Crying is a survival skill that is particularly necessary for human babies, given their helplessness at birth, and it’s the way that they attract the attention of the caregiver. How does a baby know how to cry as soon as it’s born? For a long time, scientists have asked themselves why, and many studies have been carried out to try and find an answer. Early behaviors in infants are commonly described as “innate” or “hard-wired,” but a team at Princeton University wondered how exactly those behaviors develop and when.
Behavioral similarities between humans and primates may help us find an answer. In the same way newborn humans can cry as soon as soon as they’re born, common marmoset monkeys (Callithrix jacchus) produce contact calls to seek attention from their caregivers. Those vocalizations are not improvised, researchers report in a preprint posted April 14 at bioRxiv. Ultrasound imaging of marmoset fetuses reveals that their mouths are already mimicking the distinctive pattern of movements used to emit their first calls, long before the production of sound. They’re practicing in the womb.
The secret may lie in what’s happening before birth. “People tend to ignore the fetal period,” says Darshana Narayanan, a behavioral neuroscientist who did the research while at Princeton University. “They just think that it’s like the baby’s just vegetating and waiting to be born…. [But] that’s where many things begin.”
Narayanan and her colleagues turned to marmosets because vocalization development in the monkeys is similar to that of humans. Two-person teams performed noninvasive ultrasounds on two marmosets nearly every day during four different pregnancies. Narayan says that marmosets love marshmallow fluff and the effort involved a lot of it. “They would do anything for marshmallows.”
Around 95 days into the pregnancy, a fetus’s face comes into view for the first time. Ultrasounds of a marmoset in the womb show that fetal face and mouth movements are similar to an infant’s contact call, suggesting that the fetus develops the motor skills to make its cries before it’s born and can produce sound. It was clear to the researchers early on that the mouth movements were similar to the motions a marmoset makes during a contact call—but that was before birth and before the marmoset would be doing any vocalization.
The researchers then compared the fetal movements to the contact calls produced by the infant marmosets after birth. As the fetus approaches birth, its facial and mouth movements became increasingly similar to those of the infant contact calls — evidence, the team says, that the fetus is developing the ability in order to make this call after birth. This supports the idea that early cries aren’t “magically appearing,” Narayanan says. “They have a long period of development — but in utero.” And given the similarities between humans and primates, scientists hypothesize that human babies are learning to cry in the womb in the same way and for the same reason: to attract attention from the caregiver.
