The discovery comes after two severed fins of killer whales were found on a remote Russian island. According to a recent study, both fins carried clear tooth marks from other killer whales, raising concerns that some orcas may be attacking members of their own species.

Dr. Olga Filatova, an associate professor at the University of Southern Denmark, said she first saw photographs of the fins sent by her long-time collaborator Sergey Fomin. She said the discovery was unexpected.

“When he found the first one, it was very surprising,” Filatova told BBC Science Focus, adding that scientists never expected to witness such behaviour in their lifetime.

Genetic tests showed the fins belonged to resident killer whales, a population that mainly eats fish and lives in close family groups spanning several generations. Researchers believe the likely attackers were transient killer whales, another group that lives in the same waters but hunts marine mammals such as seals, sea lions and even other whales.

Scientifically, the incident could be considered cannibalism because all killer whales belong to a single species, ‘Orcinus orca’. However, Filatova noted that resident and transient killer whales rarely mix, do not interbreed and may not even recognise each other as the same type of animal.

Researchers believe the attacks may have occurred during large gatherings when many resident whale families meet for breeding. During these events, young whales can become separated from their family groups, making them easier targets for mammal-hunting orcas.

Both fins belonged to young whales. Scientists also noted that predators often remove fins when hunting to reach the flesh underneath.

While the findings are based on only two fins, researchers say the evidence could help explain why resident killer whales maintain such strong family bonds, possibly as protection against predators from other killer whale groups.