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.
(Color online) Spectrograms of (a) the
stereotyped whistle of BD1 and (b) F13's imitation. BD1 emitted the
whistle in pairs
stereotypically, but F13 produced variable
numbers of whistles in the four sequences collected during the study.
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.
