The Geomagnetic Field and its Role in Directional Orientation
307
Amphibians that orient along the axis perpendicular to the shore, the so-called
Y -axis (see Ferguson 1967) also follow acquired courses. These courses are determined by the lay of the land in the home area; they are modified when the animals
move or are placed in an aquarium with a different distribution of land and water.
Salamanders memorize the Y -axis with respect to the magnetic field, using their
magnetic compass to move shoreward or waterward (Phillips 1986b ).
The most frequent cases of variable courses, however, concern homing, i.e.
animals returning from foraging excursions or exploratory trips to their hiding
places, burrows or nests. Homing animals determine the current home course by
navigational processes. Homing behavior has been extensively studied in birds by
displacement experiments with carrier pigeons, so that mechanisms of navigation
in birds are far better known than in any other animal group.
Avian navigation is based on a multitude of factors that provide information on
direction and position. These factors are integrated by learning processes to form
complex mechanisms. The magnetic compass plays a crucial role in avian
navigation, in particular in very young, inexperienced pigeons before the learned
mechanisms become functional. Young birds depend almost exclusively on their
magnetic compass, because it is the only mechanism available right from the
beginning. They navigate by recording the net direction of the outward journey
with the help of their magnetic compass; reversing this direction yields the home
course (R. Wiltschko and W. Wiltschko 1978). Similar route-based mechanisms
based on magnetic information have been indicated by displacement experiments
with woodmice, Apodemus sylvaticus (Mather and Baker 1981 ), and domestic
horses, Equus cabal/us (Baker 1989). It is unclear, however, how widespread this
use of the magnetic compass might be among animals.
Birds use the simple route-based strategy described above only during a short
transient stage. As soon as young pigeons become more experienced, they begin to
integrate various factors and, by learning processes, establish the navigational
'map', which is assumed to be a directionally oriented mental representation of the
distribution of various environmental gradients in the home region (for details, see
Wallraff 1974; R. Wiltschko and W. Wiltschko 1999). The map is then preferentially used. Similarly, experienced migratory birds no longer rely on innate
information, but take advantage of their experience from previous migrations,
establishing a navigational map of their migration route. The nature of the
respective learning processes escapes experimental analysis. It seems not unlikely,
however, that the magnetic compass here, too, serves as directional reference for
integrating the various factors forming the map.
4.3 The Magnetic Compass as Reference System
Section 4 of this chapter gave a few examples for the various uses of the
geomagnetic field as a compass by animals. The magnetic compass is involved in
a wide variety of other behaviors, like in the subterranean orientation of mole-rats
307
Amphibians that orient along the axis perpendicular to the shore, the so-called
Y -axis (see Ferguson 1967) also follow acquired courses. These courses are determined by the lay of the land in the home area; they are modified when the animals
move or are placed in an aquarium with a different distribution of land and water.
Salamanders memorize the Y -axis with respect to the magnetic field, using their
magnetic compass to move shoreward or waterward (Phillips 1986b ).
The most frequent cases of variable courses, however, concern homing, i.e.
animals returning from foraging excursions or exploratory trips to their hiding
places, burrows or nests. Homing animals determine the current home course by
navigational processes. Homing behavior has been extensively studied in birds by
displacement experiments with carrier pigeons, so that mechanisms of navigation
in birds are far better known than in any other animal group.
Avian navigation is based on a multitude of factors that provide information on
direction and position. These factors are integrated by learning processes to form
complex mechanisms. The magnetic compass plays a crucial role in avian
navigation, in particular in very young, inexperienced pigeons before the learned
mechanisms become functional. Young birds depend almost exclusively on their
magnetic compass, because it is the only mechanism available right from the
beginning. They navigate by recording the net direction of the outward journey
with the help of their magnetic compass; reversing this direction yields the home
course (R. Wiltschko and W. Wiltschko 1978). Similar route-based mechanisms
based on magnetic information have been indicated by displacement experiments
with woodmice, Apodemus sylvaticus (Mather and Baker 1981 ), and domestic
horses, Equus cabal/us (Baker 1989). It is unclear, however, how widespread this
use of the magnetic compass might be among animals.
Birds use the simple route-based strategy described above only during a short
transient stage. As soon as young pigeons become more experienced, they begin to
integrate various factors and, by learning processes, establish the navigational
'map', which is assumed to be a directionally oriented mental representation of the
distribution of various environmental gradients in the home region (for details, see
Wallraff 1974; R. Wiltschko and W. Wiltschko 1999). The map is then preferentially used. Similarly, experienced migratory birds no longer rely on innate
information, but take advantage of their experience from previous migrations,
establishing a navigational map of their migration route. The nature of the
respective learning processes escapes experimental analysis. It seems not unlikely,
however, that the magnetic compass here, too, serves as directional reference for
integrating the various factors forming the map.
4.3 The Magnetic Compass as Reference System
Section 4 of this chapter gave a few examples for the various uses of the
geomagnetic field as a compass by animals. The magnetic compass is involved in
a wide variety of other behaviors, like in the subterranean orientation of mole-rats