SOME shark species make mental maps of their home ranges, allowing them to pin-point destinations up to 50km (30 miles) away, research suggests. US-based scientists analysed data from tiger sharks tagged with acoustic transmitters, and found that they took directed paths from place to place. Other species such as blacktip reef sharks did not show this behaviour. Writing in the Journal of Animal Ecology, researchers suggest this shows a capacity to store maps of key sites. In addition, it is further evidence that the great fish can navigate, possibly using the Earths magnetic field. Earlier research in Hawaii had shown tiger sharks swimming across deep channels and finding shallow banks rich in food 50km away. In this project, researchers used statistical techniques to show the journeys were not made by accident; the sharks were following some kind of path. Blacktips, however, did not. A third species, thresher sharks, also showed directed walking like the tigers, but on much smaller scales. Our research shows that, at times, tiger sharks and thresher sharks dont swim randomly but swim to specific locations, said research leader Yannis Papastamatiou from the Florida Museum of Natural History in Gainesville. Simply put, they know where they are going. A key question is how they know where they are going. BBC