Killer whales can be vulnerable to predation from other killer whales
Francois Gohie/VWPics/Alamy
Biologists have seen signs of killer whale 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), found in the North Pacific. Transient or Bigg’s killer whales, as the name suggests, are nomadic and gather in floating pods to hunt seals, dolphins and other cetaceans. In contrast, killer whales live in large groups based on maternal bonds, staying with the same family throughout their lives. Residents will disperse and eat fish individually and regroup when resting or traveling.
The two subspecies are thought to rarely interact, but Sergey Fomin at the Pacific Institute of Geography in Russia has seen evidence of violent encounters. Strolling along the beach on Bering Island in eastern Russia, he sometimes finds the gnawed dorsal fins of Baird’s beaked whales and minke whales—animals that have fallen prey to ravenous killer whales. In the summer of 2022, however, he was surprised to find an orca fin on the beach, bloody and covered in teeth marks. Two years later he found another.
Genetic testing showed that the fins came from killer whales in the south. Fomin and his colleagues hypothesize that the whales were probably eaten by Bigg’s killer whales.
Most toothed whales are organized as transient killer whales, forming pods that can change over time. Why southerners form large family groups has been a scientific puzzle. “I wondered about the social structure for a while because it’s quite unique, and there aren’t that many species that have something like this,” says Olga Filatova at the University of Southern Denmark.
Once she learned about the two dorsal fins and the likely cannibalism, it clicked: perhaps resident orcas stay in large groups for safety in numbers. Together with Fomin and her colleague Ivan Fedutin, she has published a paper outlining this idea.
As top predators, killer whales are rarely bullied, but they have been known to be chased away by bullying groups of smaller pilot whales. And they can be aggressive towards each other: In 2016, Jared Towers of the marine research firm Bay Cetology in Canada witnessed a pod of Bigg’s orcas chasing and killing a newborn. This was probably to force the mother to become sexually receptive, Towers explains, as the calf was not eaten.
Towers agrees that the residents’ unique social structure is likely a defense, although he is less certain that the Bering Island whales were cannibalized. Scientists cannot rule out that the fins were torn off by sparring podmates, or that the whales died and were cannibalized post-mortem. However, this is less likely, as killer whales sink once dead.
Scientists can only speculate as to why killer whales might turn to cannibalism, but Filatova surmises that it may just be a matter of circumstance. On Bering Island, common food sources are fur seals and sea lions, but when the mountains are empty, the whales can turn to other prey. “If they can’t find food, and it’s a young tasty killer whale alone, why not?” she says.
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