The cute, Labrador retrievers of the sea, ocean seals, may not be as benevolent as they look. In fact, a dark side to seals is emerging, with behaviors ranging from penguin rape to violent mutilation of porpoises along the coast of Britain and Holland.

Experts warn that these violent behaviors could translate into attacks on human swimmers. While rare, seal attacks are not unheard of. In 2003, British marine biologist Kristy Brown, 28, was drowned by a leopard seal off the Antarctic Peninsula. The seal grabbed her in its jaws and dragged her into the icy deep while she was snorkeling. Other reports of seal attacks include an attempt in 1985 by a leopard seal to drag a man from pack ice into the chilly water and a harbor seal that bit the hand of a five year-old in Canada.

Aggressive behavior in leopard seals is not unusual, but the same sinister activities in other types of seals is a more recent observation. Scientists at the Institute for Marine Resources and Ecosystem Studies in Holland examined the remains of over 1,000 mutilated porpoises that washed up on the North Sea shorelines over the past 10 years. For many years, the cause of the mutilations remained a mystery.

Some speculated that the porpoises were caught in boat propellers or maimed by scavengers after being caught in fishing nets. Recently, however, scientists have traced the crimes to grey seals through DNA evidence.

“Many of the mutilated porpoises were found on shores used frequently by human bathers and surfers and there would appear to be no reason why humans may not be at risk from grey seal attacks,” said biologist Mardick Leopold, lead author on a new report published in the Proceedings of the Royal Society b.

Approximately 180,000 grey seals reside along the Scottish coastline and surrounding area of the British Isles. They can reach 11 feet long and weigh over 660 lbs, making them Britain’s largest carnivorous animals. For reason’s yet unknown, the seals have begun attacking porpoises that come to the surface of the water to rest. They grab them by the bony beaks and tear blubber from their bodies.

While scuba divers have reported being approached by the grey seals, no attacks on humans have been documented.