A new study led by Dr Sabrina Engesser from the University of Zurich, Switzerland, has revealed that the chestnut-crowned babbler (Pomatostomus ruficeps) – a highly cooperative bird of the Australian arid zone – is able to generate new meaning by rearranging combinations of meaningless sounds in its calls. This babbler bird communication is reminiscent of the way humans form meaningful words.
Chestnut-crowned babblers (Pomatostomus ruficeps), SW Queensland, Australia. Image credit: Aviceda / CC BY-SA 3.0.
“Although previous studies indicate that animals, particularly birds, are capable of stringing different sounds together as part of a complex song, these songs generally lack a specific meaning and changing the arrangement of sounds within a song does not seem to alter its overall message,” said Dr Engesser, who is the first author of a paper published online in the journal PLoS Biology.
“In contrast to most songbirds, chestnut-crowned babblers do not sing. Instead its extensive vocal repertoire is characterized by discrete calls made up of smaller acoustically distinct individual sounds.”
Prof Andy Russell from the University of Exeter, UK, a co-author on the study: “we think that babbler birds may choose to rearrange sounds to code new meaning because doing so through combining two existing sounds is quicker than evolving a new sound altogether.”
The scientists noticed that chestnut-crowned babblers reused two sounds (A and B) in different arrangements when performing specific behaviors.
When flying, the birds produced a flight call AB, but when feeding chicks in the nest they emitted BAB prompt calls.
When the team played the sounds back, the listening birds showed they were capable of discriminating between the different call types by looking at the nests when they heard a feeding prompt call and by looking out for incoming birds when they heard a flight call.
This was also the case when the scientists switched elements between the two calls: making flight calls from prompt elements and prompt calls from flight elements, indicating that the two calls were indeed generated from rearrangements of the same sounds.
“This is the first time that the capacity to generate new meaning from rearranging meaningless elements has been shown to exist outside of humans. Although the two babbler bird calls are structurally very similar, they are produced in totally different behavioral contexts and listening birds are capable of picking up on this,” said study senior author Dr Simon Townsend of the University of Zurich.
The team reports that in the chestnut-crowned babbler, the first sound element B is what seems to differentiate the meaning between flight and prompt vocalizations, akin to cat and at in English, where the c represents the meaning differentiating element.
“Although this so-called phoneme structuring is of a very simple kind, it might help us understand how the ability to generate new meaning initially evolved in human. It could be that when phoneme structuring first got off the ground in our hominid ancestors, this is the form it initially took,” Dr Townsend said.
_____
Engesser S et al. 2015. Experimental Evidence for Phonemic Contrasts in a Nonhuman Vocal System. PLoS Biol 13 (6): e1002171; doi: 10.1371/journal.pbio.1002171
