Development of species identification in ducklings. XIII: A comparison of malleable and critical periods of perceptual development.

The present experiments sought to determine the relationship of the malleable period--when the mallard duck embryo is most susceptible to alien forms of maternal auditory stimulation--to the critical period, when the embryo must hear its own vocalizations in order to respond normally to the species-typical maternal call after hatching. It was found that a preference for either a wood duck or a chicken maternal call could be induced in devocalized mallard duck embryos by exposing them to either alien call some time between day 24, 0600 to day 26, 0600 hrs. Further experiments showed the height of the malleable period to be between day 24, 1400 hrs and day 25, 0800 hrs of embryonic development. Since the embryonic critical period for maintenance of the species-typical maternal auditory preference spans day 24, 0800 hrs to day 26, 0800 hrs, the malleable period for the induction of a preference for species-atypical maternal stimulation falls inside the critical period. While it is often assumed that malleable and critical periods overlap, this is the first behavioral study to actually demonstrate the temporal relationship of the two periods. It would seem important to further examine malleable and critical periods of development in a broader array of developmental contexts and species to determine whether malleable periods for atypical or abnormal development and critical periods for species-typical or normal development always coincide.