Today video footage of the killings will be shown on a BBC2 programme about Scottish wildlife, Operation Survival.
The bottlenose dolphins of Moray Firth gang up to hunt a passing porpoise, battering it with their skulls and long, hard noses, biting it and flinging it into the air.
It is a highly uneven contest: the three- to four-metre dolphins weigh five times as much as the metre-long porpoise. The smaller cetaceans die of multiple injuries.
The savagery seems an ocean apart from the benign nature we have dreamed up for the ever-smiling dolphin. Playful and highly intelligent, they may be but not gentle, for they fight each other as well.
``What we've found does rock their image rather,'' said Ben Wilson, of Aberdeen University, who studies the ecology of the Moray Firth bottlenoses. ``They're real wild animals and we shouldn't forget it.''
He believes that attacks by the 150 dolphins which inhabit the 5,000 square kilometres of the firth are a significant cause of death among the much larger number of porpoises living there.
Suspicions were first aroused some six years ago when examination of washed up, dead porpoises revealed tooth marks, broken bones and crushing of internal organs.
Dr Wilson said the cause of the attacks would not be known until the dolphins carrying them out had been identified.
He and fellow scientists can pick out many of the individual bottlenoses from the shape of and marks on their dorsal fins.
Once it was known whether females, or males, or mixed groups were behind the killings this could help lead to an explanation, he said. But the video footage obtained so far has not been good enough to pick out the individual attackers.
One theory is that the killings happen because the two mammals compete for similar food. This lethal behaviour is seen among other predators.
Otters are thought to kill or chase off the smaller mink. The biggest of the big cats, like tigers, sometimes kill smaller ones like leopards.
Intense competition for smaller prey may even explain the abiding, instinctive hatred between cats and dogs.
The Independent and the World Wide Fund for Nature have produced an illustrated book on wildlife conservation in Britain, Going, Going, Gone, which features the porpoise. It is published by Bookman at pounds 6.50, ISBN 189871839-3.Reuse content