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Wolfgang Wiltschko and Roswitha Wiltschko
3.1.2 Magnetoreception based on Magnetite
The other prominent hypothesis assumes that particles of magnetite, Fe 3 0 4 , are
involved in the processes mediating magnetic information. Obtaining directional
information by ferromagnetic particles, which may act as miniature magnets, is an
obvious possibility. Yet permanently magnetic material of biogenic origin was
unknown until Lowenstam (1962) first described magnetite in the radula teeth of
chitons. By remanence measurements and Mossbauer spectroscopy, magnetite
crystals could be identified in the tissue of a wide variety of animals (see
Kirschvink et al. 1985, R. Wiltschko and W. Wiltschko 1995 for summary), which
promoted speculations about their possible role in magnetoreception.
The magnetic properties of magnetite particles depend on their size. Particles in
the range of 0.05 to 1.00 !lm are mostly single domains with a stable magnetic
moment. The magnetic energy of orientation often just barely exceeds the
background thermal energy, however. This problem could be overcome by an
orderly array of particles in chains, clusters or lattices where adding their moments
would increase their interactions with the geomagnetic field. Crystals of even
smaller size are superparamagnetic; they have no stable moment, but may easily
be aligned by an external field. Such particles might also be part of a receptor
mechanism when arranged in a suitable array and properly aligned (see
Kirschvink and Gould 1981, Shcherbakov and Winklhofer 1999).
Permanent magnetic material responds to polarity; hence mechanisms based on
magnetite should be able to indicate the polar direction of the magnetic field. Yet
magnetite-based mechanisms that produce axial responses are likewise possible, in
particular when superparamagnetic particles are involved (e.g. Kirschvink and
Gould 1981, Shcherbakov and Winklhofer 1999). Magnetoreception by magnetite
could thus be involved in polarity compass as well as inclination compass
mechanisms. Theoretical considerations also show that magnetite-based
mechanisms would not only be able to mediate the direction of the magnetic field,
but also be sensitive enough to detect small differences in intensity, provided that
a sufficient number of magnetite particles were involved (e.g. Yorke 1979,
Kirschvink and Gould 1981, Kirschvink 1989, Shcherbakov and Winklhofer
1999).
3.2 Experimental Evidence
The light-dependent initial processes of the photopigment model offer the
possibility to test this hypothesis by a simple technique: If the assumptions of the
model were correct, magnetoreception should be impossible in the absence of
light.
Tests with homing pigeons indicated that magnetic compass information was
disrupted in total darkness (W. Wiltschko and R. Wiltschko 1981). The same was
true for the aquatic newt Notophthalmus (Salamandridae; Phillips and Borland
Wolfgang Wiltschko and Roswitha Wiltschko
3.1.2 Magnetoreception based on Magnetite
The other prominent hypothesis assumes that particles of magnetite, Fe 3 0 4 , are
involved in the processes mediating magnetic information. Obtaining directional
information by ferromagnetic particles, which may act as miniature magnets, is an
obvious possibility. Yet permanently magnetic material of biogenic origin was
unknown until Lowenstam (1962) first described magnetite in the radula teeth of
chitons. By remanence measurements and Mossbauer spectroscopy, magnetite
crystals could be identified in the tissue of a wide variety of animals (see
Kirschvink et al. 1985, R. Wiltschko and W. Wiltschko 1995 for summary), which
promoted speculations about their possible role in magnetoreception.
The magnetic properties of magnetite particles depend on their size. Particles in
the range of 0.05 to 1.00 !lm are mostly single domains with a stable magnetic
moment. The magnetic energy of orientation often just barely exceeds the
background thermal energy, however. This problem could be overcome by an
orderly array of particles in chains, clusters or lattices where adding their moments
would increase their interactions with the geomagnetic field. Crystals of even
smaller size are superparamagnetic; they have no stable moment, but may easily
be aligned by an external field. Such particles might also be part of a receptor
mechanism when arranged in a suitable array and properly aligned (see
Kirschvink and Gould 1981, Shcherbakov and Winklhofer 1999).
Permanent magnetic material responds to polarity; hence mechanisms based on
magnetite should be able to indicate the polar direction of the magnetic field. Yet
magnetite-based mechanisms that produce axial responses are likewise possible, in
particular when superparamagnetic particles are involved (e.g. Kirschvink and
Gould 1981, Shcherbakov and Winklhofer 1999). Magnetoreception by magnetite
could thus be involved in polarity compass as well as inclination compass
mechanisms. Theoretical considerations also show that magnetite-based
mechanisms would not only be able to mediate the direction of the magnetic field,
but also be sensitive enough to detect small differences in intensity, provided that
a sufficient number of magnetite particles were involved (e.g. Yorke 1979,
Kirschvink and Gould 1981, Kirschvink 1989, Shcherbakov and Winklhofer
1999).
3.2 Experimental Evidence
The light-dependent initial processes of the photopigment model offer the
possibility to test this hypothesis by a simple technique: If the assumptions of the
model were correct, magnetoreception should be impossible in the absence of
light.
Tests with homing pigeons indicated that magnetic compass information was
disrupted in total darkness (W. Wiltschko and R. Wiltschko 1981). The same was
true for the aquatic newt Notophthalmus (Salamandridae; Phillips and Borland