16 June 2010

SWARMING, SCHOOLING, FLOCKING, CROWDING



Swarm Mentality is an excellent short film from Scienceline parsing the collective decisions made as the sum of individual choices, which manifest in behaviors we call swarming, schooling, and flocking.

I wrote about schooling and flocking in some detail in my book The Fragile Edge. Here's an except:  
Meanwhile, out on the lagoon, a juvenile bigeye trevally caught by a black noddy falls back to the surface to be reingested by the ocean. Sometimes, with a big enough flock of noddies and an armada of aggressive frigatebirds, the little fish fall like rain, their reentry points marking the surface with the bulls-eye targets of ripples. If you happen to be snorkeling or diving in the vicinity of a feeding frenzy while this is going on, you can see the little fish streaking like lightening bolts for the company of the school. With a homing sense as true as any bird’s, they dash towards their beleaguered fellows, gathering until they form a subschool of survivors. If the subschool grows large enough, it may temporarily abandon the mother school, particularly if the battle is not going well for the latter. 
Watching, you think it must be hopeless: that every last living creature in the sea or in the air above desires these little fish as bright as newly-minted silver coins. Tossed cruelly into the hordes of the greedy, hounded day and night, they live lives of unending terror under conditions of perpetual warfare. Yet they survive, many of them, somehow.

Photo of a school of goldband fusiliers, Pterocaesio chrysozona, by Mila Zinkova, courtesy of Wikimedia Commons.

Back to the excerpt:
A school of predatory jacks, which the Tahitians calls paaihere, are dogging their heels from below. The noddies are skipping across their spines from above. The frigatebirds lurk in the shadows of clouds. Bunching together, the juvenile trevallies spill and pool and reunite like loose mercury. When the paaihere launch an attack, the little fish perform a defensive maneuver known as the fountain effect: the school instantaneously splitting into two, each subschool reversing direction and circling behind their attackers on opposite sides. This behavior awards the trevallies a running headstart from their enemies.
Capitalizing on the momentary advantage, the little fish dive, trying to escape the reach of the birds above. But the paaihere are relentless hunters with big appetites and phenomenal accelerating abilities. They wheel around and come at the trevallies from below, herding them towards the sunlight, where the reflections of their newly-minted bodies sparkle in the irises of the dancing noddies.
Trapped, the little fish unloose another defensive stratagem known as the flash expansion. Without warning or any apparent means of coordination, the school explodes, and members scatter shrapnellike out from an imaginary center. The whole reaction takes place in as little as twenty milliseconds, as each fish accelerates to speeds of twenty bodylengths per second. Amazingly, the trajectory of each trevally carries it away from the center and away from its neighbors, and is accomplished without any collisions—which would surely prove fatal under the circumstances. More astounding is the fact that the entirety of the flash expansion occurs at speeds faster than the rate of nerve impulses travelling from the little fishes’ eyes to their brains and back to their muscles. In other words, this defensive play has nothing to do with sight, and the little fish are as good as blind throughout it.


Here's a video showing flash expansion in whirligig beetles.

Again, more of the excerpt:
Curiously, computerized fish programmed to behave like a school of a fish cannot perform as well as the real thing unless they are subject to a field, which is itself influenced by all the individuals in the school, and which in turn links them together. For living fish, sight alone is not enough to maintain tight schooling. Neither are the lateral line systems that enable them to sense minute pressure changes in the water. Real fish that have been temporarily blinded with opaque contact lenses and others have had the nerves to their lateral lines cut still manage to school effortlessly.
—from The Fragile Edge: Diving & Other Adventures in the 
South Pacific, Chapter 12, The Infinity Pool
 

Photo of schooling bannerfish by Jon Hanson, courtesy of Wikimedia Commons.
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