Like teenage boys hanging out on a street corner or fans cheering at a football game, animals behave differently when they're in a large group than they do when they're by themselves.
The mechanics and patterns of nature's aggregations - schooling fish, flocking birds or swarming insects - provide valuable understanding for how such groups behave in, and survive, trying conditions, says a University of Washington zoologist.
What looks like a complex dance - an entire group suddenly changing directions or exploding and reforming - is actually a series of interactions between members of the group reacting to outside influences, says UW research assistant zoology professor Julia Parrish.
"There's a beautiful, aesthetic, very artistic side of it, but there's also a very mathematical and a very evolutionary aspect of animal aggregation," says Parrish, who writes about the complexity and patterns of animal aggregations in the April 2 issue of the journal Science. The paper, co-authored by Leah Edelstein-Keshet, an associate mathematics professor at the University of British Columbia, is part of a package of Science articles that explore the uses of complexity theory in natural and social science.
Pattern that emerges from aggregation is not limited to living systems, Parrish says. Snowflakes are a classic example. A single flake falls and is beautiful to look at. A stormful of flakes stick together and are carved by the wind into elaborate ridges and cornices. A winter's worth of flakes slip and slide and adjust to gravity, eventually producing an avalanche.
But animal aggregations have an evolutionary side.
How individuals react to outside influences can determine their own survival, as well as the survival of other group members. A herring that turns right when the school turns left faces certain death, Parrish says. But a herring that always cooperates with the group and never competes might die of starvation. Finding the threshold between cooperation and conflict eventually could provide scientists with the proverbial "canary in the coal mine" that allows humans to grasp the effects that their actions today will have on the world a century from now.
For example, a slight increase in water temperature because of global warming or a change in the ocean's chemical balance because of coastal pollution could alter the point at which schooling breaks down. Given the added stress of overfishing - humans consume 40 million to 50 million metric tons of schooling organisms each year - fish might end up in groups too small or too unfamiliar to survive. People wonder how massive flocks of passenger pigeons could ever have become extinct, Parrish says. As flocks got smaller, social interactions between the birds broke down. Hundreds or even thousands of birds were simply too few to form the flock sizes needed for the species to survive, she says.
Documenting how animal groups behave allows computer models to predict what will happen under various conditions in the future. A school of fish, for instance, can sense the approach of a predator and take evasive action. The group might scatter to avoid being consumed, though stragglers or individuals at the outer edges of the group might be devoured. But once the danger has passed, the group reforms. With a computer model, scientists can change the intensity of predation to see at what level the school is slow to reform or doesn't get back together at all.
Likewise, the models can assume conditions that don't yet exist - higher water temperature, for instance, or lower fish populations, possibly because of overfishing. The scientists study the models to see how fish react to those conditions.
"As resources are strained, it creates greater competition within the group. That has implications for all things human," Parrish says.
Humans are among the most social species and display all sorts of crowd behavior, no matter whether the individual knows the person in the next seat.
"With models, we may be able to predict the switch from standing ovation to rampage and adjust the outside influences accordingly," Parrish says. "Fish and humans are not so different. Fish have just had a lot longer to practice."
For more information, contact Parrish at 206-616-2958 or jparrish@u.washington.edu, or Edelstein-Keshet at 604-822-5889 or keshet@math.ubc.ca
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Science