Researchers studying how human infants develop speech take great interest in marmoset monkeys because of how they vocalize. Now researchers from the University of Southern Denmark and Princeton University can show that not only the brain but also the physical alteration of the larynx change how these monkeys vocalize.
How does the ability to speak develop? Is it social contact, the brain or gradual changes in the properties of the larynx?
According to a new study, the answer is all of the above.
Researchers from the University of Southern Denmark and Princeton University studied the vocal development of marmoset monkeys from infancy to adulthood. The research sheds new light on vocal development, not only among marmoset monkeys but also among people.
“We usually consider the brain the supreme organ controlling the development of all the other organs and functions, but this is not always the case. Increasing evidence indicates that neurology cannot ignore the rest of the body and its ability to solve problems independently of the brain,” explains Coen Elemans, Associate Professor, Department of Biology, University of Southern Denmark, Odense.
The study was recently published in Nature Communications.
Very few animals can learn to speak
Humans are almost entirely alone among animals in our ability to speak.
Only a few species of marine mammals, songbirds, bats and, according to Coen Elemans, three individual elephants are equipped with the brain, organs and experience to speak.
Human infants start vocalizing early. Then they catch the right frequencies, and over time they form words that eventually turn into sentences. This takes years to master.
Researchers need animal models to conduct experiments to study how speech develops or how diseases affect the ability to speak. However, this is easier said than done.
Our closest relatives, chimpanzees, cannot learn to speak, no matter how hard researchers try to teach them. Further, a chimpanzee will always end up sounding like a chimpanzee, even if it has never been in contact with other chimpanzees.
In other words, it cannot learn language.
People are different. A person who has never been in contact with other people will never learn to speak. Learning language requires social reinforcement from other people.
“There has always been debate about which animals can learn to speak and thus be good model animals for studying how people develop speech. If only mice could learn to speak, then we could use them to study how this kind of communication evolves, but unfortunately they cannot,” explains Coen Elemans.
Marmoset monkeys learn to speak through social reinforcement
A few years ago, researchers from Princeton University discovered that marmoset monkeys, like humans, significantly transform how they vocalize.
They make one kind of sound as infants and develop another kind as adults. More specifically, they develop the ability to vocalize some high-pitched calls as they get older.
The researchers from Princeton University showed in a previous study that developing these higher-pitched vocalizations requires social reinforcement from other marmoset monkeys.
“This shook up the research community, because now we might have a good model for studying how people develop speech,” says Coen Elemans.
Physiological changes lead to a shift in calls
In their new study, Coen Elemans and colleagues investigated what happens physiologically when the marmoset monkeys change as infants who make contact calls to their mother to producing louder vocalizations with higher pitch.
Most of their vocal development is actually caused by physiological changes in the larynx and not so much social behaviour or input from the brain. The larynx matures during puberty.
This finding is important because it provides an alternative explanation for how marmoset monkeys’ voices evolve from infancy to adulthood.
“This undermines previous results and is a very good example of how neuroscience often forgets that the brain is linked to a body that can develop and change behaviour,” says Coen Elemans.
Blowing air through the larynxes of dead marmoset monkeys
Coen Elemans and colleagues recorded the cries of marmoset monkeys to identify their various acoustic features so that the cries could be recreated in the laboratory.
They then closely examined larynxes extracted from dead marmoset monkeys at various development stages.
The researchers used an air supply system to blow air through these larynxes and manipulated the various muscles electrically to produce the same cries the marmoset monkeys vocalized naturally. They also recorded the larynx movements with a high-speed camera.
The recordings showed that the infants create their earliest cries by vibrating the vocal chords, whereas the high pitches at later stages were created by getting the membranes at the apex of the larynx to vibrate.
This discovery suggests that the body itself, and not the brain, causes the transition from one way of communicating to another.
Difficult to conduct similar experiments on people
The study suggests that speech develops in different ways throughout life.
Some aspects are controlled by the brain and others by social contact with parents and others. But physiological changes also play a role.
According to Coen Elemans, continuing to study marmoset monkeys is still interesting because conducting similar experiments with people is very difficult.
Larynxes from adult humans who have died could be studied, but studying larynxes from children at various stages of development is far more difficult.
“Marmoset monkeys are therefore still a good model for studying how people develop the ability to speak,” says Coen Elemans.
“Vocal state change through laryngeal development” has been published in Nature Communications. In 2017, the Novo Nordisk Foundation awarded a grant to co-author Coen Elemans for the project Give Voice to Your Body: Decoding Vocal Motor Control.