Flipper fashion

Natural History, Sept, 2005 by Graciela Flores

Not long ago in Shark Bay, off the coast of western Australia, a female bottlenose dolphin broke a chunk of sponge off the seafloor and wore it as a mask over her snout while she probed the sediment for fish. Today "sponging" is a foraging fad among dolphins in Shark Bay--but, with one exception, exclusively among females. Moreover, though the Shark Bay dolphins adopt a dozen foraging tactics, sponging is the only one that involves a tool.

Biologists have resisted giving the label "culture" to the perpetuation of the practice of sponging. But Michael Krutzen, a molecular ecologist formerly at the University of New South Wales in Sydney, Australia, and his colleagues maintain that the term fits, and they've ruled out alternative explanations. Both spongers and nonspongers forage in deep water, so sponging is not a response to habitat. And samples of nuclear DNA from adult spongers rule out the possibility that a propensity for sponging is a genetic trait.

The only remaining explanation, Krutzen and his colleagues argue, is that sponging is socially learned--the first established example of the cultural transmission of tool use in marine mammals. But if it's social learning, it remains (almost) all in the family: according to an analysis of the spongers' mitochondrial DNA, all but one of them are descended from a single matriarch. Thus they most likely learned the practice from a female relative, probably Mom. The single sponging male examined by the investigators is kin, and would have spent the same amount of time with his mother as a daughter would have. So why don't other males sponge? That's still a puzzle. (PNAS 102: 8939-43, 2005)