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In Laguna, Brazil, fishermen get help from a wild partner. Local bottlenose dolphins have learned to herd mullet into shallow water and to signal fishermen so they know when to cast their nets. After signaling, the dolphins wait with their mouths agape, ready to catch mullet not snagged in the nets. Only some dolphins participate in this cooperative hunting, and as new research shows, these helpful animals have other behaviors that are distinct from their less-friendly fellows.

A team of researchers from Brazil’s Federal University of Santa Catarina recently discovered that not only do the cooperative dolphins have tighter social bonds and spend more time with each other than with non-cooperative dolphins, they also vocalize differently. Specifically, the dolphins that forage with fishermen generally emit whistles that are shorter, have higher frequencies, and have more inflections than those of non-cooperative dolphins.

There are numerous examples of dolphins throughout the world forming distinct social groups or developing new foraging techniques. And other cetaceans, such as sperm whales, develop region-specific accents. Robin Perrtree, a marine science technician at Savannah State University in Georgia who found that dolphins foraging in the wake of shrimp trawlers form social clusters, says the Laguna dolphins are notable because they’re not just taking discarded fish or stealing from fishermen, they’re working in tandem with humans.

Records in Laguna indicate dolphins and fishermen have been teaming up there since the 1840s, and generations of fishermen have learned the technique from local elders. Luke Rendell, a marine mammalogist who co-authored a book on cetacean culture, says it’s likely the behavior is passed on in a similar way by the dolphins, too, from mother to calf.

Considering how many generations of cooperative and non-cooperative dolphins have spent time apart, it’s not surprising that their whistles became distinct.

Rendell doubts that the dolphins have become dependent on fishermen, because they’re also capable of foraging alone. But it’s unclear why the behavior has yet to spread to the entire dolphin population. It’s equally hazy, Rendell says, what the long-term results of the social separation will mean for the bottlenose population.

“I think it’s a really cool sort of phenomenon,” Rendell says. “This is a really nice step forward to looking at how these kinds of processes can influence population structure and may cause populations to divide.”