Orcas Are Considered One Species. Should They Be?

Killer whales are some of the most cosmopolitan creatures on the planet, swimming through every one of the world’s oceans. They patrol the frigid waters near both poles and periodically pop up in the tropics, in locations from western Africa to Hawaii.

Although their habitats and habits vary widely, all killer whales are considered part of a single, global species: Orcinus orca. (Despite their common name, killer whales are actually part of a family of marine mammals known as oceanic dolphins.)

Now, scientists have drawn upon decades of research to suggest that two killer whale populations often observed off the Pacific Coast of the United States and Canada are actually so different from each other — and from other orcas — that they should be considered separate species.

In a paper published in the journal Royal Society Open Science on Tuesday, the scientists proposed giving new species designations to two groups of animals, one known as resident killer whales and the other often called Bigg’s killer whales. Although both types live in the eastern North Pacific, they have different diets: the resident orcas eat fish, with a particular predilection for salmon, while the Bigg’s orcas hunt marine mammals such as seals and sea lions.

The proposal documents numerous other behavioral, physical and genetic differences between the two orca populations, which have been evolving away from each other for hundreds of thousands of years, the scientists noted.

“These two types are genetically two of the most distantly related types in the whole world,” said Phillip Morin, a geneticist at the Southwest Fisheries Science Center at the National Oceanic and Atmospheric Administration, or NOAA, and an author of the study. “They’re not just behaving differently. They really are on these evolutionary trajectories which we consider to be different species.”

There is no single definition for what qualifies as a species, and the lines between animal populations are often fuzzy. But these sorts of taxonomic distinctions can have implications for conservation, scientists said, allowing experts to make more informed decisions about how to manage different orca populations.

“They very much do face different threats,” said John K. Ford, an orca expert and scientist emeritus at Fisheries and Oceans Canada who was not an author of the new paper.

In recent decades, for instance, rebounding seal and sea lion numbers have helped fuel a population boom for the Bigg’s orcas, he said. Resident orcas, on the other hand, have been threatened by dwindling wild salmon stocks.

Dr. Ford said that the authors of the new paper made a “very strong case,” pulling together a growing body of evidence that the resident killer whales and Bigg’s killer whales are distinctly different creatures. “It’s these multiple lines of evidence all pointing in the same direction,” he said.

The next step will be to submit the proposal to a committee of taxonomy experts at the Society for Marine Mammalogy, which maintains “the most authoritative list” of species, Dr. Morin said.

In recent years, scientific advances have allowed scientists to conduct more sophisticated analyses of the orcas’ genomes. The data suggests that the Bigg’s killer whales branched off from other orcas between 200,000 and 300,000 years ago. The residents, for their part, diverged from other orcas about 100,000 years ago. Genetic and behavioral analyses also suggest that there has been little interbreeding between the Bigg’s orcas and the resident orcas in recent years.

“It’s very compelling evidence to suggest that they represent different species,” said Kim Parsons, a geneticist at NOAA’s Northwest Fisheries Science Center and an author of the study.

Overall, the genomes were different enough that the scientists could predict, with high accuracy, whether a killer whale was a Bigg’s orca or a resident orca based on its DNA alone.

Skull shape is similarly predictive. The Bigg’s orcas have larger, wider skulls, with more deeply curved jaws, than the residents do — traits that might help them wrangle their larger prey. The Bigg’s orcas are also slightly larger than the residents overall, with wider, more pointed dorsal fins and different black-and-white patch patterns.

There are behavioral differences, too. The resident orcas live in large, stable groups, and are known to be chatty, communicating readily as they pursue fish. The Bigg’s killer whales, on the other hand, live in smaller groups and hunt quietly. When they do vocalize, their whistles sound different from those of the residents.

The paper’s authors proposed giving the resident killer whales the new scientific name Orcinus ater. If the Society for Marine Mammalogy accepts the proposal, the scientists said that they planned to consult with Indigenous groups in the Pacific Northwest to select a new common name that reflects the orcas’ cultural importance.

The scientists suggested that the Bigg’s orcas keep that common name, which honors Michael Bigg, an influential orca researcher, but receive the new scientific name Orcinus rectipinnus.

Further analysis might reveal other orca populations that qualify as distinct species, the scientists said.

“There is so much diversity in the oceans that we don’t know about,” Dr. Morin said. “Even with animals that are the size of a school bus.”