By Paul Rincon BBC News science reporter

Female dolphin Dodger was already "sponging" by the age of two

Female bottlenose dolphins are taught by their mothers to use marine sponges to look for food, according to a study.

The finding represents the first case of material culture observed in a marine mammal species.

An international team looked at wild dolphins from Western Australia and used DNA analysis to investigate if the behaviour could be inherited.

They tell the journal Proceedings of the National Academy of Sciences it is most probably transmitted socially.

It has been shown in captivity that they can socially learn - they can imitate Michael Krützen, University of Zürich

Biologists observing the dolphins in eastern Shark Bay saw the animals break marine sponges off the seafloor and wear them over their snouts to probe into the seafloor for fish.

Sponging was mostly confined to females - only one out of the 13 regular spongers looked at by the study was male.

"It looks like the animals use the sponge as a kind of glove to probe the [sediments]. It might just give them protection against some noxious critters hiding in there," said Michael Krützen, of the University of Zürich in Switzerland, co-author of the study.

"But they might also be able to chase other fish living on the seafloor. That's what it looks like from the surface. We can't go in the water; Shark Bay was given its name for a reason," he told the BBC News website.

All in the genes?

If genetic explanations for sponging could be discounted, it was likely the behaviour was passed from mother to daughter through learning.

To investigate possible genetic factors, the team took small tissue samples from the dorsal fins of 185 sponging and non-sponging dolphins for DNA analysis.

Demi the dolphin has a mother and daughter that are also spongers

Results from mithochondrial DNA (mtDNA) - which is passed down the maternal line only - suggested spongers were part of a close-knit family group with a recent common ancestor - a "sponging Eve" if you like.

This suggested transmission of sponging within a single "matriline" (a related group of animals linked by descent through the female line), but did not resolve whether it was learnt or inherited.

A complex pattern of inheritance based on multiple genes that can be shuffled during sexual reproduction was considered unlikely. "Why, then, would it be confined to one maternal line?" Dr Krützen proffered.

So the researchers investigated whether sponging might be inherited via a single gene. None of the 10 possible single-gene scenarios they considered matched what researchers saw in the field.

'Recent switch'

"It's really hard to make genetic arguments for recent switches in behaviour, because things don't happen that quickly in populations," commented Grant Pogson, an evolutionary biologist at University of California, Santa Cruz, US.

Andrew Read, professor of conservation biology at Duke University in Beaufort, US, said: "There have been similar insights from sea otters, so it wouldn't particularly surprise me if it were true.

"In bottlenose dolphins in Saratoga [US] there have been some suggestions that individually specific foraging behaviours are likely to be transferred from mother to daughter because of the long time they spend together."

It is unclear why the behaviour is confined solely to females, but clues may come from a recent study of chimpanzees.

A paper published in Nature last year suggested female chimps learn from their mothers how to gather termites much faster than males - who prefer to spend more of their time playing.

Dr Krützen added: "Those who work on these animals know that if there is a prime candidate for socially transmitted behaviour - culture - in the marine mammal world, it is bottlenose dolphins."

"It has been shown in captivity that they can socially learn - they can imitate. If one dolphin can do it then others should be able to."