Photograph by Chris Stenger, Minden Pictures/Nat Geo Image Collection
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A gray seal (Halichoerus grypus) was recently spotted attacking and eating a younger member of its own species, like this pup in Helgoland, Germany.

 

Photograph by Chris Stenger, Minden Pictures/Nat Geo Image Collection

Cannibalism is more common than thought in gray seals

“If gray seals had a police force, this guy would be locked up,” one researcher says.

The vast majority of the time, gray seals are happy eating fish. But new research shows that they may, from time to time, eat other mammals—including members of their own species.

A new study published in the Journal of Sea Research details the macabre case of an adult male seal capturing, killing, and beginning to eat a younger seal, off the coast of Germany’s island of Helgoland.

The new observation is only the third published paper on cannibalism among gray seals. However, it is the first time that scientists witnessed the whole act up close and performed a necropsy after the fact, describing the wounds left behind.

The case

In late March, 2018, Abbo van Neer of Germany's University of Veterinary Medicine Hannover witnessed the cannibalistic attack off Helgoland.

Van Neer and colleagues watched as a 5-year-old male, identified later by a flipper tag attached to the animal, latched on to a juvenile seal. The male then bit into its throat and pushed its body underwater. (See also: Why some animals become cannibals.)

While the younger seal managed to escape the bull’s grip once, it didn’t last. After just 10 minutes, the water turned red and the juvenile stopped moving. Then, for about 90 minutes the bull began to feed, biting into the skin to expose the energy-rich blubber.

Once the seal had his fill, van Neer quickly collected the carcass to document the tears and damage, before scavengers could get to it. “This is the first time we have the whole story,” he says.

This allowed van Neer to establish a distinctive pattern that seals cause when they cannibalize each other. Looking at a database of seal deaths that dates back to the 1990s, he thinks there are other cases where seals may have cannibalized each other.

These fatalities were previously attributed to shark attacks or propeller collisions.

The geographic spread of carcasses washing ashore suggests there are a small number of seal cannibals. “We hypothesize it’s probably a few specialized individuals, he says. “It’s not going to be the majority of the gray seal population.”

But why?

Cannibalism in nature happens for a variety of reasons. “It’s exciting in a morbid kind of way, because when you start noticing these types of behaviors, it can lead to new scientific questions,” says Amy Bishop, a biologist with the Alaska SeaLife Center in Seward, who was not involved with the study.

In 2014, Bishop witnessed a bull gray seal attack and eat six pups. Male seals fast while on the beach to breed with females, so maybe this is way for them to stay in the colony longer and have more mating opportunities, she says.

Van Neer can’t yet prove why gray seals are eating other seals on Helgoland, but suspects it’s related to energy. For example, gray seals would need to eat about 28 grams of seal blubber to take in the same amount of calories as 100 grams of herring, he says. “From an energetic point of view, it just makes sense.”

But not everyone agrees.

“I have a hard time believing that he did it because he was hungry,” says Andrew Trites, a marine mammal biologist at the University of British Columbia who was not involved with the study. If it was predation, says Trites, the animal should have eaten more from the carcass and even defended the carcass from other predators and scavengers.

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To him, this male seems to be the Hannibal Lecter of seals. Just as humans have a range of personalities and personality disorders, the same thing occurs in other species. “If gray seals had a police force, this guy would be locked up.”

The study adds to a growing number of reports of seals eating other mammals. Several studies show the gray seals can sometimes attack and eat harbor porpoises and harbor seals. (Read more: Gray seals can maul, suffocate seals and porpoises.)

One of the attacks, a gray-on-porpoise-seal killing, also took place off Helgoland, in 2013.

“Marine mammals are not so different from people in the sense, of being a mammal, having a brain, and not all of them work properly all the time,” Trites says.