Killer whale fin chewed up on Russian coast points to possible cannibalism, scientists say, could explain why some pods are so tightly bound together

The fins of killer whales found washed up in the North Pacific Ocean have distinctive tooth markings, suggesting that killer whales occasionally engage in cannibalism. Scientists say this may explain why some killer whales live in large family groups.

There are several different species of killer whales (Orcinus orca), which are sometimes considered separate subspecies. In the North Pacific, two of these species occur in roughly the same area. Resident killer whales (Orcinus orca ater) live in large family groups and eat fish. The more common and ephemeral big killer whale (Orcinus orca rectipinnus), on the other hand, lives in smaller groups and hunts other mammals such as whales, dolphins, and seals.

It was generally thought that the two types avoided each other. But Olga Filatova, a whale researcher at the University of Southern Denmark, and her colleagues have found evidence that this isn’t necessarily the case. They published their research results on February 24th in the journal Marine Mammal Science.

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In August 2022, study co-author Sergei Fomin, a researcher at the Russian Institute of Pacific Geography, discovered an orca fin on the coast of Bering Island in eastern Russia. The fins were bloody and had teeth marks on them.

It’s not that rare to find fins with tooth marks like this. However, previously such fins belonged to beaked whales (Berardius bairdii) and minke whales (Balaenoptera acutorostrata), which were attacked and eaten by big killer whales.

“He immediately thought, ‘Oh, this looks familiar,’ and thought that an orca, which kills mammals, had killed this,” Filatova told Live Science. However, to my surprise, it turned out to be an orca’s fin.

Two years later, in July 2024, scientists discovered a second dorsal fin on an orca. This one belonged to a young male and was slightly larger, but it had the same killer whale tooth impression.

“At that moment, I started thinking this was a pattern,” Filatova said. She added that killer whales discard their fins because they are too hard to eat and prevent predators from eating the muscle and fat underneath.

Genetic testing revealed that the fin belonged to a southern killer whale that lives in waters near Washington state and British Columbia. Killer whales are known for putting salmon on their heads and massaging each other with kelp.

Filatova and her colleagues believe that the southern killer whale was probably attacked and eaten by Big’s killer whale.

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“At least now we know that cannibalism does occur, but I don’t think it’s that common,” Filatova said.

Researchers suggest that this occasional predation by big, mammal-eating killer whales is why resident orcas form large, close-knit family groups. Animals that form large groups or herds often do so to protect themselves from predators.

Although it is generally believed that killer whales have no natural enemies, killer whales are known to be aggressive towards each other. For example, in 2016, big killer whales were seen chasing and killing newborn babies in an attempt to force sexual acceptance on their mothers. But they did not eat the calf.

Teaming up as defenders may also help explain observations that large groups of resident killer whales drive away smaller groups of big killer whales, Filatova said. In her research, she said she has seen evidence of Big’s orcas avoiding resident orca pods and returning to an area only after the residents have moved out. “So this defense strategy really seems to be working,” she said.

But not everyone is convinced. “Although our observations of the dental markings on carcasses of fish-eating whales are interesting and we think the idea is worth further investigation, there is not yet enough evidence to build a solid explanation for the social evolution of fish-eating killer whales,” biologist Luke Rendell of the University of St. Andrews in Scotland, who was not involved in the study, told Live Science via email.

Rendell said the potential benefits of foraging together and passing on knowledge of specific habitats and prey could also be important drivers for creating large groups tied to specific locations.

Other animals are also suspected of forming close-knit groups to protect themselves from killer whales. For example, pods of long-tailed pilot whales (Globicephala melas) have also been known to confront and chase off killer whales. This is mainly due to the highly social nature of killer whales. Additionally, killer whales may run away when they hear the sounds of pilot whales.

“The similarities between the social structure of short-finned pilot whales and that of resident killer whales, and their apparent responses to big killer whales, suggest that both may be responding to potential predation pressures,” Michael Weiss, research director at the Center for Whale Research in Washington, who was not involved in the study, told Live Science in an email.

“I think it’s certainly possible that the big killer whales were preying on these two whales,” he says. But he added that scavenging by the big killer whale or attacks from other killer whales during its lifetime could also have caused the rake marks on its washed-up dorsal fin. So it’s not a clear indication of cannibalism or predation, Weiss said.

Filatova acknowledged that scavenging could not be ruled out, as killer whales are known to feed on whale carcasses obtained during whaling. But she said fresh killer whale carcasses usually sink quickly, making them inaccessible, and only begin to float after a few days as they begin to decompose. “You have to be really hungry to eat this,” she said.

Filatova also said she doesn’t think fin marks are related to conflicts with other residents, as they are often found on the sides of animals.

She believes that predation pressure prompted the formation of tight-knit social groups of resident killer whales, perhaps 100,000 years ago, after killer whales that had evolved separately in the Pacific and Atlantic Oceans began to clash with each other. It stuck because the social structure proved efficient.

However, she noted that for these marine mammals, eating another orca may not seem like cannibalism, and some have called for orcas to be named as a separate species. “They’re never sociable, they never spend time together. To them, it’s just a whale. So why not eat it?” Filatova said.

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