302
Wolfgang Wiltschko and Roswitha Wiltschko
magnetic stimulation. However, the units seemed to respond to changes in the
intensity rather than to changes in the direction of the magnetic field (Semm and
Beason 1990; Walker et a!. 1997). This speaks against the associated receptors
providing compass information.
Attempts to interfere with magnetite-based receptors involved treating animals
with a short, strong magnetic pulse designed to change the magnetization of single
domain magnetite. The treatment had marked effects on the orientation behavior
in migrants as well as homing pigeons (e.g. W. Wiltschko et a!. 1994, 1998a;
Beason et a!. 1995, 1997; Munro et a!. 1997). The manifestation of these effects,
however, suggests that magnetic components of the navigational system rather
than the magnetic compass were affected (see Beason and Semm 1996; Munro et
a!. 1997; W. Wiltschko et a!. 1998a for discussion), an interpretation that is in
agreement with the electrophysiologica1 findings.
A case where a magnetic pulse might affect a compass response is reported from
mole-rats, Cryptomys sp. (Bathyergidae). The pulse caused a roughly 90° counterclockwise shift in the nest position, which persisted for at least 3 months after the
treatment (Marhold et a!. 1997b ). In arthropods, some histological, electrophysiological and behavioral data on magnetite are available from honeybees, yet the
findings do not add up to a consistent picture (seeR. Wiltschko and W. Wiltschko
1995 for discussion).
Taken together, our knowledge on possible magnetite-based receptors and what
type of information they may provide is still rather limited. The source of
magnetic compass information in animals that use light-independent mechanisms
remains unclear. Magnetite-based receptors are still among the prime candidates.
However, a modified radical pair mechanism, where the light-dependent initial
step is replaced by another process producing excited state molecules (Ritz et al.
2000) seems also possible when an inclination compass is involved.
4 Behavioral Relevance of Magnetic Compass
Orientation
In order to understand the biological significance of the magnetic compass as
orientation mechanism, one must analyze its role in the various orientation tasks.
Two points are to be considered: (1) Normally, the magnetic field is only one of a
multitude of orientation cues available. It is not even the only source of directional
information, as most animals can derive the same type of compass information
from celestial cues. (2) A compass is only helpful if animals know what courses to
take, i.e. whether their goal lies in the north, east etc. Therefore, the question of
the origin of the respective compass courses cannot be separated from
considerations on compass mechanisms. The final part of the present chapter is a
brief overview on the widespread use of the magnetic compass and its interaction
with other orientation cues.
Wolfgang Wiltschko and Roswitha Wiltschko
magnetic stimulation. However, the units seemed to respond to changes in the
intensity rather than to changes in the direction of the magnetic field (Semm and
Beason 1990; Walker et a!. 1997). This speaks against the associated receptors
providing compass information.
Attempts to interfere with magnetite-based receptors involved treating animals
with a short, strong magnetic pulse designed to change the magnetization of single
domain magnetite. The treatment had marked effects on the orientation behavior
in migrants as well as homing pigeons (e.g. W. Wiltschko et a!. 1994, 1998a;
Beason et a!. 1995, 1997; Munro et a!. 1997). The manifestation of these effects,
however, suggests that magnetic components of the navigational system rather
than the magnetic compass were affected (see Beason and Semm 1996; Munro et
a!. 1997; W. Wiltschko et a!. 1998a for discussion), an interpretation that is in
agreement with the electrophysiologica1 findings.
A case where a magnetic pulse might affect a compass response is reported from
mole-rats, Cryptomys sp. (Bathyergidae). The pulse caused a roughly 90° counterclockwise shift in the nest position, which persisted for at least 3 months after the
treatment (Marhold et a!. 1997b ). In arthropods, some histological, electrophysiological and behavioral data on magnetite are available from honeybees, yet the
findings do not add up to a consistent picture (seeR. Wiltschko and W. Wiltschko
1995 for discussion).
Taken together, our knowledge on possible magnetite-based receptors and what
type of information they may provide is still rather limited. The source of
magnetic compass information in animals that use light-independent mechanisms
remains unclear. Magnetite-based receptors are still among the prime candidates.
However, a modified radical pair mechanism, where the light-dependent initial
step is replaced by another process producing excited state molecules (Ritz et al.
2000) seems also possible when an inclination compass is involved.
4 Behavioral Relevance of Magnetic Compass
Orientation
In order to understand the biological significance of the magnetic compass as
orientation mechanism, one must analyze its role in the various orientation tasks.
Two points are to be considered: (1) Normally, the magnetic field is only one of a
multitude of orientation cues available. It is not even the only source of directional
information, as most animals can derive the same type of compass information
from celestial cues. (2) A compass is only helpful if animals know what courses to
take, i.e. whether their goal lies in the north, east etc. Therefore, the question of
the origin of the respective compass courses cannot be separated from
considerations on compass mechanisms. The final part of the present chapter is a
brief overview on the widespread use of the magnetic compass and its interaction
with other orientation cues.