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Magnetoreception - Introduction
vector field, whose detailed properties as well as its spatial and temporal variations are
extensively discussed in Skiles (1985) and Lanza & Meloni (2006).
1.3 Using the Earth’s Magnetic Field
Moving through the Earth’s magnetic field, an animal induces an electric field across its body.
This field is oriented perpendicular to the plane containing the direction of movement and the
magnetic field. Its strength is maximal when the animal moves perpendicular to the magnetic
field. The moving animal could principally thus relocate its direction until its self-generated
electric field is both of appropriate direction and strength (cf., Dusenbery 1992).
Both the regular and gradual magnetic parameters of the Earth’s magnetic field can be
used by animals to orient. To this end, two types of information are supplied: the magnetic
field vector (F) provides directional information that can be used as a compass, while total
intensity and/or inclination (see below) give information that supports position determination
on a magnetic map. For compass orientation, three components of the Earth’s magnetic field
are of importance. An individual can obtain differential information either from the spatially
varying gradients of intensity of the magnetic field or inclination (i.e. the angle between the
magnetic field vector and the horizontal plane, see below), or from directional cues provided
by horizontal polarity (i.e. direction of the magnetic field lines pointing towards magnetic
North, as visualised by a compass needle). Additionally, geological formations may disturb the
local magnetic field and serve as characteristic magnetic landmarks (Dusenbery 1992).
The functional mechanism of the magnetic compass, e.g. in birds, is not, however, in
contrast to the technical compass, based on the magnetic field’s polarity, but on its inclination:
the streamlines that leave the Earth at the northern geomagnetic pole in the southern
hemisphere and reenter the globe in the northern hemisphere at the southern geomagnetic
pole form inclination angles, which alter systematically with geographical width. These
inclination angles amount to 90° at the poles and 0° at the equator. By perceiving the
streamlines and using these angles, an animal can perceive the course of the magnetic axis.
The angles supply the animal with information on the direction the animal is heading, i.e.
polewards or equatorwards. That means that e.g. migrating birds interpret the inclination
angles and their course for orientation; this system is thus called an inclination compass
(Wiltschko & Wiltschko 1972, 1995; Wiltschko et al. 1993). With this compass, birds do not
discriminate between “North” and “South”, but receive information on the “poleward” and
“equatorward” direction, respective to the magnetic poles and the magnetic equator. Birds
from the northern and southern hemisphere thus possess the same “migration programme”
enticing them to migrate “equatorwards” in their respective autumn (Wiltschko et al. 1993).
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