New research has found that just one local group of about 20 dolphins works with the fishermen, while the other local dolphins don't cooperate, finding other sources of food. The researchers aren't sure what separates these groups.

Scientists had known that dolphins work together to herd groups of mullet, a fish that's an important source of food for local fishermen, toward a line of fishermen in boats or knee-deep water. Then the dolphins signal with specialized head or tail slaps when and where the fishermen should throw their nets. The cooperation is helpful to both parties, researchers said. The two wouldn't survive without each other.

"About 200 local artisanal fishermen are almost entirely reliant on the dolphins for catching their fish," study researcher Fábio Daura-Jorge, of the Federal University of Santa Catarina, in Brazil, wrote in a statement sent to LiveScience. "The fishermen do not fish without the assistance of the dolphins and know the individual animals from their natural marks and can recognize them by eye."