Are Bottlenose dolphins hungry for cetaceans?

Scientists who autopsy cetaceans that wash up dead on British beaches have come to a grim conclusion: some species are being killed by bottlenose dolphins.Dead porpoises (and other cetaceans) turn up regularly on beaches around Britain.

According to a Defra report last year (pdf), the cause of death of 15 out of 56 porpoise bodies found - the majority - was "physical trauma (bottlenose dolphin attack)". The photo above shows the rake marks on a harbour porpoise caused by a bottlenose dolphin.The killings were first reported from the north-east coast of Scotland, then off the coast of Wales, and this week the body of a Risso's dolphin was found further south still, in Cornwall.

The Risso's dolphin (see photo below) was said to have been killed over food shortages and dolphins were described as being so hungry they are turning on each other.

Autopsies show some of the dead animals (13 out of 56) have died of starvation. But not all, and the bottlenose killers are not eating the porpoises they kill. In the case of the Risso's dolphin, it is not even a competitor for the same prey (Risso's feed on squid). So why are they killing other cetaceans?"We can't state confidently that the killings are tied to declining fish stocks," says dolphin expert Nick Tregenza, who advises the Marine Strandings Network in Cornwall.

His guess? "They could be doing it for fun."Bottlenose dolphins are known to spread behaviour culturally and there are extensive records of violence between and within dolphin pods."Killing for fun" could be another culturally transmitted behaviour.Tregenza likens it to the spread of milk-bottle opening by blue tits."It could be a form of play rather than food competition," he suggests.

Alternatively, he says, a bottlenose dolphin might have tried to help the weak Risso's dolphin and become angry when the Risso's failed to respond.In a sick way, I kind of like the "killing for fun" explanation. Tregenza says the Cornwall Wildlife Trust has reported that dolphins have been seen picking up stones from the sea bed and throwing them on the surface.

"They were thought to be throwing stones at seals on rocks not far away," Tregenza says.Stone-throwing and killing for fun. Dolphins are even more like us than we thought.Rowan Hooper, online news editor