Many bacteria are able to perceive magnetic fields and follow lines of strength
of those fields when they move. Magneto-orientation may provide bacteria a way
of achieving straight-line movements, which otherwise would be difficult or impossible.
They also may combine magnetic field information with other environmental factors,
such as oxygen concentration, to find favorable habitats (Frankel et al. 1997).
Magneto-receptors, which are iron rich, membrane bound structures in bacterial
cells, apparently evolved more than once in bacteria (Delong et al. 1993).
In multicellular animals, the first suggestion of magenetic field perception came
from work on homing pigeons. Pigeons seemed to retain their navigation abilities
under adverse conditions for compass orientation, such as cloudy skies. Keeton
found that, at least in some experiments, equipping pigeons with magnets on their
heads disrupted their orientation. He took this to implicate perception of the
earths magnetic field in pigeon orientation. Magnetite crystals were later
found at the base of pigeon skulls, suggesting a magnetic perception organ (Gould
1980).
The realization that fluctuations of the earths magnetic field are correlated
with systematic errors in honey bee dances (missweisung) led investigators to
explore whether honey bees, as well, might have the ability to perceive the earths
magnetic field. This prompted the discovery of magnetite crystals in the abdomen
of honey bees (Gould 1980).
We dont know how widespread or important magnetoperception organs are in
the animal kingdom. The typical experimental approach is to place magnets in or
on an animal and monitor the it for changes in orientation. Recent experiments
with salmon (Diebel et al. 2000) show that they have magnetite crystals in their
olfactory organs, but implanting magnets in salmon did not affect their orientation
(Yano et al. 1997). Similarly sea turtle migration was not disrupted by magnets
placed on the animals (Papi et al. 2000). In field orientation experiments animals
have access to many possible inputs, and may discard magnetic information in favor
of other information when the magnetic field is disrupted.