Bottlenose dolphins appear to whistle their “names” to each other, according to a new study. Researchers say that this type of referential communication is extremely rare in nature, and until now had not been clearly shown in a non-human animal.
Many animals communicate with sound, in some cases to convey distress or maintain contact in threatening environments. For example, all the penguins in a group will use the same call to maintain group cohesion.
Some marine mammals, such as humpback whales, are vocal underwater, but unlike the whistles made by dolphins, these songs are mating calls produced by the male whales to woo female partners and are similar within each group.
By contrast, the high-pitched whistles made by the dolphins are unique to each animal and last only about a second, explains marine mammal researcher Laela Sayigh of the University of North Carolina at Wilmington.
Each dolphin begins developing its own signature whistle during infancy and keeps it throughout its lifetime. Researchers had previously shown that dolphins respond more to the calls of their close kin. They hypothesised that the animals identify one another by the briefly whistled tunes.
But one question undermined this idea: What if dolphins simply identify one another by the resonance of the whistled sounds?
Sayigh likens this to recognising a voice on the phone before hearing the person’s name. Variations in the vocal tract give each person’s voice a characteristic sound. As a result it’s possible to identify different friends simply by hearing their “hello”.
Similarly, each dolphin’s blow hole produces a characteristic sound.
What’s in a name?
Researchers were eager to find out whether each dolphin has a “name” that is independent of its unique vocal features. So Sayigh and her colleagues recreated bottle-nosed dolphin signature tunes electronically – keeping the pitch changes while eliminating any voice features.
The team then played the synthetic tunes of relatives and close companions to 14 bottlenose dolphins underwater. (Listen to a real dolphin whistle, and a synthetic dolphin whistle.)
These dolphins are among a population of about 140 in Sarasota Bay off of the coast of Florida, US, that is extremely well studied by researchers. As a result, experts have quantified how much a given dolphin associates with others in the population.
Among the 14 dolphins studied, nine turned around more frequently when they heard the synthetic signature whistle of a relative than when they heard the synthesized whistle of an equally familiar companion.
Sayigh says that this supports the idea that dolphins do have whistled “names” that refer uniquely to each individual in the group.
Three of the animals showed no preference and two turned more frequently toward the synthetic signature whistle of their non-relatives.
Journal reference: Proceedings of the National Academy of Sciences (DOI: 10.1073/pnas.0509918103)