Cannibalism may explain why some killer whales stay in family groups

Biologists have seen signs of orca-on-orca predation in the North Pacific, and such cannibalism may explain why some killer whales travel in large family groups.

Two distinct subspecies of killer whales, also called killer whales (Orcinus orca), are located in the North Pacific. Transient or Bigg’s killer whales, as their name suggests, are nomadic and gather in liquid hunting pods to hunt seals, dolphins and other cetaceans. In contrast, resident killer whales live in large groups based on maternal bonds and stay with the same family throughout their lives. Residents disperse and feed on fish individually and return together when resting or traveling.

The two subspecies are thought to rarely interact, but Sergey Fomin of the Pacific Geographical Institute in Russia has seen evidence of violent encounters. Walking along the beach on Bering Island in eastern Russia, he occasionally finds the gnawed dorsal fins of Baird’s whales and minke whales — animals that have fallen prey to voracious killer whales. However, in the summer of 2022, he was surprised to find a killer whale’s fin on the beach, bloodied and covered in teeth marks. Two years later he found another.

Genetic testing revealed that the fins came from southern killer whales. Fomin and his colleagues believe that whales likely ate Bigg’s killer whales.

Most toothed whales are organized as transient killer whales that form pods that can change over time. Why southerners form large family groups has been a scientific conundrum. “I’ve been thinking about the social structure for a while because it’s quite unique and there aren’t that many species that have something like that,” he says. Olga Filatova at the University of Southern Denmark.

Once she learned about the two dorsal fins and the likely cannibalism, it clicked: perhaps the resident killer whales stay in large groups for safety in numbers. Together with Fomin and her colleague Ivan Fedutin, she published an article outlining this idea.

As apex predators, killer whales are rarely bullied, but have been known to be driven off by mobbing groups of smaller pilot whales. And they can be aggressive towards each other: in 2016 Jared Towers at the marine research firm Bay Cetology in Canada witnessed Bigg’s killer whales chasing a pod and kill newborns. This was likely to force the mother to become sexually receptive, Towers explains, because the calf was not eaten.

Towers agrees that the population’s unique social structure is likely for defense, though he’s not so sure that Bering Island whales were cannibalized. Scientists cannot rule out that the fins were accidentally torn off by sparring or that the whales died and were eaten postmortem. However, this is less likely because killer whales sink once.

Researchers can only speculate as to why killer whales might turn to cannibalism, but Filatova speculates that it could just be a matter of circumstance. On Bering Island, seals and sea lions are common food sources, but when nesting sites are empty, whales may turn to other prey. “If they can’t find any food and there’s a young tasty killer whale alone, why not?” she says.

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