Magnetoreceptors

• sea turtles, birds, amphibians, fish (especially common in those that migrate), honeybees, and mice

Some examples:

• Homing pigeons become disoriented when magnets are placed at the sides of their head. However, this disorientation occurs only on cloudy days, suggesting that their ability to navigate by magnetic cues is a backup system.
• Woodmice, taken from their home territory to a new location 40 meters away, normally orient toward home (left side of figure). But if, while they are being moved, they are exposed to a magnetic field that is just the reverse of the earth's magnetic field — and they are not allowed to see the surrounding terrain — they orient away from their home (right side of figure).

  Left: orientation taken by individual woodmice after being removed from their home in a closed box. Right: same experiment except that the mice were subjected to a reversed magnetic field as they were moved from their home. Each dot represents the orientation taken by one mouse. The arrow within each circle indicates the average for all the mice. Mice transported in an open box so they can see landmarks orient correctly whether or not they are exposed to an abnormal magnetic field. (Based on the work of Mather and Baker, Nature, 291:152, 1981.)

• Thrush nightingales (Luscinia luscinia) migrate in the fall from northern Europe to equatorial Africa. They interrupt their migration with a stopover in northern Egypt where they feed and gain weight. This stopover presumably provides them with the energy stores they need to fly without feeding across the Sahara Desert.

  Fransson and colleagues report in the 1 November 2001 issue of Nature that when they confined naive birds (born in Sweden that spring) in Sweden but exposed them to a magnetic field characteristic of northern Egypt, the birds proceeded to put on weight as though they had arrived in Egypt (and three times more than control birds kept in the normal magnetic field of Sweden).

How Do They Do It (i.e. what receptors detect magnetic fields)?

The location and mechanism of action of the receptors in these animals is still a puzzle. Microscopic grains of magnetite (FeO^.Fe₂O₃), a magnetic material, have been found in honeybees and pigeons, but whether and how these might function as receptors is not known.

Certain bacteria orient themselves in magnetic fields as weak as those of the earth and this is mediated by grains of magnetite within the cell.

There is also evidence that birds and amphibians can respond to the earth's magnetic field by its interaction with blue light-stimulated cryptochrome molecules in their retina.