Schools of these fish can zip all over the place, making them challenging to study. To overcome that obstacle, the scientists had small groups of tetras swim in a ring-shaped tank so that the fish's directional shifts would be obvious. The tetras could only go in two directions: clockwise or counterclockwise.
The researchers paid special attention to what happened during U-turns — whenever the fish would suddenly switch their direction of movement. They noted how each fish reacted to the movement of its neighbors and measured the brief time delay that occurs when a fish detects what its neighbors are doing and its application of that information to its own swimming behavior.
"Those delays define whether and when a fish has responded and thus has been influenced by another individual," co-author Luca Giuggioli of the University of Bristol explained to Seeker.
The scientists further developed a computational model to rule out any individual fish movements that might have just occurred by chance.
The analysis revealed that tetras follow a limited number of influential neighbor fish that are not necessarily the ones closest to them.
Giuggioli indicated that humans often behave in a similar way. When people are skiing downhill, for example, one skier may be faster, yet farther away from others. In this instance, he said some of the skiers who may unwittingly collide with the faster one could benefit from paying attention to the speedy individual.
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Senior author Guy Theraulaz of Université Paul Sabatier in Toulouse, France, added that no single fish consistently makes decisions that the group follows. As a result, schooling fish have no long-term, defined leader.
"When a fish is incidentally a leader," Theraulaz said, "the average number of its neighbors decreases significantly and, after a certain period of time, this lack of perception of other fish triggers a U-turn, suggesting that each fish wants to stay in contact with other fish."
The same phenomenon happens in flocks of starlings: Birds that initiate changes in collective traveling direction are found at the edges of the flock. It’s true then that it can be lonely at the top — and edges — given that those in the middle have plenty of others to follow and rely upon.
The researchers believe that fish and birds are mostly responding in a conscious way to the movement of others. Their quick adjustments, in other words, are not just reflex actions. Should an even faster response be needed, however, a reflex response could occur.
This is similar to a person driving, paying basic attention to the road and other drivers, and then suddenly having a rock hit the windshield. In that moment, a reflex action could cause the driver's head to move and eyes to close.
Although approximately 50 percent of all fish species exhibit some form of schooling, it is rare to find groups that can coordinate their movements as smoothly as tetras do. Even "properly swarming insects are rare, including bees and locusts," Perna said, "and their swarming rarely depends on rapid alignment among close neighbors."
This ability to align is part of "swarm intelligence," which researchers are working to apply to robotics.