Credit: Gabriel Miller
Bees do it. Birds do it. So do fish and wildebeests. They are all able to gracefully flock or swarm in a particular direction even though not every member of the group knows where they are going.
Even human beings will tend to follow each other with a herd mentality -- say, out of a crowded theater. New research provides some surprising insight into what's going on, including a group penchant for democratic decisions.
"Groups of animals move purposefully, yet often only relatively few individuals have pertinent information as to where to travel," said Iain Couzin of the University of Oxford.
Biologists have often wondered if there is some complex communication that goes on between the informed and the uninformed. But Couzin and his collaborators have shown in simulations that a simple set of behavioral rules can control a group.
"There's no explicit signaling in our model," Couzin told LiveScience. "No one is saying, 'I know something - come follow me.'"
The only requirement seems to be a balance between a need to stay in the group and a desire on the part of some to go off in their own preferred direction. These goal-oriented individuals look just like their naïve colleagues.
"No inherent differences, genetic or otherwise, such as dominance or body size, need to be invoked to explain leadership," Couzin said.
The fact that the followers in the simulation have no way to recognize who is leading them may explain how animals efficiently move in crowded environments, where they can only see their nearest neighbors.
"[This study] demonstrates the power of the little guy," said Daniel Rubenstein of Princeton University, who did not participate in the study. "You don't need avowed leaders, you don't need complex signaling."
The results, published in the Feb. 3 issue of the journal Nature, might be useful in developing swarms of robots for exploring the oceans or other planets.
Follow the virtual leader
In computer simulations, Couzin and his colleagues programmed virtual animals with the instinct to stay near others - an important survival trait in many species. The researchers then endowed some members in the flock with a preferred direction - be it toward a food source or a new nesting site.
They then determined how close the group would come to arriving at this goal.
Accuracy increased as more of the members knew where to go. But at a certain point, adding more informed individuals did not increase the accuracy by very much. To give an example, a group of 10 gets about the same advantage from having five leaders as having six.
The minimum percentage of informed individuals needed to achieve a certain level of accuracy depended on the size of the group. If 10 virtual buffaloes need 50 percent of the herd to know where the watering hole is, a group of 200 can get by with only 5 percent.
In nature, it is likely that the number of leaders is kept as low as possible. Couzin gave the example of bees, for which scouting out a new nest site is dangerous, as well as time-consuming. Studies have shown that only five percent of a hive's population gets involved with scouting.
As is the case in human interactions, there will sometimes be a disagreement between those who are in the know.
For instance, there may be five individuals who know of a food supply to the east, but four others who have spotted food to the north. The researchers found that the entire group will tend to settle on the direction with the greater number of informed individuals.
"In the real world, you do have individuals with different information, needs and preferences," Couzin explained. "What we show is that - using very simple rules - the group will choose the majority. It's almost like a democratic decision."
To test whether these simple rules actually apply in real animals, Couzin's team has begun experiments in which certain fish are trained to associate one direction with a reward. These informed individuals will then be mixed with untrained fish to see if the group can be led.
Informed humans and robots
The scientists also plan to look at human crowds. Couzin thinks there may be a similar sort of mechanism to explain, say, how we walk along a busy street.
"We do it more or less on autopilot," he said.
Perhaps we are subconsciously reconciling two simple commands: get to work on time and avoid stepping on anyone's shoes.
"The mechanism of coordination we propose is very simple and requires only limited cognitive ability," Couzin said. "This simplicity, generality and the effectiveness of the mechanism lend support to its being selected for in populations."
The simple network of commands may also be an efficient way to program teams of robots. Couzin has previously worked with researchers at Princeton University, who are designing underwater robots that can act autonomously.
Robots that learn the location of a certain target could lead other robots to it without any human supervision.