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.