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Several years ago, scientists recorded the sounds of sea lions barking underwater in Nootka Sound off Vancouver Island in British Columbia.

At least they thought sea lions were making the sounds. But closer analysis revealed something strange. Some of the barks included harmonic frequencies over 10 kilohertz. Sea lions don’t usually produce harmonics over 4 kilohertz. And during many of the recordings, no sea lions were observed in the area. But a lone killer whale was.

Now, Andrew D. Foote of the University of Durham in Britain and colleagues say they are certain it was the whale that was doing the barking. The imitative behavior, they write in the journal Biology Letters, shows that the mammals (which are not whales but members of the dolphin family) are capable of vocal learning.

The killer whale in question, named Luna, was separated from its family group, or pod, at an early age. Pod members tend to have a repertoire of normal calls that they use to varying degrees. The researchers studied recordings of Luna and another separated killer whale named Springer, and found evidence in both animals that they used their repertoire of calls differently than the pod they were born into.

But it was the imitation barking by Luna (who was killed last March by a tugboat) that was most unusual. Many birds are capable of this kind of learning, but it is relatively rare in mammals. Most recently it was recognized in an elephant that imitated the sounds of trucks on a highway.