Sensitivity to first-order relations of facial elements in infant rhesus macaques.

Faces are visually attractive to both human and nonhuman primates. Human neonates are thought to have a broad template for faces at birth and prefer face-like to non-face-like stimuli. To better compare developmental trajectories of face processing phylogenetically, here we investigated preferences for face-like stimuli in infant rhesus macaques using photographs of real faces. We presented infant macaques aged 15-25days with human, macaque, and abstract faces with both normal and linear arrangements of facial features, and measured infants' gaze durations, number of fixations, and latency to look to each face using eye-tracking technology. There was an overall preference for normal over linear facial arrangements for abstract and monkey faces, but not human faces. Moreover, infant macaques looked less at monkey faces than at abstract or human faces. These results suggest that species and facial configurations affect face processing in infant macaques, and we discuss potential explanations for these findings. Further, carefully controlled studies are required to ascertain whether infant macaques' face template can be considered as broad as human infants' face template.