The case for light-dependent magnetic orientation in animals

Light-dependent models of magnetoreception have been proposed which involve an interaction between the magnetic field and either magnetite particles l ocated within a photoreceptor or excited states of photopigment molecules. Consistent with a photoreceptor-based magnetic compass mechanism, magnetic orientation responses in salamanders, flies and birds have been shown to be affected by the wavelength of light. In birds and flies, it is unclear whe ther the effects of light on magnetic orientation are due to a direct effec t on a magnetoreception system or to a nonspecific (e.g. motivational) effe ct of light on orientation behavior, Evidence from shoreward-orienting sala manders, however, demonstrates that salamanders perceive a 90 degrees count erclockwise shift in the direction of the magnetic field under long-wavelen gth (greater than or equal to 500 nm) light. A simple physiological model b ased on the antagonistic interaction between two magnetically sensitive spe ctral mechanisms suggests one possible way in which the wavelength-dependen t effects of light on the salamander's magnetic compass response might aris e. Assuming that the wavelength-dependent characteristics of the avian magn etic response can be attributed to an underlying magnetoreception system, w e discuss several hypotheses attempting to resolve the differences observed in the wavelength-dependent effects of light on magnetic orientation in bi rds and salamanders. By considering the evidence in the context of photorec eptor- and non-photoreceptor-based mechanisms for magnetoreception, we hope to encourage future studies designed to distinguish between alternative hy potheses concerning the influence of light on magnetoreception.