Abstract:
How animals navigate over long distances is not understood despite decades of
study. This thesis used lemon sharks as a new experimental model to study navigation
because they home reliably after displacement and their responses to the Earth’s
magnetic field could be compared with those of homing pigeons.
I hypothesised that if movements by homing pigeons, aligned parallel and
perpendicular to the intensity contours of the Earth’s magnetic field, are used in
navigation, then other species of navigating animals should perform similar movements.
Tracks of lemon sharks revealed movements similar to the magnetic alignments made
by pigeons during homing. In sharks, this behaviour continued for much of the
homeward journey, whereas in pigeons it occurs almost exclusively in the first few
kilometres.
The effects of magnets and the magnetic characteristics of release sites were
examined with pigeons to explore how homing animals overcome magnetic
disturbances while navigating over long distances. The magnets did not appear to affect
the pigeons’ bearings and alignments, but the field angle variance and the difference
between the aspect and home directions at the release sites were associated with the
amount of disorientation displayed by the birds. This study was the first to use a
multivariate analysis to examine the behaviour of homing pigeons and successfully
distinguished magnetic field characteristics that vary at most release sites and that
influence the birds’ behaviour from those that do not.
This study identified similar responses to the Earth’s magnetic field in two
vertebrate classes. The similarity of the behaviours was impressive considering the
large differences between the animals, the speeds at which they move, and the fluid
media in which they live. The behavioural similarities also suggest that the use of the
magnetic field in long distance navigation may be common in vertebrates. The analysis
of the effect of release site characteristics on the behaviour of navigating pigeons
demonstrated the importance of a multivariate approach in navigational studies. Future
research should expand this study with tracks from both pigeons and sharks, and may
lead to a model of how navigating sharks and pigeons will react to magnetic fields upon
displacement.