Samplings: June 2008

Print

Email

©iStockphoto.com/Ben Slater

Predators hunting randomly spaced prey should not themselves move randomly. That’s the advice of many biologists, anyhow, to foragers needing an efficient search strategy. Instead, they recommend the “Lévy walk,” which involves alternating clusters of short moves with much larger jumps—a pattern formulated by the late French mathematician Paul Pierre Lévy. Alas, evidence that predators actually walk the walk has so far been equivocal. Now the theory has been bolstered by a new study from David W. Sims of the Marine Biological Association in Plymouth, England, and colleagues. Reasoning that Lévy walks, often conceived as a series of horizontal moves, could also apply to aquatic predators hunting for prey at various depths, Sims’s team attached depth recorders to the bodies of thirty-one marine foragers from seven species. The data revealed that basking sharks, bigeye tuna, Atlantic cod, leatherback turtles, and Magellanic penguins swim up and down in the water column according to a Lévy pattern. Two other species—small-spotted catsharks and southern elephant seals—do not. An analysis of how swarms of krill and zooplankton change depth through time, combined with computer simulations of the movements and distributions of predators and prey, also confirmed that “Lévy dives” beat purely random searches. Although five adult basking sharks followed a Lévy pattern, one juvenile that the team tracked for seven months did not, so Sims thinks Lévy searches may be learned. (Nature)