Scientists in west Wales are puzzled by what appears to be a spate of violent attacks by Cardigan Bay's famous bottlenose dolphins on smaller porpoises.
Already this summer there have been at least four incidents.
In May, volunteers from New Quay-based Cardigan Bay Marine Wildlife Centre (CBMWC) rescued a porpoise that stranded on the beach after being chased by dolphins. Then last month researchers witnessed three dolphins killing a porpoise, with another similar incident a week later.
And then last week (July 7) the crew of CBMWC's research vessel Anna Lloyd watched in astonishment as three dolphins repeatedly pounced on a porpoise, forcing it underwater then throwing it in the air only yards from the boat.
Researcher Milly Metcalfe, who was on the boat, said the attack lasted about 20 minutes.
“One of the dolphins in particular was attacking the porpoise while the others joined in from time to time.
“Although we were close by, they took no notice of us, intent on the attack.
“Afterwards we took the porpoise's body on board to take it back for post mortem. There were no obvious external injuries but blood was coming from its mouth, so we think it may have had punctured lungs or other internal injuries”.
CBMWC's science officer Sarah Perry said that bottlenose dolphins have long been known to attack porpoises, but until recently it was rare actually to see it happening in Cardigan Bay.
“But over the past couple of months there's been a spate of these attacks, and we don't know why.
“One possibility is that they see the porpoises as competitors for food, especially if there's a shortage of prey in the area.
“Although porpoises normally go for smaller prey, dolphins will eat anything.
“However there's been no other indication recently of any shortage”.
Other theories advanced in the past are testosterone-fuelled energy caused by a shortage of females in the mating season. But there's also evidence that males sometimes kill young dolphins so they can mate with the victim's mother, and porpoises are similar in size to a dolphin calf.
It's not clear if the same dolphins have been responsible for all the attacks.
“One animal we’re familiar with, Nick, a female, was seen in the area when we rescued the porpoise last month, and she was also seen nearby in last week's attack” said CBMWC's Sarah Perry.
“Our records show that she's been seen before with the animals we think were responsible for the attack, so It may even be that she’s teaching the others.”