-mineral-based sensors and magnetite crystals located in the upper beak of some migratory birds point in that direction. We have further evidence of this magnetite-based magnetoception in migratory birds (in European robins in this case) was shown by Heyers et. al. PNAS 2010 2 Here it is shown that magnetic field variations leads to significant changes in neuronal activation in brain areas connected via trigeminal nerve with the upper break.