When a human baby is born, its first cry is a normal sign of good health. Having never taken a breath before, the baby signals its first inhalation and exhalation—in the form of a screech.
How do babies know to create a sound they’ve never made before? And is their first yelp truly the start of speech development?
As it turns out, human babies may be practicing how to cry long before they ever make a sound. That is, if they’re anything like marmosets, humans’ primate cousins.
In a recent study published in the journal eLife, scientists used dozens of successive ultrasounds of pregnant common marmosets to show that their fetuses began making cry-like facial expressions nearly two months before birth.
The researchers were also able to separate these expressions from other mouth movements the babies performed in utero, and they matched them to the face shapes made after birth when the infants begin calling out to their parents. The expressions appeared in such consistent patterns and durations that researchers had strong confidence they were practice cries, even though the animals weren’t yet capable of making sounds.
Marmosets are among the smallest of monkey species, with adults weighing just 8 to 9 ounces, or about as much as a cup of sugar. There are more than 20 marmoset species, all native to South America. Despite being physically different from humans, they are primates, which means they are much more closely related to Homo sapiens and more instructive in understanding human development and behavior than more common research subjects like lab mice.
Back in the 1970s and 1980s, some ultrasound studies of pregnant women appeared to show babies making faces consistent with crying while still in the womb, says Daniel Takahashi, a co-author of the study and an animal behaviorist at the Brain Institute at the Federal University of Rio Grande do Norte in Brazil. But such findings were difficult to track over time, because of the inconvenience of conducting frequent and repeated ultrasounds on pregnant women.
“But marmosets are monkeys that we know vocalize a lot, and they share a lot of features with humans,” says Takahashi, who worked with the Princeton Neuroscience Institute while conducting the research.
For instance, both male and female parents raise their offspring together, and unlike other primates, marmoset babies are relatively helpless when they’re born, like human infants. (Read how marmoset dads go above and beyond.)
As to how all of this translates to humans, Takahashi says the central finding will help illuminate when speech development begins, and that studying pre-birth—rather than the moment of birth—may help identify speech or motor development problems earlier.
“There are a lot of things going on in the womb that might be relevant to what’s going on afterwards,” he says.
Marmosets like marshmallows
Before the researchers could submit the marmosets to an ultrasound wand, they first had to train the animals to sit still. While that may sound like a nearly impossible task for an animal that spends its days swinging feverishly around the rainforest canopy, the scientists had a trick up their sleeves.
“Marmosets like treats with high energy contents,” says Takahashi. “And what they really love is marshmallows.”
With treats in hand, each of four pregnant marmosets were scanned two to three times a week, for up to 45 minutes at a time. Ultrasounds began on the 95th day of pregnancy, when the infants’ faces were first becoming distinguishable, and continued until birth at 146 days.
Marmosets typically have twins, and frequently carry triplets and quadruplets, however the scientists had no way to tell the babies apart while they were still inside the mothers’ wombs. (Read more about astonishing animal pregnancies.)
This meant that in three of the four pregnancies, where twins and quadruplets were involved, the scientists merged results from whatever faces they could scan.
Building blocks of spoken language
Perhaps one of the most remarkable aspects of the study was how the number of movements that qualified as practice cries increased over time, says Takahashi.
The practice cries also changed.
For instance, in the earliest scans, the fetuses would only make the practice cry expression at the same time as they moved their heads. But as they grew, the two motions were slowly decoupled, says Takahashi, until the point where they could open their mouths in mock-cry without moving their heads at all.
“Of course, we cannot study the full complexity of language in other species, because every species has its own communication system,” says Andrea Ravignani, a comparative bioacoustician and research group leader at the Max Planck Institute for Psycholinguistics. “But we can look for the building blocks of spoken language, and that’s what I think these authors did in this paper.”
Ravignani, who was not involved in the study, has previously discovered hidden rhythms in lemur vocalizations. He finds the notion of looking for evidence of sound production even before sounds are possible “extremely compelling.”
In fact, he said there may be an analogy to how human babies start walking on all fours before they transition to walking upright.
Ravignani says there’s debate among the scientific community about how instructive marmoset development and behavior may be for humans. However, for the vocal traits examined in this study, he thinks they’re an excellent fit.
“In my humble opinion, they could offer us even more insights than chimpanzees,” he says. (See beautiful pictures of monkeys around the world.)
That may sound paradoxical, since chimpanzees are more closely related to humans than marmosets.
But while marmosets are born with a largely fixed vocal repertoire that is likely pre-programmed by genetics, recent research shows the animals also go through stages of development in which they can be highly flexible and open to learning.