In 2022, a Russian whale researcher made a startling discovery on Bering Island off Russia’s Pacific coast: a severed killer whale fin marked with the teeth of another killer whale. Two years later, it happened again, just two kilometres from the original find. 

The evidence initially pointed to a whale on whale event, suggesting that killer whales engage in cannibalism. But on closer observation of the killer whales’ social groups, researchers discovered they were witnessing an evolutionary process in motion.

Bering Island is home to groups of resident killer whales that feed on fish and are characterised by their exceptionally strong family structure. Each family is led by a female and may include up to four generations of descendants. Both sons and daughters remain with their mother’s group for life, leaving only for a few hours to mate before returning.

These waters are also home to transient groups, who live in smaller, looser social units and hunt other marine mammals such as seals, sea lions, porpoises and other whales. Despite living in the same area, and technically belonging to the same species, the two groups of whales never mix, socialise or interbreed.

“In the real world, killer whales are so different from each other that many researchers argue they should be divided into several subspecies,” explains Olga Filatova, associate professor at the Department of Biology at University of Southern Denmark.

“The hunting killer whales”, added Filatova, “most likely do not perceive the ones they feed on as belonging to their own species.”

Together with Fomin and their colleague Ivan Fedutin, who conducts research both at the Department of Biology and at the combined tourist attraction and research institution Fjord&Bælt in Kerteminde, Denmark, she has published a scientific article in Marine Mammal Science describing the two fins and discussing what the discoveries can tell us.

The researchers said they cannot necessarily rule out the fact that the whales could have died of other causes before being eaten, but they say this is unlikely since dead killer whales tend to sink very quickly. Active hunting is the much more likely explanation, they added.

Hunting killer whales always eat the animals they kill, typically leaving behind only a few low-energy parts, like the fins of the animal.

“If it was just aggression, they wouldn’t bother to tear off the fin,” said Filatova.

Resident killer whales’ family members occasionally disperse for an hour or two to seek contact with other resident families before returning to their families. Multiple families gather in large numbers which provides opportunities for young females and males to mate outside their own family. 

The events span several square kilometres, meaning family members are further apart than usual, and exposing them to attacks from hunting killer whales.

One such gathering took place just days before Fomin discovered one of the severed fins with bite marks.

These groups may have first encountered each other hundreds of thousands of years ago, when the specialised mammal-hunting killer whales arrived from the Atlantic when the waters became ice-free. 

Researchers believe that resident killer whales’s close-knit family structure may have developed as an evolutionary necessity to provide protection and increase chances of survival.

“We are witnessing an evolutionary process: These two groups, which never mix, are becoming increasingly distinct. At some point, they will be so different that they will become separate species,” said Filatova.

Researchers say it’s not just these two groups that may deserve their own subspecies names. There’s great variation in killer whales across the world: some feed almost exclusively on minke whales, others on seals, Antarctic toothfish, tuna, penguins, or sharks.

Their habitats also vary widely: some are coast-dwellers, some prefer the open seas, and others have no fixed habitat at all. 

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