This is just one incident that needs further evaluation. But the consensus seems to be that such acts are very rare in the animal kingdom, but NOT for dolphins. Many such observations have been made.
The moment two porpoises were attacked and killed by dolphins who threw the animals around ‘for fun’ has been captured on camera.
The incident was seen by Caroline Weir who noticed the commotion from her bedroom window in St Cyrus in Aberdeenshire and took photographs.
The animals were later washed up on the shore dead, and had suffered serious injuries to their heads including deep gaping holes around their eyes.
Experts at the Scottish Marine Animal Strandings Scheme believe the pair of dolphins, thought to be young males, were probably playing a game similar to ‘cat and mouse’ when they inflicted the injuries.
A postmortem is now to be held to determine exactly how the two porpoises died.
What is the cause of this specific action? Is it widespread among young male (if that is confirmed) dolphins as a sort of “play” or learning?
Violence in animals is typically related to obtaining food or defense, not casual display.
Marc Bekoff, Professor emeritus of Ecology and Evolutionary Biology, University of Colorado, writes in this Huff Post piece:
So, do animals fight with one another? Yes they do. Do they routinely engage in cruel, violent, warlike behaviors? Not at all; they’re extremely rare compared to other patterns of social behavior.
Humans and chimps are the clear front runners in violent behavior outside of necessity. But dolphins have been observed in several heinous acts of violence.
Here is another observation of dolphins attacking porpoises. Californian dolphin gang caught killing porpoises – New Scientist.
And this piece, written by a deep sea researcher, shows that’s not all we tend to ignore about the “friendly” dolphin. 10 Reasons Why Dolphins Are A$$holes | Deep Sea News.
They gang rape females of their own species and are sexually aggressive towards humans on occasion. They kill their own babies and those of other species. Therein lies a clue to the porpoise interaction. This paper which documents the many attacks from bottlenose dolphins on harbor porpoises suggests that the porpoise resemble dolphin babies and suggests this is an explanation for the inter-species attacks but also infanticide.