Are orcas really a single species? Maybe not – 04/02/2024 – Science

Orcas are some of the most cosmopolitan creatures on the planet, swimming throughout the world’s oceans. They appear in the icy waters near the poles and sometimes appear in the tropics, in places ranging from west Africa to Hawaii.

Although their habitats and habits vary widely, all orcas are considered part of a single species: Orcinus orca —despite also being called the killer whale, it is part of a family of dolphins.

Now, scientists have drawn on decades of research to suggest that two orca populations commonly 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 an article published in the journal Royal Society Open Science last Tuesday (26), scientists proposed giving new species designations to two groups: resident orca and Bigg’s orca. Although they both live in the eastern North Pacific, they have different diets: the resident eats fish, with a predilection for salmon, and the Bigg hunts animals such as seals and sea lions.

The proposal presents other behavioral, physical and genetic differences between the two orca populations, which have evolved separately from each other for hundreds of thousands of years.

“They’re not just behaving differently, they’re on evolutionary trajectories that we consider to be different species,” said Phillip Morin, a geneticist at the National Oceanic and Atmospheric Administration’s (NOAA) Southwest Fisheries Science Center. ) and author of the study.

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

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

In recent decades, for example, increasing numbers of seals and sea lions have helped drive a population boom for Bigg’s orcas, according to Ford. On the other hand, residents find themselves threatened by the decline in wild salmon stocks.

For Ford, the authors of the new paper made a very strong argument, bringing together a growing body of evidence that the Residents and the Biggs are different creatures.

The next step will be to submit the proposal to a committee of taxonomy experts from the Society for Marine Mastozoology, which maintains the most authoritative list of species, according to Morin.

In recent years, scientific advances have allowed scientists to perform more sophisticated analyzes of orca genomes. The data suggests that Biggs branched off from other orcas between 200,000 and 300,000 years ago. Residents, in turn, diverged from other orcas about 100,000 years ago. Genetic and behavioral analyzes also suggest that there has been little interbreeding between Biggs and residents 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 on the study.

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

Skull shape is equally predictive. Biggs have larger, wider skulls with more deeply curved jaws than their resident counterparts — features that may help them handle their larger prey.

Biggs are also slightly larger than general residents, with wider, more pointed dorsal fins and different black and white spot patterns.

There are also behavioral differences. Residents live in large, stable groups and are known to be chatty, communicating as they chase fish. The Bigg, on the other hand, live in smaller groups and hunt silently. When they vocalize, their whistles sound different from those of the residents.

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

Scientists have suggested that the Biggs keep their common name, which honors Michael Bigg, an influential orca researcher, but receive the new scientific name Orcinus rectipinus.

Additional analysis may reveal other orca populations that qualify as distinct species. “There is so much diversity in the oceans that we don’t know about,” Morin said. “Even with animals the size of a school bus.”