It's a look that's been painted and photographed untold times: a mother gazing deep into her infant's eyes while the two smile and kiss. Psychologists believe this interplay helps a child's emotional and cognitive development. The behavior was thought to exist only in humans and to a lesser extent in our closest kin, chimpanzees. Now, scientists have discovered similarly intense shared gazing and facial expressions in monkeys. And that means, the researchers say, that this kind of maternal communication dates back at least 30 million years.
Although scientists have studied rhesus macaque monkeys (Macaca mulatto) in the lab and field for more than 50 years, they missed this key behavior. "Previous researchers were looking more at what happens when a mother and infant are separated," says Pier Francesco Ferrari, a neuroscientist at the University of Parma in Italy, not what happens when they're together.
But plenty occurs between the two, as Ferrari and his team observed. In a semi-free-range environment at the Laboratory of Comparative Ethology, part of the National Institutes of Health in Bethesda, Maryland, the scientists filmed 14 mother-and-infant pairs during the first 2 months of the youngsters' lives, beginning when the infants were a few hours old. The team watched each pair one to three times a day for 15 minute sessions while they were awake. Infants sleep 50% to 75% of the time, which may be another reason the emotional gazing was previously missed.
As the macaque mothers looked into their babies' eyes, they "actively searched for the infants' gaze and tried to engage their babies," says Ferrari. For instance, a mother might hold her baby's head and pull the child's face toward her own or bounce her own head up and down, all while gazing directly into the infant's eyes. Other times, mothers would smack their lips in an exaggerated manner and kiss their babies' faces--reminiscent of the way human mothers get their infants' attention. The infants often responded by imitating their mothers' lip-smacks or using lip-smacks of their own to get her attention, the team reports today in Current Biology. As in humans, these actions are likely important in the baby rhesus macaque's emotional development, says Ferrari. Unlike in humans, however, the shared behaviors begin to disappear in macaques after the first month as the babies become more independent.
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"It's a very interesting study of a neglected, or misjudged, topic," says Frans de Waal, a primatologist at the Yerkes National Primate Research Center at Emory University in Atlanta, Georgia. It demonstrates that "the true observer can still discover things no one knows about" even in species that have been extensively studied, he says.
Because rhesus macaques are born with the capacity to communicate, Ferrari says, “We can now explore the roots of interpersonal communication, maybe even the mutual appreciation of others' intentions and emotions."