Facial expressions in common marmosets (Callithrix jacchus) and their use by conspecifics.

Facial expressions have been studied mainly in chimpanzees and have been shown to be important social signals. In platyrrhine and strepsirrhine primates, it has been doubted that facial expressions are differentiated enough, or the species socially capable enough, for facial expressions to be part of their communication system. However, in a series of experiments presenting olfactory, auditory and visual stimuli, we found that common marmosets (Callithrix jacchus) displayed an unexpected variety of facial expressions. Especially, olfactory and auditory stimuli elicited obvious facial displays (such as disgust), some of which are reported here for the first time. We asked whether specific facial responses to food and predator-related stimuli might act as social signals to conspecifics. We recorded two contrasting facial expressions (fear and pleasure) as separate sets of video clips and then presented these to cage mates of those marmosets shown in the images, while tempting the subject with food. Results show that the expression of a fearful face on screen significantly reduced time spent near the food bowl compared to the duration when a face showing pleasure was screened. This responsiveness to a cage mate's facial expressions suggests that the evolution of facial signals may have occurred much earlier in primate evolution than had been thought.