Experts were divided over what caused the injury and deaths of hundreds of harbor porpoises that were washed up on beaches over the past decade. Some attribute this to the animals becoming entangled in fishing nets while others blame boat propellers.

Findings of a new study, however, revealed that the seemingly cuddly gray seals were responsible for the mysterious injuries and deaths of the porpoises. Study researcher Mardisk Leopold, a biologist from the Wageningen University and Research Center in The Netherlands, and colleagues have found evidence that gray seals have learned to prey on mammals.

For their study published in the journal Proceedings of the Royal Society B on Nov.26, Leopold and colleagues conducted DNA analysis of the bite wounds on porpoises that were stranded along the Dutch coastline between 2003 and 2013.

In porpoises with relatively fresh wounds, the researchers found that majority of these animals had bite and claw marks that characterize attacks of gray seals. The DNA of gray seals was also found in some of the bite marks, providing evidence that the seals were behind the attacks.

"We conclude that the majority of the mutilated harbor porpoises were victims of gray seal attacks and that predation by this species is one of the main causes of death in harbor porpoises in The Netherlands," the researchers wrote.

The marine animals, which are known to primarily prey on fish, appeared to be turning to bigger animals and the researchers warned it is possible that they could attack humans as well.

Leopold said that the new behavior does not indicate that there is lower supply of fish. The gray seal just appeared to have learned a new trick and made a switch from eating fish to eating mammals. He also said that the behavior can be replicated by other seals elsewhere given the animal's social behavior.

"They are very social and when they go hunting, they do it alone, so it was surprising they can transfer this behavior to others," Leopold said. "It's not just one seal. This is everywhere along the Dutch coast."

The researchers likewise found that many of the porpoises that the seals attacked were found on beaches that people use. It is possible that the marine animals can pose threats to humans as well. Leopold said that there are already reports of seals attacking humans in northern Europe, albeit the animals may have been provoked.