for National Geographic News
When researchers first saw something strange on the snout of a dolphin in Shark Bay, Western Australia, they thought it was a massive tumor. Now they say it provides the first evidence of a tool-use culture in marine mammals.
The object turned out to be a marine sponge broken off from the seabed. Later other bottlenose dolphins in Shark Bay were observed holding sponges over their beaks, and appeared to use them as a fishing tool.
Researchers now report this odd hunting technique originated in a single female and is passed from mother to daughter.
Basing their findings on genetic analysis, the team suggests that this so-called sponging behavior represents the first known example of tool-related culture in cetaceans. Cetaceans are a group of mostly marine animals, including whales and dolphins.
Details of the study were published online this week in the Proceedings of the National Academy of Sciences.
The question of whether animals are, like humans, capable of culture has been hotly debated in recent decades. A growing body of research suggests that some animal populations do invent behaviors that are passed to other generations.
Outside humans, the best evidence for culturedefined as distinct and complex behavior originating in local populations and passed on through learningcomes from chimpanzees.
Research published in the journal Nature in 1999 summarized the findings of various long-term field studies, including those of renowned primatologist Jane Goodall. The research found significant cultural variations in wild chimp populations, with 39 behavioral patterns peculiar to local groups. Fifteen behaviors involved foraging using tools, such as probing for ants with sticks and cracking nuts with stones.
The new study says sponging by bottlenose dolphins in Western Australia is best explained as a form of culture.
"We believe they use [marine sponges] to probe in the [seafloor soil] for fish," said co-author Michael Krützen, of Zurich University's Anthropological Institute and Museum in Switzerland.
"The sponges probably act as a protective glove so the dolphins don't get stung by stonefish," Krützen added. (The stonefish is a bottom dweller with highly venomous spines.) The sponge also appears to disturb fish hiding on the seabed. The dolphins then snap the fish up.
The hunting tactic was almost wholly confined to a small group of females and their daughters among the Shark Bay population, with just a single male showing the same behavior. The challenge for the study team was to find out whether sponging is acquired through social learningand therefore evidence of cultureor is transmitted genetically.
The researchers analyzed the mitochondrial DNA (DNA passed down by females) of 13 spongers and 172 nonspongers. They found the trait appeared to be passed on mostly within a single family line from mother to daughter and that sponging most likely originated in a recent ancestor.
"This is the first study of cultural transmission that actually looks at different modes of inheritance both on the family and the population level," Krützen said. "If sponging was coded on the Y chromosome [found only in males], only males would sponge. If sponging was coded on one of the nonsex chromosomes, then one would expect as many males as females to be spongers. This does not seem to be the case. We ruled out that sponging might be a genetic propensity."
Instead, the researchers conclude, sponging is learned by daughters from their mothers.
Why male offspring rarely acquire the same skill remains unclear, though the team put forward one possible explanation: Male bottlenose dolphins tend to form close bonds with other males, and such alliances aren't suited to seabed foraging, since it is a time- consuming, solitary activity.
Dolphins and Whales
While it's difficult to study dolphins and whales closely in the wild, some biologists suspect that cetaceans rival chimpanzees when it comes to culture.
Hal Whitehead, a cetacean expert at Dalhousie University in Nova Scotia, Canada, points to bottlenose dolphins in Laguna, Brazil. These marine mammals coordinate their fishing efforts with local people. The dolphins herd schools of fish toward the beach, then signal to fishers to throw their nets. Any fish that escape swim straight into the mouths of the waiting bottlenoses.
"This feeding technique has been passed down culturally in both species [humans and dolphins] since 1847," Whitehead said.
Another suggested example of cetacean culture involves killer whales that grab seals from beaches in Argentina. Females have been seen to rush beaches with their offspring even when no seals are arounda highly dangerous hunting tactic. The mother helps her young back into the sea if they get stranded. "This looks very much like teaching," Whitehead said.
The haunting songs of humpback whale males have been compared to our pop charts, with the whales changing their tunes in unison across entire oceans.
Michael Krützen referred to a "cultural revolution" in Australian humpback whales, "where one particularly popular song was replaced by a new one at sweeping speed."
Krützen added, "We are still lagging behind primatologists, but given the necessary time and number of long- term studies, I think we will discover many new socially learned behaviors in cetaceans. They must need their big brains for something."
This research was supported in part by the National Geographic Society's Committee for Research and Exploration.
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