How a Shark Finds Its Next Meal

When hunger pains strike a shark, does it simply swim around aimlessly until it encounters a meal? Scientists who have studied the diving behavior of sharks, turtles, fish, and other marine predators now suggest that these animals are a bit more strategic.

They appear to follow a special type of random motion, known as a Levy walk, that is a superior strategy for vast oceans in which food is patchily distributed, the researchers report online today in Nature.

Ecologists have long studied how animals find food, with some exploring whether simple rules could explain foraging behavior. In 1996, one team claimed that wandering albatrosses perform Levy walks, which are characterized by a particular distribution of movement lengths that include many short-distance hops and rare long jumps.

Others then found that many animals, as well as hunter-gatherer tribes and fishing ships, used this search strategy, too. But last fall, scientists–including authors of the original 1996 report–revealed that the original albatross data were flawed.

They also cast doubt on most claims of Levy walks by animals, criticizing the small data sets analyzed and a key statistical method many groups used (Science, 2 November 2007, p. 742).

The tables have turned again, however. Behavioral ecologist David Sims of the Marine Biological Association laboratory in Plymouth, U.K., and his colleagues have found evidence of Levy walks in data from depth-sensing recorders they had attached to 31 large marine predators from seven species, including sharks, bony fish, sea turtles, and penguins.

As part of the European Census of Marine Life, they analyzed the vertical motion in more than a million dives as the animals apparently hunted for food.

Using various statistical methods, they showed that during foraging, five of the seven species examined seemed to perform vertical Levy walks, with many short dives with an occasional much longer dive, some hundreds of meters deep.

The team also conducted computer simulations showing that if prey is dispersed in the ocean in clumps separated by large intervals, predators should find it more efficiently using a Levy walk search strategy than simple random motion.

Sims contends that his team’s vast data set and the use of diverse statistical analyses make their study “the most credible evidence for Levy-like movement in foraging animals.” Levy walks “are most likely an evolved pattern of behavior in response to complex prey distributions,” he says.

The study is compelling, says ecologist Frederic Bartumeus of Princeton University. He says that he’s impressed with the amount of data Sims’s team analyzed and predicts that the study will shift the debate from whether animals perform Levy walks to when they use this strategy and why: “It’s going to make biologists less suspicious about the existence of these patterns.”