A tiger shark.
Credit: Klaus Jost
Sharks aren't relegated to swimming randomly in search of prey to chomp on. New research indicates two species — tiger sharks and thresher sharks — somehow orient themselves and navigate toward a target across long distances.
It's not clear how those sharks orient themselves, but they appear to intentionally travel long distances. Thresher sharks travel from under a half-mile to 1.2 miles (400 to 1,900 meters), while tiger sharks make journeys of at least 3.7 miles (six kilometers) and possibly more than five miles (eight kilometers). [Top 10 Most Incredible Animal Journeys]
The tiger shark's travels, in particular, indicate something sophisticated is going on in the animal's head.
"Over those distances, it implies they have some sort of mental map to be able to find their way around," said lead study author Yannis Papastamatiou, a marine biologist at the Florida Museum of Natural History.
The researchers reanalyzed tracking data that had been collected while following tiger, thresher and blacktip reef sharks for at least seven hours in different locations in the Pacific Ocean. The technique used, in which a transmitter is attached to the shark and then someone in a boat or kayak follows it, is limited by the stamina of the person doing the tracking. But the method produces a great deal more detail about the animal's movements than data generated by satellite tracking, Papastamatiou said.
While both tiger sharks and thresher sharks showed directed movement on long journeys, blacktip reef sharks had small home ranges and did not appear to orient themselves for large-scale movements, according to the study, which was published online March 1 in the Journal of Animal Ecology.
Since they didn't know what the sharks had in mind, researchers determined whether the animals were moving randomly or intentionally by analyzing the routes they took. This included comparing the distance traveled with that predicted by an equation describing a comparable random trip, as well examining each shark's path on different scales, from farther away to closer up.
Tiger sharks' journeys were particularly interesting, because they swam across deep water at night, indicating they were not using their vision for navigation. Smells or sounds possibly provided cues, but given the long distances over which the sharks traveled, those senses were unlikely to be adequate.
It's possible the sharks are sensitive to the Earth's magnetic field and use this sense to orient themselves for long journeys, Papastamatiou said.