On a narrow island in eastern Canada battered by freezing winds, hundreds of gray seal pups lie dead every breeding season, bearing massive, oddly shaped wounds. The injuries start at their mouth and corkscrew down to their chest, tearing down to the bone but leaving the rest of the body intact. For years, scientists blamed local sharks and boat propellers for the bizarre deaths. But research published in the latest issue of Marine Mammal Science reveals an even grimmer truth. The freshly weaned animals were brutally killed by their own kind: adult male gray seals.

“Witnessing mortality on such a massive scale—and in such a violent way—is deeply disturbing,” says Claudia Hernández-Camacho, a marine biologist at the Interdisciplinary Center for Marine Sciences at Mexico’s National Polytechnic Institute who was not involved in the study. “Studies like this are so important,” she says, because they remind scientists that sometimes the killer isn’t humans or the environment—but something much closer to home.

Scientists first documented the “corkscrew” wounds in the 1980s on Sable Island, the world’s largest gray seal breeding area, where 80,000 pups are born each winter. The culprit remained mysterious, although researchers would later speculate that the deaths were caused by boat propellers or attacks by Greenland sharks. Still, no one had actually witnessed such attacks, and the waters around the island are too shallow for boats.

In 2016, scientists reported a single male gray seal in Scotland killing multiple pups by tearing their blubber out, leaving behind corkscrew injuries. But seal scientists in Canada doubted they had cannibals in their colonies. Nell den Heyer, a marine biologist at Fisheries and Oceans Canada who’s worked on Sable Island for 20 years, says her team had never seen the seals eating each other—“and we’re out there every day” during the breeding season.

That changed in 2024, when Izzy Langley, a marine ecologist at the University of St. Andrews, was equipping gray seals on Sable Island with acoustic receivers to help track the movement of tagged codfish. Unexpectedly, she witnessed an adult gray seal male attacking a pup.

Intrigued, she, den Heyer, and their colleagues took to their ATVs to scour the island’s beaches twice a week during the breeding season, examining and ear tagging every pup corpse they could find that bore the famous spiral mark. Taking a deeper look inside the cadavers, the team found bite marks from large fangs and traces of claws raking through blubber—the clear work of gray seals. “They’re amazingly dexterous with their flippers,” Langley says.

The team’s total count: 765 corkscrew-ravaged pups in that 2024 breeding season alone. In 2025, the scientists sighted a whopping 359 carcasses on a single day. Although it’s possible the attacks were increasing, Langley suspects the researchers were simply getting better at their job. “Once you know what you’re looking for, you see it more and more."

Cannibalistic attacks between gray seals leave telltale “corkscrew” injuries spiraling from mouth to chest.Hanne Siebers

Thus far, all evidence points to adult males taking advantage of unsuspecting youngsters. That’s because all worldwide sightings thus far, including 2023 Sable Island drone footage the team just reanalyzed, involve adult bulls. “These pups don’t recognize the adult males next to them as a threat, which likely makes the attacks especially shocking to witness,” Hernández-Camacho says. The males may be seeking added nutrients in high-calorie blubber to boost their mating value during the breeding season, a time when bulls usually fast, Langley speculates. “It’s almost as if an individual learns this behavior,” she says, “and then becomes kind of specialized on exploiting this to top up its energy reserves.”

The scientists aren’t recommending intervention, even if the perpetrators tend to be the same few individuals. “We don’t know how natural it is,” says Ursula Siebert, a veterinary pathologist specializing in wildlife population health at the University of Veterinary Medicine Hannover who was not involved with the work. “It can definitely be hard to watch,” Langley adds. “But the life of a seal—and indeed any wild animal—is tough.”