Cradled within the silent world of the womb, fetuses may be preparing to wail out.
In the same way that newborn humans can cry as soon as they are born, common marmosets (Callithrix jacchus) produce contact calls to seek care from their caregivers. Those vocalizations are not improvisation, the researchers report in a preprint published April 14 in bioRxiv. Ultrasound imaging of marmoset fetuses reveals that their mouths are already imitating the distinctive pattern of movements used to make their first calls, long before sound production.
Early behaviors in babies are commonly described as “innate” or “programmed,” but a Princeton University team wondered how exactly those behaviors develop. How does a baby know how to cry as soon as she is born?
The secret may lie in what happens before birth. “People tend to ignore the fetal period,” says Darshana Narayanan, a behavioral neuroscientist who conducted the research while at Princeton University. “They just think it’s like the baby is vegetating and waiting to be born… [But] That’s where a lot of things start.”
Research shows, for example, that chicks inside their eggs are they are already learning to identify the call of their kind (Serial number: 09/16/21). “A lot is developing much earlier than we previously thought,” says developmental psychobiologist Samantha Carouso-Peck, executive director of the Grassland Bird Trust in Fort Edward, New York, who was not involved in the research. But, she says, “we haven’t really looked at the production side of this very much. Most of what we know is the auditory side.” Carouso-Peck studies vocal learning in songbirds and how it applies to how humans acquire language.
Narayanan and his colleagues turned to marmosets because the development of vocalization in monkeys is similar to that of humans (Serial Number: 8/13/15). Two-person teams performed noninvasive ultrasounds on two marmosets nearly every day during four different pregnancies. The effort involved a lot of marshmallow fluff, Narayanan says. “They would do anything for marshmallows.”
Around 95 days of pregnancy, the face of a fetus first appears. The researchers observed that each young fetus moved its mouth and other parts of its face along with its head. As the pregnancy progressed, the facial features and head began to move independently. The uncoupling of these different motor regions presumably prepares the fetus for tasks such as feeding or vocalizing.
So it was a matter of lip reading. Did the fetus simply move its mouth or did the movements mimic particular sounds? The researchers were clear from the start that the mouth movements were similar to the movements a marmoset makes during a contact call, distinguished by their long duration and multiple syllables.
“The contact call is so unique that it really can’t be confused with any of the other calls,” says Princeton biologist Asif Ghazanfar.
But to confirm, Ghazanfar, Narayanan and their team tracked fetal jaw movements frame by frame to determine their duration. They also measured the number of “syllables” a fetus makes by calculating the number of movements separated by less than 500 milliseconds. The researchers then compared the fetal movements to the contact calls produced by the baby marmosets after birth. As the fetus approaches birth, its facial and mouth movements become increasingly similar to those of the baby’s touch calls, evidence, the team says, that the fetus is developing the ability to make this call after of birth.
This supports the idea that early cries don’t “magically appear,” says Narayanan. “They have a long period of development, but in the womb.”