They are the same group of bottlenose dolphins in the Moray Firth in north-east Scotland that two years ago were revealed as attacking and killing their small cousins, harbour porpoises. Now the animals have been shown to be killing their own young, in the first example of infanticide recorded in cetaceans - dolphins, porpoises and whales.
Their behaviour was uncovered by a joint team of zoologists from Aberdeen University and vets from the Scottish Agricultural College in Inverness.
The vets carried out post-mortem examinations on five baby dolphins washed up on shore, and found that they had suffered injuries identical to those seen on the porpoises killed by dolphins. They had injuries consistent with being battered, and toothmarks on their sides.
Subsequently, Ben Wilson, one of the Aberdeen researchers studying the dolphins of the Moray Firth, saw two adult dolphins in the sea with a newly dead calf: one of them repeatedly lunged at it, butted it clear of the water with its head, and gripped it in its jaws.
The revelation that dolphins are not always as friendly as we might like to think will be reported in full in the July edition of the Proceedings of the Royal Society; the conclusions of the research are reported in next month's edition of BBC Wildlife magazine, out today.
The Moray Firth resident colony of bottlenose dolphins, thought to number about 130 animals, is one of only two around Britain's coasts, the other being in Cardigan Bay. It has become clear in the past few years that they were occasionally attacking the much smaller harbour porpoises, throwing them clear of the water and battering them to death.
The researchers point out that the dolphin calves that have also been killed are almost identical in size to the porpoises - about a metre and a half long - and think the two types of attacks may be linked, though they do not know why.
Infanticide is relatively common in some of the larger mammals, such as lions and some primates, where males sometimes kill the young of their rivals to increase the chances of their own reproductive success.
It has never been observed in cetaceans before, but in fact it might be widespread, said Paul Thompson, another of the researchers. "They're difficult animals to study. You're only seeing a very small part of their lives. These behaviours could be very widespread but you don't spot them till someone points them out."
The researchers warn that "it may have serious consequences for the viability of small populations".
Asked if people might find the discovery disturbing, Mr Thompson said: "I don't think it devalues people's opinions of dolphins. They're quite remarkable as wild animals and we should respect them for what they are, not what we think they are."