Additionally, the team says, these barks were interspersed with recognizable killer whale calls or echolocation clicks and occurred when no sea lions were present.
A killer whale call sounding like a bark had previously never been reported.
The team says Luna's barks are examples of vocal learning, whereby an animal is able to alter the way it makes sounds after listening to other noises.
Unequivocal evidence for vocal learning, the study authors write, "can include the production of novel sounds not found in the animal's natural repertoire."
Rare among animals, vocal learning is known in humans, dolphins, elephants, and certain species of birds.
Luna's barking was likely learned from the sea lions he was frequently seen around, Foote says.
"He was exposed to the sea lions for several weeks at a time and often interacted with them, which may have triggered the mimicry," he said.
Foote adds that tapes of the solitary killer whale revealed another type of unusual call, which may have had a human source.
"The call type apparently resembles the whistle of a local fisherman who used to whistle to Luna when he saw him."
The study suggests that Luna also mimicked other killer whales he occasionally came across. Sound recordings link his calls to a different pod than those of the one he was born into.
Luna, which was born in 1999, may have become separated from his natal pod before he had time to learn the calls that identify it, Foote says.
"Each pod has a unique call repertoire," Foote said. "Although most calls are used by all the pods occasionally, many of the calls are predominantly used by one pod."
The study suggests that vocal learning may enable killer whales to develop and learn specific calls that help to strengthen family bonds and keep a pod together through group recognition.
A similar explanation has been proposed for other animal mimics that show evidence of vocal learning.
A study published last year in the journal Nature reported on an orphaned female elephant from a wildlife reserve in Kenya, East Africa. She mimicked the sound of rumbling traffic on a nearby highway.
Elephants "may well be able to produce unique calls, like a vocal signature, that are particular to that individual or to its family or to very closely bonded individuals," said lead researcher Joyce Poole, scientific director of the Amboseli Elephant Research Project in Kenya.
Poole says this ability may be particularly important to animals like elephants that often communicate over long distances.
Another study, which appeared last May in the journal Proceedings of the National Academy of Sciences, suggests that bottlenose dolphins use vocal learning to create distinctive whistles.
These so-called signature whistles are thought to act like names, helping bottlenose dolphins locate and identify others in their group.
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