Dolphins shake-up foraging skill

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

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

"Maybe they're playing, maybe they're socialising, maybe males are presenting a gift to a female, maybe the animals are eating the animal inside," said researcher Simon Allen.

But researchers were more intrigued when they studied the photos and found the back of a fish hanging out of the shell, realising the shaking drained the water out of the shells and caused the fish 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 about two decades. Researchers saw it at least seven times during the four months starting in May, Allen said.

"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 it might be spreading before our eyes," he added.

The Shark Bay dolphin population has two foraging techniques. One involves the dolphin briefly beaching itself to grab fish after driving them up onto the shore.

The other is "sponging" - 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.

Both of these spread "vertically", mainly through the female dolphin population, from mother to daughter. But with this new behaviour with the shells, it might be spreading "horizontally", Allen said.

"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 were known for having preferences in terms of companions and whom they spent time.