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Animals - fish, birds, mammals, often congregate in huge groups that move together. No one really knows why. But it's long been thought that gathering in these flocks, schools and herds help protect animals from being eaten by predators.

NPR's Nell Greenfieldboyce reports scientists recently tested that idea by essentially getting fish to play a video game.

NELL GREENFIELDBOYCE, BYLINE: The fish who unwittingly played this video game were not the ones being chased but rather the big bad predators. They were bluegill sunfish that researchers caught in a local lake and brought back to a tank in the lab.

IAIN COUZIN: And the nice thing about bluegill sunfish is they're generalist predators. They all tend to hunt pretty much anything that's alive in the environment that's smaller than them.

GREENFIELDBOYCE: Iain Couzin studies animal behavior at Princeton University. He says these fish are such eager hunters that it turns out they'll even hunt something that isn't real. His team used a computer to create fake prey, little images that were projected onto a thin transparent film inside the fish tank.

COUZIN: So to us they look like little red dots moving around. To the fish they look very much like little zooplankton, little creatures that live in the water body that move around that we know group together in the natural environment.

GREENFIELDBOYCE: And animal groups are what Couzin wants to understand.

COUZIN: Why do we see the evolution of groups in nature? Why is grouping so ubiquitous?

GREENFIELDBOYCE: He says there are real questions about how animals evolve to form groups and coordinate their behavior.

COUZIN: And this has actually been almost impossible to address using conventional biological experiments.

GREENFIELDBOYCE: Because it would be very hard to take say, a bird flock and get each one to do different things to see how that might effect their chance of being eaten by a predator.

But he and his colleagues could manipulate the behavior of the computer-generated prey in the fish tank. They programmed each little dot to follow a different set of rules. For example, some dots would cozy up to other dots, others kept their distance. Some would start to travel in the same direction as their neighbors; others, not so much.

COUZIN: And then we would ask, which of these strategies is most successful at avoiding being attacked by our real predators?

GREENFIELDBOYCE: They did that by watching to see which dots the live fish went for. The dots that survived tended to be the ones that both stayed close and moved in concert with their neighbors.

COUZIN: And so, forming groups that not only aggregate, but actually move together, allows you to minimize risk of attack from real predators.

GREENFIELDBOYCE: The results were published online by the journal Science.

Couzin says this is some evidence that group behaviors can evolve as a response to predation. But this experiment is still far from what happens in the real world. He's already developing a new fish video game that's even more life-like. This time they'll see what happens when the bluegill sunfish see virtual prey that can move in 3-D and try to escape.

Nell Greenfieldboyce, NPR News.

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