Dead dolphins are frequently found stranded on shorelines around the world and the reasons for this are not hard to find. Some are debilitated by disease or old age; others have been injured by predators or have become accidentally entangled in fishing gear. Harder to comprehend are the single or mass strandings of apparently healthy live animals, recorded as far back as the days of ancient Greece, which almost invariably results in their death from such secondary effects as overheating, suffocation and pneumonia.
Many theories have been put forward to explain why live strandings occur. These have ranged from the unlikely suggestions that the animals are attempting mass suicide or following some atavistic urge to return to land, to the more plausible ideas that their echo-ranging systems are impaired, whether by parasitic infestation of the inner ear or by the confusing nature of the coastal topography. Another theory is that healthy individuals may be following a seriously ill or injured dominant member of the pod or herd.
In the early 1970's, the growing general interest in the biology and conservation of cetaceans led to the establishment of stranding programs in many countries. Data from such programs require care in interpretation, since the more people there are watching for and reporting strandings, the more strandings there appear to be.
However in the UK such records of both live and dead strandings have been kept by the British Natural History Museum, in co- operation with the Receiver of Wrecks and HM Coastguard Service since 1913. It is on the basis of these that scientist Margaret Klinowska has developed a novel explanation as to why live strandings occur.
She argues that cetaceans travel by following geomagnetic contours. By comparing the distribution of live strandings in the UK with the magnetic characteristics in particular areas, she has found a high correlation between the incidence of strandings and the occurrence of sites where the magnetic contours are oriented perpendicular to the coast rather than parallel to it. In following the contours, the dolphins are led to their deaths on the shore.
She has also pointed out a pattern of daily variation in the total geomagnetic field which may function as a biological 'travel clock' for cetaceans. Solar activity can affect this pattern, causing irregular fluctuations which disturb the clock.
If the clock is out, they are not where they 'think' they are on their magnetic contour map and, as a result, can make turns too early or too late and get into unfamiliar areas, where simple contour following may lead them onto land. This can explain why they 'willingly' go on to the beach - they 'think' it is the right way. It also explains re-stranding, because again their basic strategy has let them down in an unfamiliar area.
Joseph Kirschvink and his scientific colleagues in the US have obtained similar results which also support this 'magnetic travel' hypothesis.
This new hypothesis has enriched the scientific debate about the causes of live strandings but a great deal more research in many other parts of the world will be needed if we are to reach a full and final explanation of this most intriguing and dramatic of natural phenomena.
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