Killer whales are among the few animals on Earth capable of vocal learning, or the ability to pick up new vocalizations by imitating someone else's. It's the basis for language, and it lets pods of killer whales — aka orcas — develop "dialects" that are likely passed down from generation to generation.

According to a new study, though, killer whales don't necessarily stop at imitating each other. They're also capable of learning a different species' language, the study's authors found, mimicking the clicks and whistles of bottlenose dolphins after spending time around them. 

Just six groups of animals are known to use vocal learning: parrots, songbirds, hummingbirds, bats, cetaceans and humans. Countless others vocalize, but their sounds are almost always innate, not learned. Many also use auditory learning to make associations with sounds, like a dog learning how to respond to the sound "sit." Only true vocal learners, however, can say "sit" after hearing it.

While orcas don't speak English yet, they apparently can speak bottlenose — albeit with an accent. They're actually a type of dolphin themselves; their ancestors are thought to have branched off from other oceanic dolphins several million years ago. All dolphins belong to a group of cetaceans known as toothed whales, as opposed to larger, filter-feeding baleen whales like humpbacks.

Normal orca communication is already elaborate, including clicks, whistles and pulsed calls. These vocalizations vary across pods and social groups, resulting in local dialects, but they're all still distinct from the calls made by other dolphins. And since a vocal-learning test typically requires placing animals in a novel social setting — thus prompting them to communicate in new ways — orcas who have spent time with bottlenose dolphins are in a unique position to reveal the depth of their species' social skills.

"We had a perfect opportunity because historically, some killer whales have been held with bottlenose dolphins," study co-author and marine biologist Ann Bowles says in a statement about the new research. "Killer whales seem to be really motivated to match the features of their social partners."

orca pod

Photo: Holly Fearnbach/NOAA

The researchers based their findings on three captive orcas who've spent extended periods of time with bottlenose dolphins. By studying old recordings of those animals' calls as well as the calls of orcas and bottlenose dolphins who lacked such cross-species exposure, they were able to test how much the orcas adjusted their own vocalizations to mimic their distantly related companions.

Those three orcas produced 17 times as many "click trains" and up to four times as many whistles, the researchers write, "making their relative usage of vocalization categories more similar to those of dolphin social partners." The acoustic features of their calls were also less distinguishable from those of bottlenose dolphins, and one of the orcas even learned to produce a novel chirp sequence that humans had taught to the bottlenose dolphins before she was introduced to them.

All three spoke bottlenose with an orca accent, though. They often whistled at lower rates than native speakers, and they mostly altered orca sounds to resemble bottlenose sounds rather than making totally new noises. One orca was better able to imitate the bottlenose calls, but even her attempts "contained abrupt steps in frequency that were not typical of the dolphin's stereotyped whistle." This may be because orcas have difficulty producing some bottlenose sounds, the researchers suggest.

(For what it's worth, captive bottlenose dolphins showed off a similar skill during a 2011 study. They were able to perform impressive imitations of humpback whale songs — but they did it literally in their sleep. And in the 1980s, a young beluga named "NOC" was reported to mimic human voices.)

bottlenose dolphin

Photo: Stephen McCulloch/HBOI/NOAA

The new study involved orcas in captivity, an increasingly controversial practice as evidence of their intelligence and social complexity piles up. Bowles is also a scientist at the Hubbs-SeaWorld Research Institute, an independent nonprofit arm of the SeaWorld theme parks criticized in the 2013 documentary "Blackfish." Yet the study was co-authored by researchers from the U.S. National Marine Mammal Laboratory and the University of San Diego, and published in the peer-reviewed Journal of the Acoustical Society of America. And while any use of captive orcas can be uncomfortable, this study offers potentially ground-breaking insight into these iconic but still mysterious mammals.

"There's been an idea for a long time that killer whales learn their dialect, but it isn't enough to say they all have different dialects so therefore they learn," Bowles says. "There needs to be some experimental proof so you can say how well they learn and what context promotes learning."

And beyond the captivity issue, the study's authors say there are urgent ecological reasons to investigate whales' and dolphins' vocal patterns. Orcas and many other marine mammals are threatened by a variety of human activities, including entanglement in fishing gear, boat strikes, water pollution, oil exploration and habitat loss due to man-made climate change. Depending how closely their social bonds are connected to the way they "talk," orcas' long-term success amid shifting territories and social groups may hinge on how well they can adapt their communication strategies.

"It's important to understand how they acquire [their vocalization patterns], and lifelong, to what degree they can change it, because there are a number of different [cetacean] populations on the decline right now," Bowles says. "And where killer whales go, we can expect other small whale species to go."

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