© Fábio Daura-JorgeA helping fin. A fisher in Laguna, Brazil, fishes with the assistance of a local dolphin.
Every autumn, lucky visitors to Laguna, Brazil, which is situated around a narrow lagoon on the Atlantic Ocean, catch an odd sight. Here, resident bottlenose dolphins (Tursiops truncatus) frequently turn sheepdog, herding schools of small, silver fish called mullets toward the shore - and, it turns out, toward lines of wading fishers. As soon as the dolphins get close to their human companions, they give the signal, slapping their heads or tails against the surf. In an instant, the fishers cast their nets, catching dozens of frenzied mullet.
While it's not clear whether the dolphins themselves get food out of the exchange or benefit in other ways, both people and marine mammals alike have fished side by side like this for generations, says study co-author Simon Ingram, a marine mammal biologist at Plymouth University in the United Kingdom. Still, something intrigued him and his colleagues: Only about one-third of the lagoon's more than 50 dolphins - whose genders and relatedness have yet to be determined - regularly take part in the tradition, and some of them earn nicknames from the locals such as Scooby or Caroba. 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 these animals interacted - whether some dolphins spent more time with only a few friends, isolating outsiders. For the most part, the dolphins seemed pretty egalitarian, the group discovered. A few stray animals might swim together for a time. Then they'd eventually split up, each darting off to join another group. Individuals that helped local fishers tended to also cluster more with each other, while dolphins that didn't pitch in kept more to themselves, the team reports online today in Biology Letters.
For study co-author Fábian Daura-Jorge, a marine biologist at the Federal University of Santa Catarina in Brazil, this suggests that learning may be involved. Dolphins with tighter bonds, he explains, might be more inclined to pick up each other's unique behavior. Dolphins and their close relatives certainly can swap unique behaviors from animal to animal, scientists say. Young dolphins living in Australia's Shark Bay, for instance, learn from their mothers how to cover their noses with dead sponges - a nifty trick that keeps them from getting scraped.
Something similar may be happening in Brazil, Daura-Jorge says. He's seen mothers here gently nudging their offspring toward mullet as if giving them a lesson in herding fish. If that's the case, 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 may 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." The answer, Janik says, comes down to how you define the term. It's not clear, for instance, if mother dolphins intentionally change their behavior in order to instruct their offspring - perhaps similar to how the Laguna fishers teach their children how to spot helpful dolphins. And some researchers regard that intent as critical to forming a culture.
Regardless, the close bond between humans and dolphins in Laguna makes protecting these animals in this region of Brazil that much more important, Daura-Jorge says. Many of Laguna's fishers catch fish with only the aid of dolphins, meaning that their livelihoods depend entirely on the success of these marine mammals. "If we lose the cooperative dolphins," he says, "we lose this traditional way of life."
if this isn't learned behavior passed on from long ago - by a culture that lived in a more symbiotic relationship with dolphins?