Magnetoreceptors

• honeybees, the Pacific dolphin, blind mole rats, mice, some planarians, mollusks, fish, salamanders, birds, and sea turtles

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).

The nature of the receptor in these animals is still a puzzle. Several have been found to contain magnetic material — microscopic grains of magnetite (FeO^.Fe₂O₃). In pigeons, these are probably located in the upper part of the beak area. In the trout (a fish), they are in the nose.

There is also evidence that birds and amphibians can supplement their magnetic sense using the interaction of light and magnetic lines of force on cryptochrome molecules in their retina.

The ability of Drosophila to respond to magnetic fields depends on blue light and cryptochrome.

• In the absence of blue light, the flies do not respond to a magnetic field.
• Mutant flies that lack cryptochrome are likewise insensitive to magnetic fields.
• However, mutant flies whose own cryptochrome genes have been replaced by the human gene respond normally to a magnetic field while exposed to blue light. Does human cryptochrome enable us to sense magnetic fields?