October 9, 2009 -- Rhesus macaque mothers and their babies like to get in each others' faces, exchanging looks and smacking their lips. They're neither rude nor hungry. Just as their human counterparts do, these monkeys communicate in a mutually pleasing way that prepares infants to navigate the social world, a new study suggests. Interactions between macaque moms and babies often begin with exaggerated lip smacking by the adult, who gently touches her infant's lips and face with her mouth, ethologist Pier Ferrari of the University of Parma in Italy, and his colleagues report online October 8 in Current Biology. Mothers also lower their heads and then move them up and down while looking at babies seated nearby, as a prelude to lip smacking and mutual eye contact. For their part, macaque babies imitate lip smacking displayed by their mothers -- but not by other females -- within the first few days of life, Ferrari's group reports. Infants then sometimes smack their lips at their mothers to initiate sustained eye contact. This distinctive form of communication disappears near the end of an infant's first month of life, the researchers say. Macaque youngsters become highly mobile by that time and begin to forge peer relationships.

Ferrari hypothesizes that macaque and human babies share an inborn capacity to communicate using emotional displays and gestures, to share experiences with others and to understand adults' behaviors as having a purpose. "We can trace back to macaques the evolutionary foundation of some behaviors present in the early stages of mother-infant relations that are crucial for social learning," Ferrari says. His team studied 14 pairs of macaque mothers and infants in a primate facility for the first two months of infants' lives. Daily, 15-minute observations occurred during infants' first 23 days of life, followed by observations at one and two months of age.

Scientists already knew that macaque mothers and infants smack their lips and look into each other's eyes, remarks psychologist Dario Maestripieri of the University of Chicago. But Ferrari's study is the first to record the frequency and context of such behaviors, offering clues to why they occur, he says. "We don't yet know if these interactions contribute to the mother-infant bond and to the infant's emotional and cognitive development," Maestripieri cautions. Still, the new findings indicate that an innate capacity for emotionally based interactions between mothers and infants evolved more than 30 million years ago in a common ancestor of humans, apes and Old World monkeys (including macaques), proposes psychologist Kim Bard of the University of Portsmouth, England. Macaque mothers' exaggerated lip smacking at infants corresponds to "motherese," the tendency of human mothers to exaggerate their facial expressions and talking style with babies, Bard asserts in a comment to be published along with the new study in the November 17 Current Biology.