issue samples from 139 killer whales from around the world point to at least three distinct species, say researchers in the journal Genome Research.

Researchers had suspected this may be the case. The distinctive black-and-white or grey-and-white mammals have subtle differences in their markings and also in feeding behaviour.

But until now DNA analysis has been inconclusive because of the inability to map the entire genetic picture, or genome, of the whales' mitochondria, an organelle within the cell inherited from the mother.

Phillip Morin, lead author and geneticist at NOAA's Southwest Fisheries Science Center in La Jolla, Calif., said: "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.

"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."

In all, tissue samples from 139 killer whales were analysed. Samples came from killer whales found in the North Pacific, the North Atlantic and oceans surrounding Antarctica.

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