Brutal Killer Lurks Within Cuddly Marine Mammal

More than 1,000 mutilated marine mammals thought to have been victimized by humans were attacked by another marine mammal with a cuddly reputation.

Grey seals instigated the majority of attacks that left hundreds of stranded harbor porpoises mutilated, a recent investigation found.

The conclusion is a shock to animal experts: It was previously thought that grey seals only preyed on fish. But according to the study, published in the latest issue of Proceedings of the Royal Society B, grey seals will challenge much larger animals, possibly even humans.

"People go on excursions to swim or dive with grey seals, unaware that these animals can be quite dangerous for human-sized animals. Most people consider them quite cute," study co-author Lineke Begeman, of Utrecht University's Faculty of Veterinary Medicine, told Discovery News.

Lead author Mardik Leopold, of the Wageningen University and Research Center, added: "Grey seals use their ‘hands' to hold large prey and their teeth to tear off bits and pieces, so they do not have to swallow large prey whole. This is why they can also take on large prey."

Leopold and his team assessed bite marks on 1,081 mutilated harbor porpoises that had stranded on the Dutch coastline. Among those with relatively fresh wounds sufficient for analysis, the majority had bite and claw marks consistent with grey seal attacks. Some of the animals also tested positive for grey seal DNA, providing further evidence that seals were the attackers.

To some extent, the findings come as a relief to people in the Netherlands. Early blame for the mutilations was assigned to fishermen cutting by-catch from their nets, ship propellers, suction dredgers, or some other human activity.

"These earlier assumptions caused emotional responses of those who were held responsible, and rightly so as these accusations were false," Begeman said.

In addition to the Netherlands, mutilated harbor seals have also been observed in the U.K., France, Belgium, and along the Assateague National Seashore in Maryland. Grey seals can be found in all of these regions, so it is possible that they once again inflicted the damage.

Not every attack resulted in the death of the porpoise. Seals sometimes seem to bite chunks out of their victims, perhaps going for the parts that contain the most blubber/fat. There could, however, be other reasons why grey seals don't finish their prey off.

"Some attacked porpoises evidently got away, which is to be expected with a predation attempt," Leopold said. "In a few cases, the seal may just have been aggressive, rather than hungry, or may have been disturbed during the encounter."

The study is a reminder of how large grey seals are. They weigh an average of 551 pounds, whereas harbor porpoises only weigh about 55 pounds.

A few years ago, fishermen in Scotland warned that they had seen seals engaged in "aggressive behavior," according to George Moodie, of Udny Trust. One seal savagely attacked a dog that later had to be put down. Moodie said several fishermen were frightened by the seals' behavior.

So far, however, Leopold said there have been no verified reports of fatal seal attacks on humans.

"Some people have been bitten by ‘inquisitive' or ‘sexually frustrated' grey seals, but all survived to tell the story, luckily," he explained.

The researchers suggest that signs could be put up to warn recreational water users about aggressive grey seals in the area. In the future, they also hope to place small underwater cameras on the heads of seals. The cameras may, as Leopold said, "provide video footage of an actual attack, as seen through the eyes of the predator."

Grey seals have been implicated in a high number of harbor porpoise mutilations.

The National Oceanic and Atmospheric Administration (NOAA) is celebrating "30 Days of the Ocean" in the month of June, and in honor of the organization's hat-tip to life undersea, we take a look, this final June weekend, at some of the organization's captivating marine life snapshots. Here, the eyes of a queen conch (

Strombus gigas

) peek out from under its shell in La Parguera, Puerto Rico.

PHOTOS: 30 Days of the Ocean

Kelp and sardines, just doing what they do, off Anacapa Island, Channel Islands National Marine Sanctuary.

Whales Get Fishing Tips From Peers

This school of permits contains 60-80 individuals, each more than a foot long. The school was observed in the Dry Tortugas, Florida.

PHOTOS: Shipwreck Hunt Turns Up 'Tar Lilies'

A Caribbean spiny lobster strolls on the sea floor. This photo was shot during a 2010 NOAA expedition in the U.S. Virgin Islands to map underwater habitats and the marine life they support.

Deep-Sea Methane Ecosystem Found in Atlantic

DNA testing confirmed that the eggs pictured here were those of a loggerhead turtle, a marine reptile species listed as threatened under the Endangered Species Act. The testing was done by NOAA's Center for Coastal Environmental Health and Biomolecular Research, in Charleston, S.C., the only lab in the country dedicated to forensic analysis of marine species.

PHOTOS: Sea Turtles From Shell to Surf

This manta ray coasts over a reef, in the process inviting much smaller fish to clean parasites and other debris off of it. Manta Rays are the largest type of ray in the ocean.

The World's Biggest Manta Ray Sanctuary Created

Make way for the balloonfish, also known as a porcupine or spiny puffer fish. As its name suggests, it will swell up like a balloon when attacked.

VIDEO: Flushed Fish Invading Oceans

Here we see a close-up of brain coral in the Dry Tortugas, Florida.

Off the California coast, a group of elephant seals sleeps in the sun on a sand dune on Active Point, San Miguel Island, part of the Channel Islands National Marine Sanctuary.

With its distinctive reddish and white stripes, its gracefully flowing fins, and its menacing spine, not many fish can embody the beauty, mystery, and danger of the ocean quite like the lionfish. Although it's native to the Indo-Pacific region, lionfish were introduced to the Atlantic and are now found along the U.S. coast, from North Carolina to Florida, and in the Bahamas and Caribbean. The lionfish spells trouble for the balance of ecosystems and fisheries it invades, as it can out-compete native species for food and space. It lacks predators, has a voracious appetite, reproduces quicly, and is spreading fast.