Dolphins and fishermen in Laguna, Brazil, fancy the same food: plump, silver fish called mullets. But rather than compete, the two have formed an alliance.

When the marine mammals slap their heads or tails against the water, it cues the fishermen to cast their nets, which in turn breaks up the schools and makes individual fish easier for the dolphins to catch.

Scientists have known about this mutually beneficial relationship since the 1980s. They’ve also observed in this population of about 60 dolphins—only some of which cooperate with fishermen—that the helpers tend to hang out around other helpers.

But Mauricio Cantor, a biologist at the Federal University of Santa Catarina in Brazil, and his colleagues wondered why. (Watch these