March 1, 2011 | 1
Wandering the neighborhood randomly is not usually the best strategy to find a great dinner—especially if you live in a place where such meals are few and far between.
The resulting trajectory, known in mathematics as "a random walk," does not always make for the best use of time and energy, particularly in locations where resources can be scarce, such as the open ocean. But a more purposeful "directed walk" to a destination takes a pretty sophisticated memory and spatial sense (or a device with GPS) that many animals don’t have.
New research, however, shows that thresher and tiger sharks are actually quite adept at highly directed swimming, with tiger sharks finning it over to a familiar spot from six to eight kilometers away within a home territory that covers hundreds or even thousands of square kilometers.
Demonstrating that an animal is traveling directly to a desired destination—rather than stumbling on it accidentally—is challenging, given communication barriers and the fact that even straight paths are not always part of a purposeful travel pattern. To find out whether sharks were always circling their home range randomly or were intentionally returning to a place they remembered to offer food, shelter or mates, a team of researchers used fractal analysis to assess old GPS tracking data from three shark species: tiger sharks (Galeocerdo cuvier), thresher sharks (Alopias vulpinus) and blacktip reef sharks (Carcharhinus melanopterus). Tracking for each individual shark lasted for at least seven hours.
"Our research shows that, at times, tiger sharks and thresher sharks don’t swim randomly but swim to specific locations," Yannis Papastamatiou, of the Florida Museum of Natural History and co-author of the new study, said in a prepared statement. "Simply put, they know where they’re going."
How do the sharks do it? Papastamatiou and his team suggest that the sharks with larger ranges making shorter trips could be darting to their destinations working off of clues from water currents, temperature, magnetic fields and perhaps even smell. But when sharks are making long and apparently purposeful treks, they might be making use of "cognitive maps, requiring good spatial memory and awareness of features in their home range," the researchers noted in their paper.
The researchers also found that thresher sharks in particular weren’t necessarily born knowing how to find the hotspots. Adult threshers seemed to use "directed walk" much more often than the younger ones. "We believe it shows that adults have learned to navigate more effectively than juveniles, or have developed better mental maps of the area where they live," Papastamatiou said.
The results were gleaned from 34 study sharks, and some might have been exploring new territory during the tracking, which would increase the likelihood that they would use a "random walk" approach. But other anecdotal evidence supports the idea that some sharks make the most of their large territories by developing cognitive maps of resource-rich areas.
The ability to re-find food sources is not uncommon among larger land animals. But pinpointing marine predators that can do it is a newer development. "Many people could walk to a known destination 6 to 8 kilometers away, but imagine doing it in deep water and at night," Papastamatiou said.
And like people, the sharks seemed to vary widely in their individual ability to arrive directly at their destination.
The findings were published online March 1 in the Journal of Animal Ecology.
Image of blacktip reef shark, one of the three species in the study, courtesy of Yannis Papastamatiou