When marine biologists first spotted bottlenose dolphins cavorting off the coast of Australia wearing sea sponges on their snouts, they didn't know what to make of the odd behavior.
Now, an international team of researchers has produced evidence that the animals' antics represent a form of culture, which would add the dolphins to an elite group of species that pass traditions down through generations without being compelled by their genes.
"We define culture as a behavior that is acquired by imitation and passed on in a population," said Michael Krutzen, of the University of Zurich in Switzerland, who led the new research. "We think this behavior is an example of that. It's very exciting."
Krutzen and his colleagues believe the dolphins, which live in Shark Bay off the west coast of Australia, wear the sponges while foraging for small fish, crustaceans and other food along channels in the sea floor to protect themselves against sharp coral and stinging critters such as stonefish. It's a trick that appears to be almost exclusively passed from mothers to daughters.
"They wear them like a glove," Krutzen said. "When they go down to the sea floor to probe for prey, there are lots of noxious animals down there. By using the sponge, it protects them."
Many species use tools to perform tasks. Crows, for example, fashion tools out of leaves and twigs to forage for food. And many animals learn behavior by mimicking their elders -- that's how birds develop their songs.
But those abilities stem from instincts that are inherited through genes. Aside from humans, the only other creatures known to transmit behavior purely by interacting with one another are primates. Unrelated chimps pass on techniques for using sticks to fish ants out of nests; different groups of orangutans display unique eating habits, bedtime rituals and other behaviors that researchers believe are examples of socially transmitted culture.
The bottlenose dolphins would be the first marine mammal shown to exhibit similar behavior, indicating that complex social conduct may be more common than had been thought, Krutzen said.
"The boundaries between humans and animals are becoming less and less clear," Krutzen said. "Thirty years ago, people thought humans and animals were very different from each other. No one thought animals used tools. No one thought they had any kind of culture. Those boundaries have been getting fuzzier and fuzzier. Now here's another example."
The findings prompted mixed reactions, with some scientists praising the work and others questioning how firmly Krutzen's team had made its case.
"This is an exciting addition to the catalogue of what we can be increasingly confident are culturally transmitted forms of tool use in non-human populations," said Andrew Whiten, a professor of evolutionary and developmental psychology at the University of St. Andrews in Scotland.
But Bennett Galef, an animal behaviorist at McMaster University in Canada, said the researchers had not proved that the dolphins use the sponges as tools.
"They have no idea how this behavior develops or really what it is used for," Galef said. "If you don't know that, then you're really just guessing."
The researchers said their conclusions are based on years of observing the dolphins and a detailed genetic analysis that clearly ruled out other explanations.
"We really think this is an example of a cultural phenomenon," Krutzen said.
To make its case, the team first ruled out the possibility that the dolphins were using the sponges because of something unique about their environment.
"If there was something about the habitat that forces the behavior, then it wouldn't be cultural," Krutzen said. "That was fairly easy to rule out. We looked at other animals in the same area and saw that many of them did not do this -- they never use sponges. So that excluded an ecological explanation."
But that still left open the possibility that the animals that used sponges were born with a gene that created an instinct for that behavior. To explore that possibility, the researchers analyzed DNA samples from 185 dolphins, including 13 sponge users.
In a paper published online earlier this month in the Proceedings of the National Academy of Sciences, the researchers found that while all the sponge users were related, the pattern of sponge use among them was not consistent with any of 10 possible genetic explanations.
"If you have a sponging gene, then you would expect this gene to be inherited in families in a certain way. It would follow normal genetic rules, and it would spread in the population in a specific pattern. But when we looked at all these possible transmission mechanisms on a family and population level and compared it with our actual genetic data, we found it did not agree," Krutzen said.
The researchers did find, however, that the dolphins that used the sponges were almost all females and were related, indicating that young females learned to use sponges by imitating their mothers. Moreover, the genetic analysis indicated that the behavior probably originated from a single ancestral female at some undermined point in the past.
"All the animals that are spongers have the same maternal gene, which shows that all the animals are descended from the same female," Krutzen said. "We call her the 'Sponging Eve.' "
Laela Sayigh of the University of North Carolina Center for Marine Science said the findings provided "convincing evidence that the behavior is transmitted by social learning." But Sayigh questioned whether that meant that dolphins possess a form of culture, since that can be defined in many ways. Like Galef, she noted that the researchers had only inferred that the dolphins used the sponges as tools.
"They've never actually seen the animals down at the bottom doing what they do with the sponges," Sayigh said.
But Janet Mann, an associate professor of psychology and biology at Georgetown University who helped conduct the study, said she is planning to publish new research to bolster researchers' argument that the sponges are tools.
"I'd put all the money I have on it," Mann said.
The behavior may be passed almost exclusively to females because foraging along the seabed is a solitary activity, and males tend to spend most of their time traveling in groups searching for mates.
"Males form long-term alliances of pairs and trios that cooperate to sequester individual females," Mann said in an e-mail. "It would hamper their ability to . . . consort with different females . . . if they had to stay in channel habitats where sponging occurs."