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Bottlenose dolphins that work together with humans to catch fish have their own distinctive whistle, one that may help them recognise each other.

Off Laguna, Brazil, fishers stand in a line in waist-deep water or wait in canoes while, farther out, chase shoals of mullet to the shore. The fishers can’t see the fish in the murky water, so they wait for the dolphins to give a signal — like an abrupt dive or tail slap — then cast their nets.

Fishers catch larger and more fish when they work with dolphins. “Dolphins likely reap similar benefits,” says of the Federal University of Santa Catarina in Brazil – it might be easy for them to gobble up fish disoriented by the nets.

But only some dolphins, working alone or in small groups, cooperate with humans. To explore the differences between helpful and unhelpful dolphins, Cantor and his colleagues recorded the sounds made by both types while they foraged either on their own or with people.

Whistle while you fish

Surprisingly, the whistles of cooperative dolphins were different from those of non-cooperative ones, even when foraging alone. For instance, they used fewer ascending whistles.

Dolphins from different regions often whistle differently, Cantor says, but “it is much less common to find such acoustic differences among dolphins of the same population that inhabit such a small area”.

Since cooperative dolphins also whistle differently when fishing solo, the researchers don’t think these calls carry specific messages about fishing with people. Instead, the whistles may be a way for dolphins to label themselves as members of a particular social group, Cantor says.

Alternatively, the dolphins may be using dialect or slang. Cantor says it is “as if they speak the same language but use some ‘expressions’ that are exclusive to their social community”.

Dolphins also use clicks to communicate during feeding, says of the Italian National Research Council. Researchers will need to examine clicks as well as whistles to better understand the dolphins’ communication styles.