Human voices and birds’ sound production change with age. A new study of the zebra finch may advance understanding of the motor control of the vocal organ – among both healthy and sick people. Both neural circuit development and postnatal maturation of the body can dramatically change birds’ vocal behaviour. The study shows that the size and properties of birds’ vocal instrument – the syringeal skeleton and musculature – appear to remain unchanged throughout life.
During puberty, the human voice undergoes a transition mostly caused by the growing larynx. This process differs in birds, with the transition taking place almost explosively over weeks instead of years. This is one reason why studying songbirds is extremely useful for learning to understand not only how their voices develop but also how this happens with people – as a new study of zebra finches shows.
“Birds and humans are influenced by at least three possible physical changes. The brain’s neural circuitry changes, the muscles develop and then there is the vocal instrument itself – the syringeal vocal folds. We studied these three elements separately in zebra finches. The brain and muscles change, but the vocal folds remain the same. This new method may help us to understand how physical changes alter the sound and, in the long term, this may strengthen clinical application for human vocal cord surgery,” explains Coen Elemans, Associate Professor, Department of Biology, University of Southern Denmark, Odense.
Training is key
To separate the effect of changes in the brain and muscles from those of the zebra finch’s vocal organ – the syrinx – the researchers designed an experimental set-up to artificially create sound from a syrinx surgically removed from a bird. By subsequently analysing the sounds from the finch syrinxes from day 25 to day 100 of their lives, they established that the sound generated does not differ much in young and older zebra finches.
“The syrinx produces sound very efficiently, and important acoustic parameters such as frequency range do not change with age in either males or females. Our data therefore suggest that the observed acoustic changes in the vocal development of birdsong result from changes in the brain’s motor control pathway and muscle force rather than changes in the vocal folds in the birds’ syrinx,” says Coen Elemans.
Only male zebra finches sing beautifully, whereas the females only make calls, so the question was therefore how this great difference between the males and females arises, and here too the researchers are certain.
“We are currently investigating whether muscle training is the key to being able to sing and whether this is crucial for the males searching for a mate. Constant training may enable the males to sing loudly and also very precisely and complexly, and so the urge to find a female may drive the males to train their syringeal muscles very intensely,” explains Coen Elemans.
Will map the motor control system
According to the researchers, although comparing young male finches’ brief and intense singing training with teenage boys’ breaking voices does not immediately seem relevant, there are still many useful lessons.
“Zebra finches are one of the few animals that, like humans, have the ability to imitate sounds by learning. The vast majority of animals are born with fully developed vocalizations, but marine animals, elephants, bats and also songbirds have the unique ability to learn to make new sounds – just like humans. And since songbirds are easier to study than elephants or humans, they are one of the best models we have for learning speech acquisition,” says Coen Elemans.
In addition to examining how the zebra finches’ vocal organs physically produce sound, the research on the birds also provided an opportunity for studying vocal development at a much more advanced level. For example, the birds can be equipped with cameras to enable the researchers to relate sight, sound and brain activity. A major interdisciplinary study with collaborators from the University of Maine and Emory University in the United States and the University of Southern Denmark will therefore try to integrate physics, neuroscience and biomechanics.
“In our new project, we will create computer models based on experiments and observations that can explain the mechanisms involved in producing sound that can lead to the development of computer-controlled vocal cord surgery and other advances,” concludes Coen Elemans.