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Dolphin fish-catching trick spreading

Catchy behaviour Dolphins in one western Australian population have been observed holding a large conch shell in their beaks and using it to shake a fish into their mouths - and the behaviour may be spreading.

Researchers from Murdoch University in Perth were not quite sure what they were seeing when they first photographed the activity, in 2007, in which dolphins would shake conch shells at the surface of the ocean.

"It's a fleeting glimpse, you look at it and think, that's kind of weird," says Simon Allen, a researcher at the university's Cetacean Research Unit.

"Maybe they're playing, maybe they're socialising, maybe males are presenting a gift to a female or something like that, maybe the animals are actually eating the animal inside."

But researchers were more intrigued when they studied the photos and found the back of a fish hanging out of the shell, realising that the shaking drained the water out of the shells and caused the fish that was sheltering inside to fall into the dolphins' mouths.

A search through records for dolphins in the eastern part of Shark Bay, a population that has been studied for nearly 30 years, found roughly half a dozen sightings of similar behaviour over some two decades.

Then researchers saw it at least seven times during the four-month research period starting this May, says Allen.

"There's a possibility here, and it's speculation at this stage, that this sort of change from seeing it six or seven times in 21 years to seeing it six or seven times in three months gives us that tantalising possibility that it might be spreading before our very eyes," he says.

"It's too early to say definitively yet, but we'll be watching very closely over the next couple of field seasons."

The Shark Bay dolphin population is already unusual for having developed two foraging techniques, one of which involves the dolphin briefly beaching itself to grab fish after driving them up onto the shore.

The other is 'sponging', in which the dolphins break off a conical bit of sponge and fit it over their heads like a cap, shielding them as they forage for food on the sea floor.

Sharing the behaviour

But both of these spread 'vertically', mainly through the female dolphin population, from mother to daughter. The intriguing thing about this new behaviour with the conch shells is that it might be spreading 'horizontally', says Allen.

"If it spreads horizontally, then we would expect to see it more often and we'd expect to see it between 'friends'," he added, noting that dolphins are known for having preferences in terms of companions and whom they spend time with.

"Most of the sightings from this year are in the same habitat where we first saw it in 2007, and a couple of the individuals this year are known to associate with the ones that we saw doing it a year or two ago."

The next step would be not only to observe the behaviour again in another season but also to try and gather evidence of deliberate actions on the part of the dolphins.

"If we could put some shells in a row or put them facing down or something like that and then come back the next day, if we don't actually see them do it but find evidence that they've turned the shell over or make it into an appealing refuge for a fish, then that implies significant forward planning on the dolphins' parts," says Allen.

"The nice idea is that there is this intriguing possibility that they might manipulate the object beforehand. Then that might change using the shell as just a convenient object into actual tool use."