Its fate is sealed Dr Sean Twiss/Durham University

The video captures the moment when a male grey seal went rogue. It appears to be sunbathing near a group of pups, then it looks up and sidles over to one of the hapless pups. It then catches, drowns and eats it.

Over the following week in 2014, Amy Bishop from Durham University, UK, and her colleagues saw the same male cannibalise five more pups from the breeding colony on the Isle of May in Scotland.

Additional carcasses found in the vicinity suggested 11 pups had been killed in this manner in one week, they now report.


“One potential explanation is that the male is tapping into a handy food reserve right next door to him,” says team member Sean Twiss. “It may be some new strategy to maximise reproductive success.”

The alternative to eating pups would be leaving the colony and heading out to sea in search of prey. This carries a cost, however, because males need to defend their territory within the colony in order to mate with females. “The chance of the male then regaining its territory would be close to zero,” says Twiss.

Grey seals return to roughly the same spot each year to breed, meaning that pups around a male could well be ones he sired the previous year. Twiss thinks this might explain why each time the male attacked a pup, it did so having moved well outside its territory, ignoring pups “just as fat and juicy” on the way.

These observations add to a growing body of evidence that at least some grey seals have switched from their diet of fish and crustaceans to targeting other marine mammals. No one knows why they do that, but rapid changes in the marine environment could be to blame, the researchers say.

Since 1985, seal carcasses bearing distinctive corkscrew lacerations have been washing up on UK shores. Collisions with ship propellers were thought to have caused these deaths.

But the Isle of May carcasses have similar wounds to these, so the researchers suggest that grey seal males could be the culprits.

“We are now exploring the idea that this kind of predation is behind these mysterious deaths,” says Joseph Onoufriou of the Sea Mammal Research Unit at the University of St Andrews, UK. However, he says the team still haven’t found a carcass that shows all the injuries once attributed to ship propellers.

Whether this murderous male was acting alone or represents a wider trend remains to be seen.

Journal reference: Aquatic Mammals, DOI: 10.1578/ AM.42.2.2016.137