Some shark species make "mental maps" of their home ranges, allowing them to pin-point destinations up to 50km (30 miles) away, research suggests.

研究显示，某些种类的鲨鱼对其活动范围制作出“心理地图”，这使它们可以精确达到50公里以外的目的地。

navigate¹ on scales of many kilometres - perhaps aided by internal maps" src="http://www.enread.com/upimg/allimg/110302/1_110302052320_1.gif" width="304" height="304" />
Tiger sharks can navigate on scales of many kilometres - perhaps aided by internal maps
US-based scientists analysed data from tiger sharks tagged with acoustic²（声学的） 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 Earth's 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 statistical³ 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（长尾鲨） , also showed "directed walking" like the tigers, but on much smaller scales.

"Our research shows that, at times, tiger sharks and thresher sharks don't swim randomly⁴ but swim to specific locations," said research leader Yannis Papastamatiou from the Florida Museum of Natural History in Gainesville.

"Simply put（简言之） , they know where they are going."

Maps and magnets

A key question is how they know where they are going.

Sharks are among the wide array of animals that can sense magnetic fields.

But whereas others, such as yellowfin tuna（黄鳍金枪鱼） , apparently⁵ do this using small amounts of the mineral magnetite in their heads, sharks do not appear to maintain deposits of this magnetic sensor⁶.

Alternative possibilities are that they use signals from ocean currents, water temperature or smell.

"They have to have a pretty good navigation system because the distances are great," Dr Papastamatiou told BBC News.

"Which one it is is open to debate, but the fact that many of these journeys took place at night - you and I would think there's nothing to orientate⁷（定位） to, so orientating⁸ to magnetic fields is one possibility."

Among thresher sharks, adults made much longer directed journeys than juveniles⁹.

The researchers say this suggests the fish build up mental maps as they mature.

The differences between species are probably explained by the varying ways in which they live.

Blacktip reef sharks (Carcharhinus melanopterus), although widespread around the Pacific, appear to have small ranges within their home reef system.

On the other hand, tiger sharks (Galeocerdo cuvier) can cover huge distances. Tags have been recovered from individuals more than 3,000km away from where they were attached.