One of the most widely known but not fully understood processes in the animal kingdom is when fish swim together in a pack, or as it's more commonly referred to, a "school." A pair of researchers at New York's Polytechnic Institute decided that building a robotic fish and manipulating the way it moves would allow them to create controlled experiments to figure out why seeing a school of fish is so common.
Previously, the assumptions were what you might expect - by traveling in large numbers, fish can more easily ward off predators and keep one another moving in the right direction. But there are inherent problems as well, including the fact that if you're part of a large group, it's harder for each member of the group to get enough food and sustenance. Also, if anyone in the group gets a disease, everyone else is at risk. So what did the engineers discover?
They noticed, first off, that the fish actually accepted the robot as one of their own. There were a few exceptions; some fish kept their distance from the foreign object, but for the most part it fit in. In addition, they noticed that if they made the robot fish swim, and other fish joined behind it, the other fish slightly lowered the force with which they swam. In other words, swimming in a group eases the pressure on everyone and gives the entire group an energy advantage. It's the same priciple as birds flying in a V formation. The results of this research were published in the Journal of the Royal Society Interface.