Back in May, a porpoise was rescued from a beach along Cardigan Bay, Wales, after onlookers witnessed it fleeing from a horde of dolphins. The porpoise's only defense was to beach itself. Then, less than a month later, marine researchers watched as three dolphins attacked and killed a porpoise, followed by a similar incident just a week later. 

Dolphin attacks on porpoises are not unheard of here, but they're not common either. Researchers might have been able to dismiss these events as isolated incidents ... until, that is, the brutal fourth attack.

Just last week, researchers with the Cardigan Bay Marine Wildlife Centre observed three dolphins systematically attack a porpoise for nearly 20 minutes, reports the BBC. The dolphins could be seen repeatedly pouncing upon the porpoise when it attempted to surface to breath, ramming it, and flinging it about into the air. The attack was relentless until the brutalized porpoise was killed. 

"One of the dolphins in particular was attacking the porpoise while the others joined in from time to time," explained researcher Milly Metcalfe. "Although we were close by, they took no notice of us, intent on the attack."

The porpoise's lifeless body was later taken aboard the research vessel to be examined. Blood coming from the porpoise's mouth indicated that it had likely suffered from numerous internal injuries, such as punctured lungs.

Researchers are baffled by these attacks, which now clearly represent a pattern. Why are dolphins seemingly declaring war on the porpoises of Cardigan Bay?

"One possibility is that they see the porpoises as competitors for food, especially if there's a shortage of prey in the area," speculated science officer Sarah Perry. "Although porpoises normally go for smaller prey, dolphins will eat anything. However there's been no other indication recently of any shortage."

Another theory is that male bottlenose dolphins are mistaking the adult porpoises for dolphin calves, which are both of similar size. Male dolphins will occasionally kill young dolphins in order to mate with their mother. But this scenario doesn't appear to fit the context of these attacks. In fact, one dolphin implicated in at least two of these attacks is actually a female. 

"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," said Perry. 

But teaching them to what end? It's not yet clear whether the dolphins are hunting the porpoises for food, for sport, or some other reason. It's also still unclear whether these attacks are being orchestrated by different groups of dolphins, or if a few isolated individuals are responsible for all of the attacks. Researchers will have to observe more of the behavior to know for sure. 

It just goes to show that war-like behavior is not solely a human trait. Despite their smily expressions, bottlenose dolphins, too, have a dark side.

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