Variation in gaze-following between two Asian colobine monkeys.

Gaze-following is a basic cognitive ability found in numerous primate and nonprimate species. However, little is known about this ability and its variation in colobine monkeys. We compared gaze-following of two Asian colobines-François' langurs (Trachypithecus francoisi) and golden snub-nosed monkeys (Rhinopithecus roxellana). Although both species live in small polygynous family units, units of the latter form multilevel societies with up to hundreds of individuals. François' langurs (N = 15) were less sensitive to the gaze of a human experimenter than were golden snub-nosed monkeys (N = 12). We then tested the two species using two classic inhibitory control tasks-the cylinder test and the A-not-B test. We found no difference between species in inhibitory control, which called into question the nonsocial explanation for François' langur's weaker sensitivity to human gaze. These findings are consistent with the social intelligence hypothesis, which predicted that golden snub-nosed monkeys would outperform François' langurs in gaze-following because of the greater size and complexity of their social groups. Furthermore, our results underscore the need for more comparative studies of cognition in colobines, which should provide valuable opportunities to test hypotheses of cognitive evolution.