Why stay in school? For fish, the reasons haven't been clear. On the plus side: protection from predators, conserving energy and finding mates. On the down side, though, is the competition for food.
To tease out an answer, Maurizio Porfiri, an engineer at the Polytechnic Institute of New York University, and Stefano Marras, of the Institute for Coastal Marine Environment in Italy, designed and built a robotic fish that generates a wake behind it that's similar to a real fish.
They put the robotic fish in a tank with real fish to see what would happen. The real fish followed the robo-fish around. Later, they attached the robo-fish to the bottom of a channel full of moving water (to simulate a current) and they found the living fish swimming along by the robotic fish, just as though it were leading a school.
One finding was that when swimming near the robot, real fish will fall just behind it, as they get a small boost from the robot's wake. Swimming in formation saves energy, just as it does with other animals, notably migrating geese. This shows that energy conservation is a major factor in forming schools, and possibly offsets the increased food cost.
This opens up a lot of possibilities for leading fish around where people want them to go. One application is in getting fish to swim away from oil spills, or the intakes of a power plant.
That said, there are a few unanswered questions. The robot fish may swim like a real one, but it's a lot bigger than the fish that followed it, and isn't even the same color. The fish followed anyway.
Credit: Polytechnic Institute of New York University