The Geomagnetic Field and its Role in Directional Orientation
4.1 Interrelations Between Magnetic and Celestial Compass
Information
303
Day-active animals can use the sun as a compass. Sun-derived factors such as the
pattern of polarized light also provide directional information, in particular in
insects (see Wehner 1984). These celestial features vary with time; their exact
position depends on the time of day, season and also on geographic latitude. This
makes their use far more demanding than that of the magnetic field, where
directions are immediately derived from sensory input. Most animals use celestial
compass mechanisms that are established by learning processes, which allows a
fine tuning of these mechanisms to the local situation.
The relative importance of magnetic and celestial cues has been analyzed in few
species only, many of them birds. The results indicate a general tendency of dayactive animals to prefer the sun compass.
In the case of conflict, e.g. when their internal clock was shifted, fish, anurans,
birds and amphipods mostly followed their manipulated sun compass in spite of
correct information from the magnetic field. A prominent case are homing pigeons
that respond to clock-shifting with typical deflections in spite of their magnetic
compass, indicating the preferred use of the sun compass. Yet under solid
overcast, they are equally well oriented, relying on their magnetic compass
(Fig. 7).
In certain ecological situations, the magnetic compass is of crucial importance.
Near the equator, where the changes in sun azimuth are rather rapid and large
seasonal changes occur, most litoral amphipods were found to rely on their
magnetic compass; orientation deteriorated in the absence of magnetic information
(e.g. Pardi et al. 1988, Ugolini and Pardi 1992).
Bird migration is another case where the magnetic compass plays a dominant role.
Day migrants preferably rely on the magnetic field, possibly, because the
magnetic compass, in contrast to the sun compass, is not affected by the latitudinal
changes birds inevitably experience during migration (see Munro and Wiltschko
1993 for discussion).
The preferred use of visual cues often observed seems to suggest a hierarchy of
cues, with the sun compass dominating over the magnetic compass. The two
mechanisms are not independent from each other, however. In young homing
pigeons, the magnetic compass provides the reference system for the learning
processes which establish the sun compass, i.e. the sun compass is derived from
the magnetic compass (W. Wiltschko et al. 1983, R. Wiltschko and W. Wiltschko
1990). The adjustment of the sun compass to seasonal changes of the sun's arc
might also be controlled by the magnetic compass.
4.1 Interrelations Between Magnetic and Celestial Compass
Information
303
Day-active animals can use the sun as a compass. Sun-derived factors such as the
pattern of polarized light also provide directional information, in particular in
insects (see Wehner 1984). These celestial features vary with time; their exact
position depends on the time of day, season and also on geographic latitude. This
makes their use far more demanding than that of the magnetic field, where
directions are immediately derived from sensory input. Most animals use celestial
compass mechanisms that are established by learning processes, which allows a
fine tuning of these mechanisms to the local situation.
The relative importance of magnetic and celestial cues has been analyzed in few
species only, many of them birds. The results indicate a general tendency of dayactive animals to prefer the sun compass.
In the case of conflict, e.g. when their internal clock was shifted, fish, anurans,
birds and amphipods mostly followed their manipulated sun compass in spite of
correct information from the magnetic field. A prominent case are homing pigeons
that respond to clock-shifting with typical deflections in spite of their magnetic
compass, indicating the preferred use of the sun compass. Yet under solid
overcast, they are equally well oriented, relying on their magnetic compass
(Fig. 7).
In certain ecological situations, the magnetic compass is of crucial importance.
Near the equator, where the changes in sun azimuth are rather rapid and large
seasonal changes occur, most litoral amphipods were found to rely on their
magnetic compass; orientation deteriorated in the absence of magnetic information
(e.g. Pardi et al. 1988, Ugolini and Pardi 1992).
Bird migration is another case where the magnetic compass plays a dominant role.
Day migrants preferably rely on the magnetic field, possibly, because the
magnetic compass, in contrast to the sun compass, is not affected by the latitudinal
changes birds inevitably experience during migration (see Munro and Wiltschko
1993 for discussion).
The preferred use of visual cues often observed seems to suggest a hierarchy of
cues, with the sun compass dominating over the magnetic compass. The two
mechanisms are not independent from each other, however. In young homing
pigeons, the magnetic compass provides the reference system for the learning
processes which establish the sun compass, i.e. the sun compass is derived from
the magnetic compass (W. Wiltschko et al. 1983, R. Wiltschko and W. Wiltschko
1990). The adjustment of the sun compass to seasonal changes of the sun's arc
might also be controlled by the magnetic compass.