Killer whale fins that wash up in the North Pacific are scratched with distinctive tooth marks that indicate killer whales are sometimes cannibals. Scientists say this may explain why some killer whales live in large family groups.
Orcas (Orcinus orca) come in several different types, sometimes considered different subspecies. In the North Pacific, two of these types live in roughly the same areas: Resident killer whales (Orcinus orca eats) live in large family groups and eat fish, and Bigg’s killer whales (Orcinus orca rectipinnus), which are more common and transient, live in smaller groups and hunt other mammals, such as whales, dolphins and seals.
In August 2022, study co-author Sergey Fomina 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 bloody and covered with teeth marks.
It is not so unusual to find fins with such tooth marks. But previously such fins had belonged to Baird’s beaked whales (Berardius bairdii) and minke whales (Balaenoptera acutorostrata) who had been attacked and eaten by Bigg’s killer whales.
“He immediately thought, ‘Oh, this looks familiar, and he thought mammal killer killer whales killed this,'” Filatova told Live Science. But being a killer whale was a surprise.
Two years later, in July 2024, he found a second dorsal fin from a killer whale. This one was slightly larger, from a young male, but it had the same orca teeth 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 underneath, so orcas discard them, she added.
Genetic testing revealed that the fins came from southern-dwelling killer whales, which live in waters near Washington and British Columbia and are known for with salmon on the head and giving each other a massage with kelp.
So it seems this defense strategy really works
Olga Filatova, cetacean researcher at the University of Southern Denmark
Filatova and her colleagues believe the southern killer whales were probably attacked and eaten by Bigg’s killer whales.
“At least now we know that cannibalism happens, but I think it’s not super common,” Filatova said.
The researchers suggest that such occasional predation by the mammal-eating Bigg’s killer whales is one reason why the killer whales that live here form large, tightly knit family groups. Animals that gather in large groups or herds often do so to protect themselves from predators.
Orcas are generally thought to have no natural predators, but they have been known to be aggressive towards each other. In 2016, for example, Bigg’s killer whales were witnessed chasing and kill a newbornpotentially to force the mother to become sexually receptive. However, they did not eat the calf.
Banding together as a defense may also help 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 that Bigg’s killer whales avoid groups of resident killer whales and return to an area only after the residents have moved on. “So it seems 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 is not yet enough evidence to build a solid account of the social evolution of fish-eating killer whales,” Luke Rendella biologist at the University of St Andrews in Scotland who was not involved in the study told LiveScience via email.
Rendell said the potential benefits of foraging together and imparting specific knowledge about habitat and prey could also be important drivers of creating large groups linked to specific locations.
Other animals have also been suspected of forming tight groups to defend themselves against killer whales. For example, pods of long-finned pilot whales (Globicephala melas) is also known to confront and drive away killer whales – a behavior which largely attributed to their highly social nature. And killer whales sometimes flee when they hear calls from pilot whales.
“The similarities between the social structure of short-finned pilot whales and the social structure of resident killer whales, and the similarities in how they apparently respond to Bigg’s killer whales, suggest that they may both respond to potential predation pressure.” Michael Weissdirector of research at the Center for Whale Research in Washington, who was not involved in the research, told LiveScience via email.
“I certainly think it’s possible that Bigg’s killer whales went ahead of these two whales,” he said. But he added that the capture of Bigg’s killer whales or aggression from other resident killer whales while still alive could also have caused the tear marks on the washed-up dorsal fins. Therefore, it does not definitively show cannibalism or predation, Weiss said.
Filatova acknowledged that purification cannot be ruled out, because Killer whales are known to have fed on whale carcasses from whaling. But she said fresh killer whale carcasses usually sink quickly, making them inaccessible, and they don’t start floating until a few days later, when they begin to decompose. “You must be very 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 believes predation pressure drove the formation of dense social groups in resident killer whales perhaps 100,000 years ago, after killer whales that had evolved separately in the Pacific and Atlantic oceans started bumping into each other; because the social structure proved effective, it stuck.
However, she pointed out that eating another killer whale may not seem like cannibalism to these marine mammals, and it is calls for naming them as separate species. “They never socialize, they never spend time together. To them, it’s just another whale. So why not eat it?” said Filatova.
Filatova, OA, Fedutin, ID, & Fomin, SV (2026). Predation by mammal-eating Bigg’s killer whales (Orcinus orca rectipinnus) may shape the unique social structure of “resident” fish-eating killer whales (O. o. ater) in the North Pacific. Marine Mammal Science, 42(2). https://doi.org/10.1111/mms.70142
