Cannibalism may explain why some orcas stay in family groups

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

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

The two subspecies are thought to rarely interact, but Sergey Fomin of the Institute of Pacific Geography in Russia has seen evidence of violent encounters. Walking 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 voracious orcas. In the summer of 2022, however, he was surprised to find an orca fin on the beach, bloody and covered with teeth marks. Two years later, he found another.

Genetic testing revealed that the flippers came from southern resident orcas. Fomin and his colleagues speculate that the whales were likely eaten by Bigg’s orcas.

Most toothed whales are organized like transient orcas, forming groups that can change over time. Why southerners form large family groups remains a scientific enigma. “I wondered for a while about the social structure, because it is quite unique and there are not many species that have something like that,” says Olga Filatova from the University of Southern Denmark.

Once she heard about the two dorsal fins and the likely cannibalism, it clicked: perhaps the resident orcas stay in large groups for reasons of safety in numbers. Together with Fomin and her colleague Ivan Fedutin, she published a paper outlining this idea.

As large predators, orcas are rarely intimidated, but they have been known to be hunted by groups of smaller pilot whales. And they can be aggressive toward each other: In 2016, Jared Towers of the marine research company Bay Cetology in Canada witnessed a pod of Bigg’s orcas chasing and killing a newborn calf. This was probably to force the mother to become sexually receptive, Towers explains, because the calf was not eaten.

Towers agrees that the inhabitants’ unique social structure is likely for defense, although it is less certain that Bering Island’s whales were cannibalized. Researchers cannot rule out that the fins were torn off by exercising podmates, or that the whales died and were nibbled on postmortem. This is less likely, however, as orcas sink once they die.

Researchers can only speculate about why orcas might turn to cannibalism, but Filatova speculates that it might simply be a matter of circumstance. On Bering Island, common food sources are sea lions and sea lions, but when colonies are empty, whales may turn to other prey. “If they can’t find food and there’s a lone, tasty young resident killer whale, why not? » she said.