Predatory instinct: Dolphins can attack in groups

Jan Loveridge, volunteer co-ordinator for Cornwall Wildlife Trust’s Marine Strandings Network, said the number of attacks appeared to be on the increase.

‘Despite their friendly image bottlenose dolphins can be aggressive towards one another and on the rare occasion that we see a dead bottlenose wash ashore it often has rake or tooth marks inflicted by its own species,’ she said.

‘But we have recently begun to see an increase in the numbers of young and female harbour porpoises that have clearly been attacked by bottlenose dolphins and results from the post mortems carried out on these animals confirm this.’

Concerns about their behaviour have grown after the body of a rare baby Risso’s dolphin was found on a beach in the Scilly Isles.

The baby was still young enough to be dependent on its mother. Marks and cuts on its body suggest that it was killed by adult bottlenoses.

The dead dolphin was found on St Agnes in the Scilly Isles. Risso’s dolphins are often seen around Cornwall, although not as frequently as bottlenose and common dolphins.

‘The discovery of yet another species that has suffered from these attacks is of particular interest especially as it was so young, ‘ she added.

‘The motives for such attacks are unclear, although scientists have considered that competition for declining food stocks may trigger the behaviour.’

Young Bottlenose Dolphin calves (Tursiops truncatus) having fun together in the Moray Firth, Scotland

Dolphins are deadly opponents. They are agile, fast swimming and intelligent. They hunt in groups and can use their finely honed ultrasound abilities to identify parts of the body most vulnerable to attack. They ram their opponents with their beaks and even flick them into the air.

Some marine biologists believe that adults fight each over mates or territory. Some males may kill young dolphins to allow them to mate with the mothers.

However, the reason why they are attacking porpoises in increasing numbers remains a mystery.

Dr Peter Evans, director of Sea Watch Foundation,  said it was unusual for bottlenose dolphins to attack a Risso’s. He believes they may have mistaken it for a porpoise.

‘Risso’s dolphins feed on things like squid and cuttlefish and octopus, which are rarely eaten by bottlenose dolphins, which feed predominantly on fish,’ he said.

‘Bottlenose dolphins are aggressive and quite bulky and they have frequently been observed attacking other smaller species within their vicinity.

‘They have been attacking porpoises a lot.

‘There is evidence that bottlenose dolphins have been killing porpoises more in the last 20 years than before that.  It could be that some of them went unobserved. ‘

Douglas Herdson of the National Marine Aquarium was just as perplexed, adding: ‘I’m surprised the Risso’s was attacked – it would have had adults nearby and adult Risso’s are actually slightly bigger than bottlenose dolphins.’