Vocal learning via social reinforcement by infant marmoset monkeys

For over half a century now, primate vocalizations have been thought to undergo little or no experience-dependent acoustic changes during development [ 1 ]. If any changes are apparent, then they are routinely (and quite reasonably) attributed to the passive consequences of growth. Indeed, previous experiments on squirrel monkeys and macaque monkeys showed that social isolation [ 2, 3 ], deafness [ 2 ], cross-fostering [ 4 ] and parental absence [ 5 ] have little or no effect on vocal development. Here, we explicitly test in marmoset monkeys—a very vocal and cooperatively breeding species [ 6 ]—whether the transformation of immature into mature contact calls by infants is influenced by contingent parental vocal feedback. Using a closed-loop design, we experimentally provided more versus less contingent vocal feedback to twin infant marmoset monkeys over their first 2 months of life, the interval during which their contact calls transform from noisy, immature calls to tonal adult-like “p...