Dolphin innovators hunt fish by collecting conch shells

Matt Walker BBC News 8 Sep 10;

Dolphins living in Shark Bay, Australia have developed a rare, and extraordinary new behaviour.
The dolphins have become shell-collectors, using their snouts to pick up and transport large conchs.

The dolphins seek out the shells to hunt fish that are sheltering within.

It is likely that the dolphins originally chased the fish into the conchs, and have now learnt to bring the shells to the surface, where they can flush out and eat their prey.

The foraging tactic is "quite spectacular", say researchers, who have published photographs and details of the behaviour in the journal Marine Mammal Science.

The dolphins of Shark Bay, Western Australia are well known to scientists, as they are one of the best studied dolphin populations in the world.

"Shark Bay dolphins are known as clever inventors, showing a remarkable range of foraging tactics, which are unprecedented in other cetacean populations," says biologist Dr Michael Krützen of the University of Zurich, Switzerland.

For example, these dolphins have developed a number of novel ways to catch fish.

Sometimes the dolphins 'hydroplane' into extreme shallow water, beating their tails furiously to pursue fish into the shallows where they are more easily caught.

In doing so, the dolphins can beach themselves in pursuit of the fish, wriggling their way back into the sea.

They use another trick called 'kerplunking', whereby the dolphins slap their tails over beds of seagrass, creating bubbles that flush out fish hiding within.

Most remarkable, perhaps, is the technique known as 'sponging'.

Some dolphins intentionally pick up marine sponges with their snouts (known as a rostrum), and use them as 'gloves' or 'shields' to protect their sensitive snouts when rubbing them into the sandy seabed to forage.

This is a cultural trait passed down from mother to offspring, the only known example of culturally transmitted tool use among cetaceans.

Shake it out

Now Dr Krützen and colleagues Mr Simon Allen and Dr Lars Bejder of the Cetacean Research Unit at Murdoch University, Western Australia describe another extraordinary innovation by the same group of dolphins.

An individual dolphin inserts its snout into a sizable conch, then lifts the shell up from the sea floor, carrying it to the surface.

Although rarely observed, this behaviour has been spotted by a number of researchers working in Shark Bay.

Now the marine biologists have obtained a series of photographs of the dolphins collecting shells, that reveal why they do it.

The dolphins appear to be hunting fish that have taken refuge inside the shells, as evidenced by photographs showing a relatively large fish falling from a conch being shaken by one dolphin (see above).

"We don't know whether the dolphins go searching in otherwise vacant conch shells for fish and just 'happen' to find prey therein, or whether they actively pursue fish into the vacant conch shells and then lift them to the surface, shaking them about to drain the water and the hapless fish into the waiting jaws of death," Mr Allen told the BBC.

The researchers suspect the dolphins are pursuing fish into the shells, due to the dolphins' erratic swimming before reaching the conchs and rapid surfacing once they have located the shells.

Once they reach the surface, the dolphins shake the shells to make the water and fish spill out.

"It's quite surprising that they are apparently manipulating such large, heavy, cumbersome objects in such a way as to secure a meal," says Mr Allen.

"I think the behaviour is quite unusual, otherwise it would presumably have been seen more often."

As yet, there is no evidence that the behaviour is being culturally transmitted between dolphins, but "I'd say we were lucky enough to witness a dolphin being innovative," says Mr Allen.