School rules

Fish swimming together appear to focus on nearest neighbor or two

To avoid bumping into each other, fish swimming in a school behave a lot like drivers on the road — ignoring most of the other fish and changing speed based on the movements of their nearest neighbors.

Two recent studies, including one published November 7 in the Proceedings of the National Academy of Sciences, have revealed this unprecedented fish-eye view of the world. But they disagree about how many of its neighbors each animal in the crowd keeps an eye on.

Despite that disagreement, “this work gives us a foundation by which we can understand how collective behaviors that give these animals remarkable capabilities evolved,” says Iain Couzin, a mathematical biologist at Princeton University. His team published its work describing fish behavior online July 27 in the same journal.

Scientists have simulated the patterns created by fish, birds, bacteria and other living organisms that move in groups. Typically, these computer simulations treat the creatures like magnets, their movements akin to forces that pull to keep the group together and push to prevent individuals from running into each other. Few studies, though, have looked at individual animals within the group to tease out the behaviors that give rise to these forces.

“We had a fairly good idea of what such force[s] could be … but it is important to find out what they actually are,” says Leah Edelstein-Keshet, a mathematician at the University of British Columbia in Vancouver, who studies flocks of birds.

To see things from the perspective of a single fish, James Herbert-Read at the University of Sydney and colleagues filmed and tracked groups of mosquitofish swimming around the edge of a tank in the lab. The researchers used neural networks, a technique inspired by the human brain for finding patterns, to analyze how each creature responded to other fish surrounding it.

Each fish seemed to be paying attention only to the single closest fish at any given moment, the researchers report in the Nov. 7 paper. If that neighbor was far in front or right behind, the fish sped up; if the neighbor was far behind or too close in front, the fish slowed down. And contrary to the predictions of some simulations, the fish didn’t turn their bodies to line up with each other; simple, quick changes in speed were enough to keep the group together.

“This is much in the same way as car drivers on an open highway try to keep a fixed distance from each other,” says Herbert-Read.

But Couzin’s team found that pairs of interacting fish might not be enough to explain all the patterns observed in schools. Watching golden shiners swim, the researchers found that fish in groups of three also seemed to coordinate their movements with one another.

Previous studies have suggested that pedestrians choose their paths based on two or more neighbors when moving in a crowd. And observations of starlings suggest that they watch six or seven other birds when flocking.

Keeping track of where lots of your friends are takes time, says Herbert-Read. So a simpler strategy for schooling — coordinating with just one or two neighbors — could be easier on the small brains of fish, allowing them to pull off the death-defying turns that keep them out of the jaws of predators.

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