A model for photoreceptor-based magnetoreception in birds

A large variety of animals has the ability to sense the geomagnetic field a nd utilize it as a source of directional (compass) information. It is not k nown by which biophysical mechanism this magnetoreception is achieved. We i nvestigate the possibility that magnetoreception involves radical-pair proc esses that are governed by anisotropic hyperfine coupling between (unpaired ) electron and nuclear spins. We will show theoretically that fields of geo magnetic field strength and weaker can produce significantly different reac tion yields for different alignments of the radical pairs with the magnetic field. As a model for a magnetic sensory organ we propose a system of radi cal pairs being 1) orientationally ordered in a molecular substrate and 2) exhibiting changes in the reaction yields that affect the visual transducti on pathway. We evaluate three-dimensional visual modulation patterns that c an arise from the influence of the geomagnetic field on radical-pair system s. The variations of these patterns with orientation and field strength can furnish the magnetic compass ability of birds with the same characteristic s as observed in behavioral experiments. We propose that the recently disco vered photoreceptor cryptochrome is part of the magnetoreception system and suggest further studies to prove or disprove this hypothesis.