Scientists at the University of Auckland are studying the magnetic sixth sense that helps some animals to find their way home.
For more than 100 years the magnetic sense seemed a phantom: a sense that almost should, but could not, be shown to exist. That the sense "should" exist springs from the observed navigational feats of animals and the knowledge that the earth's magnetic field provides consistent navigational information worldwide.
Scepticism that the sense exists has remained principally because it was unknown how animals might detect magnetic fields. Exciting new results are now providing clues to how animals might use the earth's magnetic field to navigate.
Rainbow trout have proved ideal for the study of the magnetic sense. The fish can be trained to press a bar for food when they detect a particular magnetic field and not another. To do this, they must have a magnetic sense with sensory cells, sensory nerves and processing centres in the brain. Responses to magnetic field stimuli have also been detected in the trigeminal nerve, which branches over the front of the head of the trout.
The identification of a nerve carrying magnetic field information to the brain narrowed down the search for the magnetic sense organ. The magnetic field detectors had to be at the ends of one or more of the many fine branches of the trigeminal nerve. Unfortunately, we had no idea what the detectors would look like.
Fortunately, candidate detector cells can be identified using a completely different approach. Chains of magnetic crystals are found in a discrete layer of tissue in the nose of the trout. The crystals are probably magnetite, or lodestone, the mineral used in the first compasses made by man. If linked to a system for detecting their movement in response to the earth's magnetic field, the chains of crystals will produce a signal that could be detected by the nervous system and the site of magnetic field detection will have been found.
That claim cannot yet be made, but fine branches of the trigeminal nerve certainly come very close to the cells containing the magnetic crystals. In other words, all the necessary components of a magnetic sense have been identified in the rainbow trout.
The understanding gained from the trout has led to new ideas about how animals might navigate using the earth's magnetic field. Their responses to magnetic field intensity are consistent with suggestions that the intensity of the earth's magnetic field is used by homing pigeons to determine their position.
Analyses during the 1970s of the effects of magnetic storms and anomalies on the homing orientation of pigeons suggested the birds can detect the very small systematic changes (about 0.01% per km) in magnetic intensity that occur between the magnetic equator and the magnetic poles. These observations implied that magnetic intensity could act as an analogue to geographic latitude used by human navigators.
It has been difficult to discover how homing pigeons might determine longitude to go with a magnetic latitude. Earlier studies linking magnetic intensity to position suggested the slope in intensity is important. The field slope lies at right angles to the magnetic intensity ("magnetic latitude") and so could act as a magnetic "longitude" if linked to information about direction obtained from either the sun or magnetic compasses.
We will soon begin testing whether these magnetic analogues of geographic latitude and longitude are used by homing pigeons. A successful outcome will mean that all the stages between the detection of magnetic fields and their ultimate use in navigation by animals will have been identified. Michael Walker
Senior lecturer in the school of biological
sciences at the
University of Auckland, New Zealand