Swarming and thresholds

Iain Couzon a Behavoural Ecologist working on emergent group behaviour was recently profiled by Carl Zimmer in NYTimes From Ants to People, an Instinct to Swarm:

The more Dr. Couzin studies swarm behavior, the more patterns he finds common to many different species. He is reminded of the laws of physics that govern liquids. “You look at liquid metal and at water, and you can see they’re both liquids,” he said. “They have fundamental characteristics in common. That’s what I was finding with the animal groups — there were fundamental states they could exist in.”

Just as liquid water can suddenly begin to boil, animal swarms can also change abruptly thanks to some simple rules.

Dr. Couzin has discovered some of those rules in the ways that locusts begin to form their devastating swarms. The insects typically crawl around on their own, but sometimes young locusts come together in huge bands that march across the land, devouring everything in their path. After developing wings, they rise into the air as giant clouds made of millions of insects.

…The scientists found that when the density of locusts rose beyond a threshold, the insects suddenly began to move together. Each locust always tried to align its own movements with any neighbor. When the locusts were widely spaced, however, this rule did not have much effect on them. Only when they had enough neighbors did they spontaneously form huge bands.

“We showed that you don’t need to know lots of information about individuals to predict how the group will behave,” Dr. Couzin said of the locust findings, which were published June 2006 in Science.

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