Perception of biological motion in common marmosets (Callithrix jacchus): by females only

The ability to perceive biological motion (BM) has been demonstrated in a number of species including humans but the few studies of non-human primates have been relatively inconclusive. We investigated whether common marmosets (Callithrix jacchus) are able to perceive biological motion, using a novel method to test non-human primates. Marmosets (7 male and 7 female) were trained to remove a cover from a container and look inside it, revealing a computer screen. Then they were presented with images on this computer screen consisting of a novel BM pattern (a walking hen) and 4 manipulations of that pattern (a static frame of this pattern and inverted, scrambled, and rotating versions of the pattern). The behavioural responses of the marmosets were recorded and used to assess discrimination between stimuli. BM was attended to by females but not males, as shown by active inspection behaviour, mainly movement of the head towards the stimulus. Females paid significantly less attention to all of the other stimuli. This indicates the females' ability to attend to biological motion. Females showed slightly more attention to the inverted BM than to the static, scrambled, and rotating patterns. The males were less attentive to all of the stimuli than were the females and, unlike the females, responded to all stimuli in a similar manner. This sex difference could be due to an inability of males to recognise BM altogether or to a lesser amount of curiosity. Considered together with the findings of previous studies on chicks and humans, the results of the present study support the notion of a common mechanism across species for the detection of BM.