Why are these orcas killing sharks and removing their livers?

When sevengill shark carcasses with pectoral tears and missing livers began washing up on the South African coast, questions abounded. Then a marine biologist found something: orca tooth impressions.

By Jessica Taylor Price
Published 23 Mar 2023, 10:12 GMT
Orcas, which live worldwide, hunt a variety of prey, from fish to seals to sharks.
Orcas, which live worldwide, hunt a variety of prey, from fish to seals to sharks.
Photograph by Brian Skerry, Nat Geo Image Collection

When 19 shark carcasses washed up on the beach just outside her home in Cape Town, South Africa, last month, Alison Towner knew right away who had killed them.

The sevengill sharks—predators in their own right­—were all found in the same condition: missing their livers, which had been sucked out through a clean tear in their shoulders. The rest of their organs remained intact.

Such near-surgical precision is the hallmark of a pair of orcas known as Port and Starboard, who have been extracting livers from sevengills and great white sharks since at least 2015. Easily spotted thanks to their dorsal fins, which bend right and left (hence their names Starboard and Port), the two male orcas were seen in the area two days prior.

"My reaction was just, 'Here we go again,'" says Towner, a shark biologist and Ph.D. candidate at Rhodes University in South Africa. "There's no stopping this."

Undoubtedly the biggest mystery surrounding Port and Starboard is whether their hunting behaviour is unique. Orcas, which live worldwide, display a wide range of diets and behaviours, including eating sharks, whose organs—especially their livers—are high in fat.

But scientists have never before documented orcas performing this kind of consistent, surgical predation on sharks. What’s more, their observations suggest that the orca pair are showing others how to remove shark livers—possibly an intriguing example of culture in the animal kingdom.

Shark killers

Located on the southwestern coast of South Africa, False Bay is normally teeming with sevengill sharks, with scuba divers spotting as many as 70 in a single dive. But on November 9, 2015, divers noticed something peculiar: The sharks had abandoned the dive area virtually overnight.

Then, carcasses of several sevengill sharks appeared on the seabed with the same clean tears in their bodies. The divers took photographs and shared them with Towner and other researchers, who debated whether the deaths were the result of fishing or an animal attack.

Orcas were one possibility. While rare in the region, they had been spotted in the False Bay area since 2009—but until that time, they were only known to eat marine mammals, such as Cape fur seals. 

"It was on everybody's mind: Could it be?'" says Towner, who studies great white shark movements. "Never in my wildest dreams did I expect it to unfold the way it has."

When, in April 2016, five more sevengill shark carcasses washed up on shore with pectoral tears and missing livers, Alison Kock, a marine biologist at South African National Parks, and her team conducted necropsies. The tears matched the photos taken in November 2015. And she found something that they overlooked in the photographs: orca tooth impressions.

The finding became the first recorded instance of orcas killing sevengill sharks in the area, and the first recorded instance of orcas carefully tearing the pectoral girdle to access the liver and leaving the rest of the shark. Though there are earlier records of orcas eating shark livers, researchers called the South African phenomenon "a novel and specialised technique."

"It's not like they're tearing the back of the shark open. They're going exactly where the liver starts. It's incredible," says Towner.

Then, in 2017, five great white sharks washed up on shore in nearby Gansbaai, also without their livers. Kock suspected Port and Starboard, which were orcas already known to live in the area, but it wasn’t until May 2022 that drone video footage confirmed the pair were hunting great whites.

Evidence of culture?

Little is known about the animals, such as their ages or where they came from, says Simon Elwen, a killer whale researcher and head of the South Africa-based conservation nonprofit Sea Search.

Their bent dorsal fins, a relatively unusual trait that could be due to diet, injury, or genetics, make “them so unusual and captivating," Elwen says. "You have these two highly identifiable individuals."

It’s also uncommon for two males to travel together, he adds—but there could be a reason why.

Elwen's colleague A.R. Hoelzel, a molecular ecologist at Durham University in the U.K., has sequenced the two orcas’ genomes and found preliminary evidence that they’re related—possibly brothers(Read how orcas can also take down blue whales, the largest animal on Earth.)

But why hunt sharks and target their livers? Some experts hypothesise that Port and Starboard are part of a subgroup, or "ecotype," of orcas that frequent the open ocean, but that this duo simply moved closer to shore, perhaps due to fishing depleting their normal food sources.

Another theory is that Port and Starboard, independently or along with their subgroup, have developed this new behaviour in order to save their teeth, which wear down when orcas bite into sharks' rough skin. A great white shark liver is large enough to provide a full meal, without the work or the wear that comes with tearing the shark apart.

Orcas around South Africa are difficult to study due to their transient nature, and because researchers rely heavily on whale watchers and citizen scientists to report sightings, much of the data is "opportunistic," Elwen says.

But the video footage could be a key piece of evidence, since it shows Starboard performing the shark-liver technique in the presence of four other orcas. Many species of whales and dolphins—orcas are the largest dolphin species—exhibit signs of culture, for example passing on certain dialects, hunting strategies, or other behaviours to the next generation. Culture is a relatively exclusive club in the animal kingdom, usually displayed by other social species with long lives and big brains, such as crows and great apes.

Regardless of whether Port and Starboard are the inventors of this shark-eating behaviour, "it's likely the behaviour will spread," Elwen says.

Bad news for sharks

And that could spell disaster for shark species, most of which are already declining, Towner says. Sevengill sharks are gone from the region, as are great whites, which has led to a collapse in Cape Town’s shark-cage tourism industry—not to mention a lack of animals for Towner to study.

Unregulated overfishing poses a much greater threat to shark populations than Port and Starboard, Towner notes, but the predators have added pressure to fish already in trouble.

The absence of sevengills and great whites, apex predators, may have a ripple effect throughout the ecosystem of coastal South Africa, she adds. For instance, it’s possible prey species, such as seals and fish, may increase in number. Other shark species, such as the copper shark, are already moving in to occupy the top spot, according to her research. (Read how reef sharks are in major decline worldwide.)

"Twenty years of stability, and then two killer whales appeared and boom, all hell breaks loose," Elwen says of the drop in shark populations.

Some on social media have expressed frustration about how the orcas have impacted local businesses, particularly great white tourism. For her part, Towner approaches the situation with a combination of fascination and hopelessness.

"I wish there was a golden nugget of hope we could give," Towner says. "It’s a reflection of how delicate the balance of nature is, and if that's to be perturbed in any way, then the ramifications can be profound."


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