Some sharks can likely tell which way humans are facing and tend to stay out of their field of vision, typically passing by or approaching people from the rear, new research suggests. 
 
 Although divers and shark scientists have noticed this tendency before, it hadn't been carefully documented, said Erich Ritter, a scientist at the Shark Research Institute in Florida. In a new study, published in December in the journal Animal Cognition, research volunteers kneeled on the seafloor for hours staring straight ahead, while interactions with Caribbean reef sharks were videotaped from above. About 80 percent of the time, reef sharks that came close to the subjects passed behind them. 
 
 Volunteers assumed this unusual posture and were told to remain still so as to not give sharks clues about their body orientation, Vitter said. But the sharks appear to have figured out which way the person was facing anyway — and favored passing behind the people, he added. [8 Weird Facts About Sharks] 
 
 "They truly do swim up from behind, be it that they want to sneak up or they don't want to be seen," Ritter told LiveScience. "It doesn't mean they sneak up in a way of having a vicious thought; mainly, they are curious but at the same time cautious." 
 
 It's unclear how the sharks are able to tell which way humans are facing, Ritter said. Perhaps they are able to see face masks from afar and know from previous experience with humans that this is where their eyes are located, he said. But since humans are an unfamiliar species to sharks, and don't resemble any shark prey species, this comes as a surprise, Ritter said. 
 
 Ralph Collier, a researcher with the Shark Research Committee in Los Angeles, said the study finding helps explain stories from commercial and sport divers, who have told him tales of turning around to find great white sharks staring them down. It also helps explain why most shark attacks take place from the rear, he added. Shark attacks remain extremely rare, only killing about 10 people globally per year. 
 
 "I think it's another piece of information that can be added to what we know about typical shark behavior," Collier told LiveScience. 
 
 But another researcher wasn't impressed by the paper. "I am not persuaded that the author's thesis was proven in this study," said George Burgess, director of the Florida Program for Shark Research at the Florida Museum of Natural History. "We don't know the past history of these sharks, and whether this interaction with humans represents a new experience," he said. In other words, the sharks may have seen people before, and it wouldn't be that surprising if they could recognize the masks of people and interpret them as eyes, since prey fish also have eyes, he said. 
 
 Email Douglas Main or follow him on Twitter or Google+. Follow us @livescience, Facebook or Google+. Article originally on LiveScience. 

 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 fish 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 differ. 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-fifths of the time, and in front only one-fifth. 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.
– Given a choice, sharks like to swim up on humans from behind, a new study suggests. But that's not necessarily because they're looking for lunch—they may just want to avoid trouble, say scientists at the Shark Research Institute in Florida. Their study in Animal Cognition backs up anecdotal evidence from divers and surfers that on the rare occasions when sharks attack, they tend to do so from the rear, reports the Economist. "They truly do swim up from behind, be it that they want to sneak up or they don't want to be seen," the lead researcher tells LiveScience. "It doesn't mean they sneak up in a way of having a vicious thought; mainly, they are curious but at the same time cautious." The finding was surprising if only because researchers weren't sure that sharks would be able to differentiate, well, your front from your back. But, in a not-so-appealing job description, "research volunteers kneeled on the seafloor for hours staring straight ahead, while interactions with Caribbean reef sharks were videotaped from above," says LiveScience. Sharks approached single kneelers from the rear four times out of five. When two volunteers kneeled back to back, the sharks showed no preference.