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.