Development comparisons of grimace and play mouth in infant pigtail macaques (Macaca nemestrina)

Five laboratory-born infant pigtail macaques (Macaca nemestrina) were studied during playroom interactions. Each infant was videotaped for a 5-minute period weekly for 5 months. The animals were scored on the frequency of the mouth display observed. Both play-type (round mouth corners, ie, relaxed open-mouth display) and fear-type (sharp mouth corners, ie, silent bared-teeth display) facial expressions were present at the onset of obsevations. Play mouths appeared with greater frequency than fear mouths throughout the study. The frequency of round-cornered mouths (play and gnaw) vs sharpcornered mouths (bite and grimace) was significantly different both across monkeys and across weeks. There appeared to be a bimodal distribution of round and sharp mouths, with infants using the different mouth types in a nonsocial context at earlier ages and in a social context as they matured. The data are consonant with a phylogentically separate origin of the fear grimace and playface in old world anthropoids including humans.