From The Economist 409.8866 (Dec13)
Quote:
He’s behind you!
Some sharks know how to stay out of sight of nearby people
HUMAN beings like to believe they are at the top of the food chain. When something else eats one it is not only upsetting to the victim’s friends and relatives, it also seems slightly improper-a reversal of the natural order of things. Such attacks are thus often portrayed as aberrations from predators’ normal behaviour. In the case of sharks, for example, the sh are assumed to have mistaken human swimmers for seals or turtles. But Erich Ritter, of the Shark Research Institute, an American charitable foundation, begs to dier. He thinks sharks know exactly what they are doing when they attack people, and he believes he has the data to prove it.
Anecdotal evidence suggests sharks generally take swimmers from behind. This would make sense from the shark’s point of view, since its approach would not be detected. But it does depend on its knowing what behind means when applied to such an oddly shaped creature as a human. And if that is the case it implies there is no mistake in the animal’s mind about what its target is.
To test this idea Dr Ritter did an experiment, the results of which have just been published in Animal Cognition. He asked some scuba divers to kneel, for a total of an hour a day each, on the seabed at a site in the Bahamas frequented by reef sharks. Since the divers were stationary, their direction of travel could not give away which
part of them was the front, and since they were kneeling their body shapes were about as un-seal-like or un-turtle-like as it is possible for a person to be.
Some divers knelt alone. Others, acting as controls, knelt back to back, in pairs. A camera at the surface, 12 metres above them, then recorded what happened.
Altogether, when they looked at the footage, Dr Ritter and his statistician colleague Raid Amin, of the University of West Florida, were able to analyse 312 encounters between sharks and divers. When a single diver was present (211 of the encounters), any approaching shark passed behind him four-fths of the time, and in front only one-fth. When there were two divers (the remaining 101), the sharks had no preference about what they did. They did not, of course, have the choice of going behind both divers’ backs. But there was nothing, either in the area or in the divers’ subliminal behaviour, that caused them to go one way round the pair rather than the other.
Reef sharks rarely attack divers, and this experiment is not proof-positive that those species which do would behave in the same way. But it does show that some sharks, at least, know perfectly well which part of a human being is the front and which, if they wish to remain undetected, is the back.
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