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Unlocking How Flocks Stop, Turn, and Swirl in Unison

James Owen
National Geographic News
March 4, 2005
 
It's one of the wonders of the natural world—to see a flock of starlings pulse, wheel, and ripple as one across an evening sky. Just how do they perform these displays with such precision?

The question thas puzzled scientists for centuries, since many group-living animals have this talent for moving together in a seemingly spontaneous yet highly coordinated way.

Anchovies, for instance, are as synchronized as starlings in their underwater ballets, especially when animated by the presence of predators.

Scientists had thought such graceful mass movements could only be achieved through complex signals.

In the 1930s some even suggested these animals must be able to instantly transfer thoughts to one another.

Or, in the case of birds, perhaps they follow a leader via electromagnetic signals.

Now a new study suggests there's a much simpler and more democratic navigation system that allows flocks, shoals, and herds to travel in unison.

For the study, published in the science journal Nature, researchers used computer models to show that large numbers of animals can move together relatively easily, even when few individuals know what's going on and there is no obvious leader.

"It demonstrates the power of the little guy," said study co-author Daniel Rubenstein, professor of ecology and evolutionary biology at Princeton University in New Jersey. "You don't need avowed leaders, you don't need complex signaling."

The researchers add that their finding may also be useful in understanding human crowd behavior, and in designing robots to explore the deep ocean and distant planets.

Bees, Ants

Some group-living animals use obvious signals to guide others. For example, after a sortie, foraging honeybees perform an elaborate "waggle dance" back at the hive that tells other bees where to find a good source of nectar or pollen. Ants pass on directions mainly through chemical communication using pheromones.

However, birds, fish, and mammals that travel together in large numbers tend to be crowded together, which means individuals can only see the animals closest to them. This means they must make collective decisions on which direction to take, otherwise they risk never reaching the desired destination.

Disunity can also prove fatal—wildebeests or caribou that stray away from the main herd are far more likely to fall victim to lions or wolves.

Using computer simulations, the study team found that group coordination arises from two factors: the need for a group to stick together and the desire of some individuals to make their own minds up about where to go.

The researchers first programmed their virtual animals with a basic urge to stick as close together as possible without actually colliding. This instinct caused animals to form close-knit, evenly spaced groups, as seen in real mammal herds and fish schools.

Some animals were then programmed to have a preferred direction in mind, as if on a migration route or heading toward a particular feeding area. These animals' desire to reach their goal was then balanced with an urge to stay with the main group.

During these simulations, the researchers found that it needed only a few individuals to set off together in a certain direction for hundreds of others to follow.

The study suggested that just a handful of animals were as effective in leading a large group as a small one. And as group size increased, the number of individuals needed to guide it accurately reduced dramatically.

Simple Mechanism

"When you see apparently complex behaviors, the mechanisms that coordinate these behaviors may be surprisingly simple," said lead author Iain Couzin, also of Princeton.

The study also suggested that the will of the majority won out when deciding which way to turn next. When subgroups of animals with different ideas on where they were headed were introduced to the simulations, the overall group went with the majority consensus, even if it outnumbered the minority by just a single vote.

Couzin said, "The direction an informed individual decides to take lies in a balance between two influences: the desire to achieve a goal such as reaching a known food source, and the interaction with the animals around them."

He said such a system can also help us understand human behavior, adding, "This even applies to the pedestrian walking down the street, who automatically balances the desire to follow the quickest route to a chosen destination with local conditions caused by the movement and position of other pedestrians around them."

Couzin said computer models allow us to better understand the processes that determine group movement. The models may also lead to a new breed of robots designed to work together to explore deep-sea regions or even distant planets.

"They could select collectively the direction associated with the best-quality information or select collectively the majority direction," he added. So while the mechanism behind it might be simple, the flocking behavior of those starlings isn't just a wonderful sight. It could help shape the way we explore space.

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