by Justine Alford
Photo credit: nix6658, "ORCA" via Flickr. CC BY-NC-ND 2.0.
Although many species communicate acoustically, the vast majority of animals use a genetically innate repertoire of sounds to exchange information. But some species, including humans, are capable of imitating sounds and adding it to their own repertoire, a process known as vocal learning. It is thought that the acquisition of this ability may have been a first step in the evolution of human language.
Although this trait is extremely rare, it is not unique to humans and has been discovered in 6 groups of animals: 3 groups of birds and 3 groups of mammals. Now, thanks to new research, we know that killer whales are capable of cross-species vocal learning. When socialized with bottlenose dolphins in captivity, the team discovered that they transitioned from their typical vocalizations and emitted more dolphin-like noises. According to the researchers, this suggests that cetaceans (dolphins, whales and porpoises) may use this trait to facilitate social interactions. The work has been published in The Journal of the Acoustical Society of America.
Killer whales, or orcas, emit three main types of vocalization: clicks, pulsed calls and whistles, with pulsed calls being the dominant form of communication. These are known to vary across social groups in terms of duration and pitch, but killer whales living together tend to produce similar calls that are distinct to that particular group, which is known as a dialect. Researchers had their suspicions that killer whales learn this dialect, but there was no experimental proof. Since dolphins produce similar vocalizations to orcas, and the two are sometimes housed together in captivity, the researchers took this unique opportunity to investigate whether killer whales could learn vocalizations from their cross-species social partners.
For the study, the researchers analyzed recordings of vocalizations produced by 10 captive orcas; three of these lived with bottlenose dolphins for several years, whereas the rest were housed with their own species. They then compared these recordings with the vocalizations produced by the dolphins.
They found that 95% of the 1551 vocalizations made by the seven orcas that were living with members of their own species were the typical pulsed calls that dominate their repertoire. The orcas that were living with dolphins, however, emitted far more whistles and clicks, just like their cross-species social partners. Intriguingly, they found that one of the killer whales even learned how to produce an artificial chirp sequence that a human trainer had taught the dolphins before they were introduced to each other.
According to the researchers, this demonstration of cross-species vocal learning suggests that orcas have substantial vocal plasticity and are highly motivated to match the vocalizations of their social associates. This is important because the fate of orcas in groups disrupted by change, such as oil spills, will be partly dependent on their ability to socialize with new populations and thus their vocal plasticity.
Killer Whales Learn to Communicate Like Dolphins
Killer whales living with bottlenose dolphins demonstrate cross-species vocal learning
Newswise — WASHINGTON, D.C., October 7, 2014--From barks to gobbles, the sounds that most animals use to communicate are innate, not learned. However, a few species, including humans, can imitate new sounds and use them in appropriate social contexts. This ability, known as vocal learning, is one of the underpinnings of language.
Vocal learning has also been observed in bats, some birds, and cetaceans, a group that includes whales and dolphins. But while avian researchers have characterized vocal learning in songbirds down to specific neural pathways, studying the trait in large marine animals has presented more of a challenge.
Now, University of San Diego graduate student Whitney Musser and Hubbs-SeaWorld Research Institute senior research scientist Dr. Ann Bowles have found that killer whales (Orcinus orca) can engage in cross-species vocal learning: when socialized with bottlenose dolphins, they shifted the types of sounds they made to more closely match their social partners. The results, published in The Journal of the Acoustical Society of America, suggest that vocal imitation may facilitate social interactions in cetaceans.
Killer whales have complex vocal repertoires made up of clicks, whistles and pulsed calls—repeated brief bursts of sound punctuated with silence. The acoustic features of these vocalizations, such as their duration, pitch and pulse pattern, vary across social groups. Whales that are closely related or live together produce similar pulsed calls that carry vocal characteristics distinct to the group, known as a dialect.
“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. There needs to be some experimental proof so you can say how well they learn and what context promotes learning,” said Bowles.
Testing vocal learning ability in social mammals usually requires observing the animal in a novel social situation, one that might stimulate them to communicate in new ways. Bottlenose dolphins provide a useful comparison species in this respect: they make generally similar sounds but produce them in different proportions, relying more on clicks and whistles than the pulsed calls that dominate killer whale communication.
“We had a perfect opportunity because historically, some killer whales have been held with bottlenose dolphins,” said Bowles. By comparing old recordings of vocalization patterns from the cross-socialized subjects with recordings of killer whales and bottlenose dolphins housed in same-species groups, Bowles and her team were able to evaluate the degree to which killer whales learned vocalization patterns from their cross-species social partners.
All three killer whales that had been housed with dolphins for several years shifted the proportions of different call types in their repertoire to more closely match the distribution found in dolphins—they produced more clicks and whistles and fewer pulsed calls. The researchers also found evidence that killer whales can learn completely new sounds: one killer whale that was living with dolphins at the time of the experiment learned to produce a chirp sequence that human caretakers had taught to her dolphin pool-mates before she was introduced to them.
Vocal learning skills alone don’t necessarily mean that killer whales have language in the same way that humans do. However, they do indicate a high level of neural plasticity, the ability to change circuits in the brain to incorporate new information. “Killer whales seem to be really motivated to match the features of their social partners,” said Bowles, though the adaptive significance of the behavior is not yet known.
There are immediate reasons to study the vocal patterns of cetaceans: these marine mammals are threatened by human activities through competition for fishery resources, entanglement in fishing gear, collisions with vessels, exposure to pollutants and oil spills and, ultimately, shrinking habitats due to anthropogenic climate change. If their social bonds are closely linked to their vocalizations, killer whales’ ability to survive amidst shifting territories and social groups may be tied to their ability to 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,” said Bowles. “And where killer whales go, we can expect other small whale species to go—it’s a broader question.”
The article, "Differences in acoustic features of vocalizations produced by killer whales cross-socialized with bottlenose dolphins," is authored by Whitney B. Musser, Ann E. Bowles, Dawn M. Grebner, and Jessica L. Crance. It will be published in The Journal of the Acoustical Society of America on October 7, 2014 (DOI: 10.1121/1.4893906). After that date, it can be accessed at: http://scitation.aip.org/content/asa/journal/jasa/136/4/10.1121/1.4893906.
The authors of this paper are affiliated with the University of San Diego, the Hubbs-SeaWorld Research Institute in San Diego, and the National Marine Mammal Laboratory.