Take a Levy walk on the wild side

I've mentioned before that this summer I’ll be part of some whale shark field work studies in Mexico. Some of it will focus on how these amazing animals find patches of their planktonic food in the ocean. There’s a pretty good likelihood that they have an incredibly sensitive sense of smell and can detect food from miles away. They’re a bit different than toothy sharks though, because they aren’t detecting “blood in the water” as such; rather, they need to be able to distinguish patches of ocean where plankton is denser from places where its less dense. How do they do that, and what chemicals are they smelling exactly? These are among the questions we will be trying to answer.

In reading up for this work, I came across the idea of Levy Walks. This is not a walk in the sense of your evening constitutional down to the Piggly Wiggly for a 6-pack and some Slim Jims. No, it really is just the name for a certain pattern of animal movement (shown at the right), one in which animals make several short “legs” of directed motion, usually in bunches, separated by longer legs with major reorientations. Its not random motion, but neither is it all that predictable, except that the pattern exists at all scales: its fractal. In other words, if we sketched the motion of an animal on paper, and drew it to scale, it would look similar if we zoomed out to the range of kilometers instead of meters and drew the pattern again. It turns out that moving by way of Levy walks increases your chances of running into patches of food, or the trails of scent they leave behind. At that point, more directed motion takes over and the animal zig zags towards the source of that delicious scent (whereupon it becomes not too different from homing in on the Slim Jims at the Piggly Wiggly after all). Sims et al. show that Levy walks are almost ubiquitous among animals that seek mobile prey; they conclude that its a sort of biological rule for finding food that has a patchy distribution.

It’s a fascinating idea; I wonder if you could apply a deliberate Levy walk pattern if you were looking for your sunglasses, trying to find Waldo, or trying to find an empty patch of beach to put your towel on. People might look at you a bit funny, but who’d have the last laugh?

Sims, D., Southall, E., Humphries, N., Hays, G., Bradshaw, C., Pitchford, J., James, A., Ahmed, M., Brierley, A., Hindell, M., Morritt, D., Musyl, M., Righton, D., Shepard, E., Wearmouth, V., Wilson, R., Witt, M., & Metcalfe, J. (2008). Scaling laws of marine predator search behaviour Nature, 451 (7182), 1098-1102 DOI: 10.1038/nature06518

Comments

[Robert Mutch]

Al, this is really interesting. Thanks for sharing. I wonder how applicable "optimum foraging models" are, from behavior ecology (and, human behavioral ecology) , although a very different env. Glad I found your blog (thru researchblogging.org).

[Daniel Bassett]

I know from my PhD work that fish will show increase their turning rates once they hit a patch of food. But usually they don't start showing the major reorientations until they know they have hit a patch. Could it be possible that this could be an explanation for this movement pattern also?