It's a no-brainer that dolphins are smart, but do the animals have their own culture? A new study suggests so by showing that dolphins use sea sponges to forage for fish and that daughters learn the trick from their mothers. If true, this would be the first clear demonstration that a marine mammal can pass cultural information from one generation to the next.
Many animals use tools to find food, but very few have been shown to pass tool-using traditions on to others. One notable exception is found in some chimpanzees, who transmit cultural information on how to use sticks to fish for termites. Some researchers have claimed evidence for culture in marine mammals, such as whale songs, but others have challenged these conclusions.
For the past 20 years, marine scientists have been monitoring a population of more than 850 bottlenose dolphins in Shark Bay, Western Australia. A small subgroup has been observed breaking sponges off the seabed and wearing them on their snouts, apparently to probe the seafloor for fish hiding there. A team led by evolutionary geneticist Michael Krützen of the University of Zurich, and including researchers from Australia, Canada, and the United States, obtained genetic data from tissue biopsies taken from 185 of the dolphins, 13 of whom were spongers. The team reports online this week in Proceedings of the National Academy of Sciences that the 13 spongers, all but one of them female, are closely related to each other and probably share a recent common ancestor.
The team considered two alternative explanations for the sponging behavior: Either it was genetically determined, or mothers somehow passed the tradition on to their daughters. An exhaustive analysis of genetic data from the 13 spongers and 172 other dolphins, which considered 10 possible ways the behavior could be genetically inherited (including via sex chromosomes and mitochondrial DNA) came up empty-handed. That leaves cultural transmission as the only viable hypothesis, the team concludes.
"This is a strong case indeed," says Harvard University primatologist Richard Wrangham. And Simon Reader, a behavioral biologist at the University of Utrecht in the Netherlands, agrees that "the pattern of transmission is extremely suggestive of social learning." But Reader says that the study leaves open the question of what exactly is being learned. "If the behavior is cultural, the sponging itself may not be what is transmitted." Instead, Reader says, sponging may be associated with another trait, such as a preference for a particular food.