The ‘open-mouthed threat face’ (or ‘staring open-mouth face’) is displayed by a range of non-human primate species (e.g. macaques, mangabeys, chimpanzees, guenons and colobus monkeys), but the configuration of this facial expression varies across species (van Hooff, Reference van Hooff 1967).

Steiner and Glaser found that neonates of humans and multiple primate species display a ‘deterred’ facial expressions to a bitter taste, which includes tongue protrusion, depressed mouth angles and gaping mouth (and additionally nose wrinkle in human infants and adult apes; Steiner et al., 2001).

Wright and Roberts tested both Human and rhesus monkeys and for both groups found an inversion effect for Human faces and not monkey faces (monkey face stimuli consisted of a mix of primates including, among others, chimpanzees, gorillas, orangutans, new and old world monkeys, lemurs, and marmosets).

Some primates are more similar to humans than the others, and their facial features—the same features that are heeded in human facial attractiveness, i.e., eyes, nose, mouth, etc.,—are often exaggerated to the extremes that may be perceived as human caricatures.

The face of a chimpanzee is decidedly different from that of a human, despite the fact that the apes are our nearest relative in the primate tree. Now researchers at the Stanford University School of Medicine have begun to pinpoint how those structural differences could arise in two species with nearly identical genetic backgrounds.