306
Wolfgang Wiltschko and Roswitha Wiltschko
controls the directional significance of stars and sunset cues during migration thus
represents a mechanism of primary importance for migratory orientation in birds
(for details, see W. Wiltschko et al. 1998b ).
A similar role of the magnetic field as reference for innate directions is indicated
in salmon fry. Newly hatched sockeye salmon, Oncorhynchus nerka, must find a
suitable nursing lake where they feed before they start their seaward migration.
Fry of different origin showed spontaneous preferences of population-specific
directions that were in agreement with the courses leading to nearby nursery lakes;
the directional tendencies changed accordingly when magnetic north was
deflected (e.g. Quinn 1980). Additionally, salmon fry show spontaneous rheotactic
responses and are affected by various visual and possibly chemical cues so that the
spontaneous tendency to swim in an innate magnetic direction is just one
component in a multifactorial system leading the young fry to their first feeding
grounds (for summary, see Groot 1982).
There is also support for innate magnetic courses in some arthropod species.
Migrating monarch butterflies, Danaus plexippus, changed their preference
accordingly when tested in a magnetic field with a deflected magnetic north
(Etheredge et al. 1999). Laboratory-raised specimen of equatorial sandhoppers
Talorchestia martensii (Talitridae) showed a spontaneous preference of the axis
land-sea; this tendency could be altered by shifting magnetic north (Pardi et al.
1988). In talitrids, such innate tendencies can be modified and adjusted to the
specific directional alignment of the home coastline (e.g. Ugolini et al. 1991 ), so
that the directional tendencies of adult animals reflect a combination of innate and
acquired information.
4.2.2 Acquired Magnetic Courses
Fixed directional relationships between start and goal are exceptions. In most
cases, the direction to the goal varies according to the given situation. The animals
use various means to determine the respective courses, with the specific processes
depending on the behavioral context.
The orientation "f newly hatched marine turtles is one of the best known
examples for animals relying on innate preferences to determine the correct
course. After emerging at their natal beaches, young turtles must cover a distance
of several metres to reach the water and then swim a course roughly perpendicular
to the shoreline to gain the open sea. Spontaneous responses to light stimuli from
brighter parts of the horizon as well horizon elevation and beach slope guide the
hatchlings towards the shoreline; once in the water, they head into the incoming
waves (for details, see Salmon and Wyneken 1994). The direction of these
movements is memorized as a magnetic course and then pursued for hours
(Lohmann and Lohmann 1994, Goff et a!. 1998), bringing the hatchling away
from the shore into the open ocean.
Wolfgang Wiltschko and Roswitha Wiltschko
controls the directional significance of stars and sunset cues during migration thus
represents a mechanism of primary importance for migratory orientation in birds
(for details, see W. Wiltschko et al. 1998b ).
A similar role of the magnetic field as reference for innate directions is indicated
in salmon fry. Newly hatched sockeye salmon, Oncorhynchus nerka, must find a
suitable nursing lake where they feed before they start their seaward migration.
Fry of different origin showed spontaneous preferences of population-specific
directions that were in agreement with the courses leading to nearby nursery lakes;
the directional tendencies changed accordingly when magnetic north was
deflected (e.g. Quinn 1980). Additionally, salmon fry show spontaneous rheotactic
responses and are affected by various visual and possibly chemical cues so that the
spontaneous tendency to swim in an innate magnetic direction is just one
component in a multifactorial system leading the young fry to their first feeding
grounds (for summary, see Groot 1982).
There is also support for innate magnetic courses in some arthropod species.
Migrating monarch butterflies, Danaus plexippus, changed their preference
accordingly when tested in a magnetic field with a deflected magnetic north
(Etheredge et al. 1999). Laboratory-raised specimen of equatorial sandhoppers
Talorchestia martensii (Talitridae) showed a spontaneous preference of the axis
land-sea; this tendency could be altered by shifting magnetic north (Pardi et al.
1988). In talitrids, such innate tendencies can be modified and adjusted to the
specific directional alignment of the home coastline (e.g. Ugolini et al. 1991 ), so
that the directional tendencies of adult animals reflect a combination of innate and
acquired information.
4.2.2 Acquired Magnetic Courses
Fixed directional relationships between start and goal are exceptions. In most
cases, the direction to the goal varies according to the given situation. The animals
use various means to determine the respective courses, with the specific processes
depending on the behavioral context.
The orientation "f newly hatched marine turtles is one of the best known
examples for animals relying on innate preferences to determine the correct
course. After emerging at their natal beaches, young turtles must cover a distance
of several metres to reach the water and then swim a course roughly perpendicular
to the shoreline to gain the open sea. Spontaneous responses to light stimuli from
brighter parts of the horizon as well horizon elevation and beach slope guide the
hatchlings towards the shoreline; once in the water, they head into the incoming
waves (for details, see Salmon and Wyneken 1994). The direction of these
movements is memorized as a magnetic course and then pursued for hours
(Lohmann and Lohmann 1994, Goff et a!. 1998), bringing the hatchling away
from the shore into the open ocean.