The Geomagnetic Field and its Role in Directional Orientation
297
3.1 Possible Transducing Processes
A wide variety of mechanisms has been proposed as transducing processes (seeR.
Wiltschko and W. Wiltschko 1995). Today, the discussion focuses mainly on two
hypotheses, namely (1) magnetoreception mediated by excited-state macromolecules, in particular photopigments, and (2) magnetoreception based on
ferromagnetic material such as magnetite crystals. Other models are likewise
considered, among them a hybrid model of the above-mentioned hypotheses
(Edmonds 1996).
3.1.1 Magnetoreception by Photopigments
Hypotheses proposing an involvement of light-dependent processes in magnetoreception assume that, as a first step, certain macromolecules are elevated to
singlet excited states by the absorption of a photon. A first hypothesis was
suggested by Leask (1977); the presently discussed model was forwarded by
Schulten and Windemuth (1986). These authors propose that the excited macromolecules may dissociate into radical pairs and, by hyperfine interactions, may be
interconverted into triplet pairs. The yield of this step depends on the alignment of
the molecules in the external field. Triplet products are chemically different from
singlet pairs and might initiate processes that ultimately lead to magnetoreception
(see Schulten and Windemuth 1986, rutz et al. 2000).
To mediate magnetic information by this mechanism, an orderly array of macromolecules oriented in the various spatial directions would be required. This
condition seems to be satisfied by the arrangement of photopigments in the
vertebrate eyes. The light-dependent mechanisms proposed by Leask (1977) and
Schulten and Windemuth (1986) would generate complex patterns of response in
the retina. These patterns should be axial and symmetrical with respect to
magnetic north and south and thus allow animals to detect the direction of the
ambient magnetic field.
Leask (1977) and Schulten and Windemuth (1986) originally designed their
models of magnetoreception with the functional characteristics of the avian
magnetic compass in mind. The fact that light-dependent processes lead to axial
rather than polar responses is in accordance with the inclination compass. Changes
in intensity would alter the patterns on the retina without affecting their symmetry,
so that birds might get used to the novel pattern and learn to interpret it after a
while (Leask 1977, Ritz et al. 2000).
The ability to explain important features of the birds' responses under various
magnetic conditions made this model rather attractive. A polarity compass as
described for some fish, rodents and two arthropod species, on the other hand,
appears incompatible with the axial response of the photoreceptor-based
mechanisms described so far.
297
3.1 Possible Transducing Processes
A wide variety of mechanisms has been proposed as transducing processes (seeR.
Wiltschko and W. Wiltschko 1995). Today, the discussion focuses mainly on two
hypotheses, namely (1) magnetoreception mediated by excited-state macromolecules, in particular photopigments, and (2) magnetoreception based on
ferromagnetic material such as magnetite crystals. Other models are likewise
considered, among them a hybrid model of the above-mentioned hypotheses
(Edmonds 1996).
3.1.1 Magnetoreception by Photopigments
Hypotheses proposing an involvement of light-dependent processes in magnetoreception assume that, as a first step, certain macromolecules are elevated to
singlet excited states by the absorption of a photon. A first hypothesis was
suggested by Leask (1977); the presently discussed model was forwarded by
Schulten and Windemuth (1986). These authors propose that the excited macromolecules may dissociate into radical pairs and, by hyperfine interactions, may be
interconverted into triplet pairs. The yield of this step depends on the alignment of
the molecules in the external field. Triplet products are chemically different from
singlet pairs and might initiate processes that ultimately lead to magnetoreception
(see Schulten and Windemuth 1986, rutz et al. 2000).
To mediate magnetic information by this mechanism, an orderly array of macromolecules oriented in the various spatial directions would be required. This
condition seems to be satisfied by the arrangement of photopigments in the
vertebrate eyes. The light-dependent mechanisms proposed by Leask (1977) and
Schulten and Windemuth (1986) would generate complex patterns of response in
the retina. These patterns should be axial and symmetrical with respect to
magnetic north and south and thus allow animals to detect the direction of the
ambient magnetic field.
Leask (1977) and Schulten and Windemuth (1986) originally designed their
models of magnetoreception with the functional characteristics of the avian
magnetic compass in mind. The fact that light-dependent processes lead to axial
rather than polar responses is in accordance with the inclination compass. Changes
in intensity would alter the patterns on the retina without affecting their symmetry,
so that birds might get used to the novel pattern and learn to interpret it after a
while (Leask 1977, Ritz et al. 2000).
The ability to explain important features of the birds' responses under various
magnetic conditions made this model rather attractive. A polarity compass as
described for some fish, rodents and two arthropod species, on the other hand,
appears incompatible with the axial response of the photoreceptor-based
mechanisms described so far.