They may all look similar, but killer whales, also known as orcas, include several distinct species, according to a new genetic study.
Tissue samples from 139 killer whales from around the world point to at least three distinct species, the researchers report in the journal Genome Research.
Researchers had suspected this may be the case - the distinctive black-and-white or gray-and-white mammals have subtle differences in their markings and also in feeding behaviour.
Orcas as a group are not considered an endangered species, but some designated populations of the predators are. A new species designation could change this and affect conservation efforts.
One of the newly designated species preys on seals in the Antarctic while another eats fish, says Phillip Morin of the US National Oceanic and Atmospheric Administration's Southwest Fisheries Science Center in La Jolla, California, who led the research.
His team sequenced the DNA from the whales' mitochondria, a part of the cell that holds just a portion of the DNA. Mitochondrial DNA is passed down with very few changes from mother to offspring.
New sequencing methods finally made it possible to do so, says Morin.
"The genetic makeup of mitochondria in killer whales, like other cetaceans, changes very little over time, which makes it difficult to detect any differentiation in recently evolved species without looking at the entire genome," he says.
"But by using a relatively new method called highly parallel sequencing to map the entire genome of the cell's mitochondria from a worldwide sample of killer whales, we were able to see clear differences among the species."
The 139 whales whose DNA was sequenced came from the North Pacific, the North Atlantic and Antarctica.
The genetic evidence suggests two different species in Antarctica and also separates out mammal-eating "transient" killer whales in the North Pacific.
Other types of orca may also be separate species or subspecies, but it will take additional analysis to be sure, the researchers say.
NOAA has designated a population of killer whales that lives in the Pacific off the coast of Washington state as endangered.