Chewed-up orca fins on Russian beach point to cannibalism, and scientists say it may explain why some pods are so tight-knit

Orca fins washing up in the North Pacific are scratched with characteristic tooth marks that suggest killer whales are occasionally cannibals. Scientists say this may explain why some orcas live in large family groups.

Orcas (Orcinus orca) come in several distinct types, sometimes considered different subspecies. In the North Pacific Ocean, two of these types inhabit roughly the same areas: Resident orcas (Orcinus orca ater) live in large family groups and eat fish, and Bigg's orcas (Orcinus orca rectipinnus), which are more common and transient, live in smaller groups and hunt other mammals, such as whales, dolphins and seals.

Generally, it was thought the two types avoided each other. But Olga Filatova, a whale researcher at the University of Southern Denmark, and her colleagues have uncovered evidence showing that's not always the case. They published their findings Feb. 24 in the journal Marine Mammal Science.

In August 2022, study co-author Sergey Fomin, a researcher at the Pacific Institute of Geography in Russia, found an orca fin on a beach on Bering Island in eastern Russia. The fin was bloodied and covered with tooth marks.

It isn't that unusual to find fins with such tooth marks. But previous such fins had belonged to Baird's beaked whales (Berardius bairdii) and minke whales (Balaenoptera acutorostrata) that had been attacked and eaten by Bigg's orcas.

"He immediately thought, 'Oh, this looks familiar,' and he thought that mammal-killing killer whales killed this," Filatova told Live Science. But for it to be an orca fin was a surprise.

Two years later, in July 2024, he found a second dorsal fin from an orca. This one was a bit bigger, from a young male, but it had the same killer whale tooth marks on it.

"At that moment, I started thinking that this is a pattern," Filatova said. The fins are tough and not good to eat and prevent a predator from eating the muscle and blubber under it, so killer whales discard them, she added. Genetic tests revealed that the fins came from resident orcas in the North Pacific.

Filatova and her colleagues think the resident orcas were probably attacked and eaten by Bigg's orcas.

"At least now we know that cannibalism happens, but I think it is not super common," Filatova said.

The researchers suggest that such occasional predation by the mammal-eating Bigg's orcas is a reason the resident orcas form large, close-knit family groups. Animals that aggregate in large groups or herds often do it to protect themselves from predators.

Orcas are generally thought to have no natural predators, but they have been known to be aggressive toward each other. In 2016, for example, Bigg's orcas were witnessed chasing and killing a newborn, potentially to force the mother to become sexually receptive. They didn't eat the calf, though.

Teaming up as a defense also may help to explain observations of large groups of resident killer whales chasing away smaller groups of Bigg's killer whales, Filatova said. She noted that, in her own work, she has seen evidence of Bigg's orcas avoiding groups of resident orcas and returning to an area only after the residents had moved on. "So, it looks like this defense strategy is really working," she said.

But not everyone is convinced. "I think the observations of tooth marks on fish-eating whale carcasses are interesting and the idea is worthy of further investigation, but there's not yet enough evidence to build a solid account of the social evolution of fish-eating orcas," Luke Rendell, a biologist at the University of St Andrews in Scotland who wasn't involved in the study, told Live Science via email.

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

Other animals have also been suspected of forming tight-knit groups to defend against orcas. For example, pods of long-finned pilot whales (Globicephala melas) are also known to confront and drive away killer whales ‪—‬ a behavior that is largely attributed to their highly social nature. And orcas sometimes flee when they hear the calls of pilot whales.

"The similarities between short-finned pilot whale social structure and resident killer whale social structure, and the similarities in how they apparently respond to Bigg's killer whales, do suggest they may both be responding to potential predation pressure," Michael Weiss, research director at the Center for Whale Research in Washington, who wasn't involved in the research, told Live Science via email.

"I certainly think it's possible that Bigg's killer whales predated on these two whales," he said. But he added that scavenging by Bigg's killer whales or aggression from other resident killer whales while they were still alive also could have caused the rake marks on the washed-up dorsal fins. Therefore, it doesn't definitively show cannibalism or predation, Weiss said.

Filatova acknowledged that scavenging can't be ruled out, because orcas are known to have fed on whale carcasses from whaling. But she said fresh killer whale carcasses typically sink fast, making them inaccessible, and they only start floating a few days later, when they start to decompose. "You need to be really hungry to eat this," she said.

Filatova also doesn't think the marks on the fins are related to fights with other residents, because those marks tend to be on the animals' sides, she said.

She thinks predation pressure drove the formation of tight-knit social groups in resident orcas perhaps 100,000 years ago, after killer whales that had been evolving separately in the Pacific and Atlantic started to bump into each other; because the social structure proved efficient, it stuck.

However, she pointed out that eating another orca may not seem like cannibalism to these marine mammals, and there are calls to name them as separate species. "They never socialize; they never spend time together. For them, it's just another whale. So why not eat it?" Filatova said.

Editor's note: This article was updated at 6.53 a.m. ET on March 9 to correct an error that stated the fins belonged to southern resident orcas. The fins were from resident orcas in the North Pacific.

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