— A newborn macaque (less than 10 days old) imitates tongue protrusion (Image: Pier Ferrari)
Monkeys imitate with a purpose, matching their behaviour to others as a form of social learning, researchers report.
Such mimicry has previously been seen only in great apes including humans and chimps but now Italian researchers have recorded wonderful footage of the phenomenon in newborn rhesus macaques.
Human newborns have a known capacity to mimic certain specific adult facial expressions, including mouth opening and tongue protrusion. The so-called imitation period lasts up to three months in human infants and two months in chimps.
Since newborns cannot see their own faces, they rely on watching adults to learn facial expressions, and mimicry is thought to be crucial to the development of a mother-infant relationship.
Particular brain cells called mirror neurons fire in a human infant when it watches an adult expression and copies it. Similar mirror neurons "light up" when rhesus monkeys watch another animal perform an action and when they copy that action. This similarity suggests a common brain pathway for imitation in humans and monkeys.
Pier Ferrari at the University of Parma, Italy, and colleagues tested 21 newborn macaques by holding each in front of a researcher who made various facial expressions.
At one day old, none of the infants showed any imitation. By day three, however, infants started to copy the researchers expressions, including tongue protrusions, mouth opening and lip smacking all typical macaque expressions. Watch footage of macaques copying tongue poking and mouth opening (each 3.5MB, avi format).
By two weeks, all imitative behaviour had ceased, showing the imitation period in the monkeys is far shorter than for great apes. However, the researchers note that macaques may copy other macaques for longer.
The study shows the capacity for imitation occurred earlier in the primate evolutionary tree than previously thought, the researchers argue, and before the rhesus monkey ancestor split from the human lineage, about 25 million years ago.
Journal reference: Public Library of Science (DOI: 10.1371/journal.pbio.0040302)