Dolphins in Brazil act like sheepdogs, helping fishermen catch mullets

ROLAND WEIHRAUCH/AFP/GETTY IMAGES - Mother and child bottlenose dolphins in captivity: Scientists think some dolphins in Brazil may learn hunting habits from their parents.

The clever dolphins of Laguna, Brazil, are known for regularly aiding local fishermen in trapping schools of tasty fish, a behavior not seen anywhere else in the world. A new study explores the interactions among Laguna’s dolphins, providing clues to how they have maintained their long-lasting collaboration with people.

Every autumn, Laguna’s resident bottlenose dolphins (Tursiops truncatus) frequently act like sheepdogs, herding schools of small, silver fish called mullets toward the shore — and toward lines of wading fishmen. When the dolphins get close to their human collaborators, they give a signal, slapping their heads or tails against the surface of the water. The fishermen then quickly cast their nets, usually catching dozens of frenzied mullet.

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While it’s not clear whether the dolphins get food out of the exercise or benefit in other ways, people and marine mammals have fished side by side like this for generations, says the new study’s co-author, Simon Ingram, a marine mammal biologist at Ply-mouth University in the United Kingdom. Still, something intrigued him and his colleagues: Only about a third of the lagoon’s more than 50 dolphins regularly take part in the tradition. The others stick to the sidelines, avoiding people. That raised the question, he says: “Why don’t they all do it, given the opportunity?”

Ingram and his colleagues set out in boats, taking photos of Laguna’s dolphins as the creatures swam together. The goal was to explore how the animals interacted — whether some dolphins spent more time with only a few friends, isolating outsiders. Individuals that helped local fishermen tended to also cluster more with each other, while dolphins that didn’t pitch in kept more to themselves, the team reported online in the journal Biology Letters.

For Fabian Daura-Jorge, a marine biologist at the Federal University of Santa Catarina in Brazil who co-authored the study, this suggests that learning might be involved. Dolphins with tighter bonds, he explains, might be more inclined to pick up one another’s unique behaviors. He has seen mothers in the waters off Laguna gently nudging their offspring toward mullet as if giving them a lesson in herding fish. If that is the mothers’ intent, then this strange behavior may have lasted for so long largely because juveniles are keen to copy what their elders do. But, he adds, the team can’t disprove that the tradition stems from genetics: Some dolphins might be biologically inclined to hunt in this manner.

“It’s an interesting finding,” says Vincent Janik, who studies marine mammal behavior at the University of St. Andrews in the United Kingdom. He adds that while genetics might play a role in keeping up the dolphins’ cooperation, “it’s hard to say how social learning wouldn’t have an influence on this [behavior].”

The study also feeds into the debate surrounding whether marine mammals can have “culture.” It’s not clear, for instance, if mother dolphins intentionally change their behavior in order to instruct their offspring, just as Laguna’s fishermen teach their children how to spot helpful dolphins. Some researchers regard that sort of intent as critical to forming a culture.

This story was produced by ScienceNOW, the daily online news service of the journal Science.

 
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