158
hemispheres. This provides a reliable, static reference system for orientation and
navigation. Alternatively, magnetic anomalies within the Earth’s crust can also be
recognized and used as reference features. Taking advantage of these properties of
the geomagnetic fi eld, some groups of animals have developed a biological magnetic
compass, similar to the magnetic compass used by humans to locate the north
magnetic pole. The magnetic compass has been described as an axial compass (also
known as Inclination compass) for migratory birds and homing pigeons (Wiltschko
and Wiltschko 1972 ; Walcott and Green 1974 ), and is based on the axial course of
the geomagnetic fi eld lines on the Earth’s surface. The magnetic compass is used as
a reference system and as a mechanism to maintain steady courses during homing
and migrations. Therefore animals able to discriminate the minute but steady
changes of the inclination angle and the intensity of the geomagnetic fi eld can
potentially establish their latitudinal position. To date, several models for position
determination based on magnetic fi eld parameters have been proposed (Davila
2005 ). Lohmann et al. ( 1999 ) proposed that sea turtles use a combination of intensity and inclination, as independent coordinates for map information. Contours of
equal magnetic intensity and inclination form a grid that can potentially be used as
a bi-coordinate position-fi nding system over areas of the Atlantic Ocean, where sea
turtles spend most of their life cycle. This model cannot, however, be generalized
since isolines of magnetic inclination and intensity intersect each other at high
angles only over local regions of the Earth’s surface (Davila 2005 ). In regions where
the isolines are near-parallel to each other, or where the magnetic landscape is
dominated by crustal magnetic anomalies, the bi-coordinate model is not viable for
position determination (Walker et al. 2002 ).
Other model of position determination supposes systematic measurements of the
intensity and in the direction of intensity gradient of the Earth’s main fi eld. The idea
that intensity may be a component of the navigational map system of animals is
based “on the observation that homing pigeons are disoriented when released at
magnetic anomalies,” (Davila 2005 ; see also Walcott 1978 ). On the other hand,
homing pigeons show area-wide distributions of counterclockwise orientation
errors, which are symmetrical about the line of intensity slope through the loft
(Gould 1982 ) supporting the involvement of this fi eld parameter in position determination (Walker 1998 ; Walker et al. 2002 ).
Yet a third model takes into account the regular patterns of magnetic anomalies
originating from the hard substrates in the ocean crust, and produced during sea- fl oor
spreading (Kirschvink et al. 1986 ). Although these anomalies are not present in the
continental crust, they could potentially be used by marine animals to guide longdistance migrations.
Despite the fact that these three models take into consideration different
magnetic fi eld parameters, they are not mutually exclusive. So far there exists no
evidence that all animals capable of magnetic fi eld perception use the same sources
of information based on the geomagnetic fi eld for navigation and orientation.
Furthermore, different groups of animals may have developed different magnetoreceptor systems, each of them designed to obtain information from different fi eld
parameters according to their necessities and the local conditions of the geomagnetic
fi eld (Davila 2005 ).
3 Biocomposites and Mineralized Tissues
hemispheres. This provides a reliable, static reference system for orientation and
navigation. Alternatively, magnetic anomalies within the Earth’s crust can also be
recognized and used as reference features. Taking advantage of these properties of
the geomagnetic fi eld, some groups of animals have developed a biological magnetic
compass, similar to the magnetic compass used by humans to locate the north
magnetic pole. The magnetic compass has been described as an axial compass (also
known as Inclination compass) for migratory birds and homing pigeons (Wiltschko
and Wiltschko 1972 ; Walcott and Green 1974 ), and is based on the axial course of
the geomagnetic fi eld lines on the Earth’s surface. The magnetic compass is used as
a reference system and as a mechanism to maintain steady courses during homing
and migrations. Therefore animals able to discriminate the minute but steady
changes of the inclination angle and the intensity of the geomagnetic fi eld can
potentially establish their latitudinal position. To date, several models for position
determination based on magnetic fi eld parameters have been proposed (Davila
2005 ). Lohmann et al. ( 1999 ) proposed that sea turtles use a combination of intensity and inclination, as independent coordinates for map information. Contours of
equal magnetic intensity and inclination form a grid that can potentially be used as
a bi-coordinate position-fi nding system over areas of the Atlantic Ocean, where sea
turtles spend most of their life cycle. This model cannot, however, be generalized
since isolines of magnetic inclination and intensity intersect each other at high
angles only over local regions of the Earth’s surface (Davila 2005 ). In regions where
the isolines are near-parallel to each other, or where the magnetic landscape is
dominated by crustal magnetic anomalies, the bi-coordinate model is not viable for
position determination (Walker et al. 2002 ).
Other model of position determination supposes systematic measurements of the
intensity and in the direction of intensity gradient of the Earth’s main fi eld. The idea
that intensity may be a component of the navigational map system of animals is
based “on the observation that homing pigeons are disoriented when released at
magnetic anomalies,” (Davila 2005 ; see also Walcott 1978 ). On the other hand,
homing pigeons show area-wide distributions of counterclockwise orientation
errors, which are symmetrical about the line of intensity slope through the loft
(Gould 1982 ) supporting the involvement of this fi eld parameter in position determination (Walker 1998 ; Walker et al. 2002 ).
Yet a third model takes into account the regular patterns of magnetic anomalies
originating from the hard substrates in the ocean crust, and produced during sea- fl oor
spreading (Kirschvink et al. 1986 ). Although these anomalies are not present in the
continental crust, they could potentially be used by marine animals to guide longdistance migrations.
Despite the fact that these three models take into consideration different
magnetic fi eld parameters, they are not mutually exclusive. So far there exists no
evidence that all animals capable of magnetic fi eld perception use the same sources
of information based on the geomagnetic fi eld for navigation and orientation.
Furthermore, different groups of animals may have developed different magnetoreceptor systems, each of them designed to obtain information from different fi eld
parameters according to their necessities and the local conditions of the geomagnetic
fi eld (Davila 2005 ).
3 Biocomposites and Mineralized Tissues