Evidence for an ability to alter their behavior in response to the earth's magnetic field has been found in many animals, including sea turtles, birds, fish (especially common in those that migrate), honeybees, mice as well as in some bacteria.
- 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).
Figure 22.214.171.124 Magnetic sense
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).
Receptors that 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 supplement their magnetic sense using the interaction of light and magnetic fields 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.
How humans detect magnetic fields
The jury is still out. There is some evidence that humans can detect the orientation of magnetic fields. Both cryptochrome and magnetite are found in humans, but their presence may have nothing to do with magnetoreception.