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.

The evolution of self-control.

Cognition presents evolutionary research with one of its greatest challenges. Cognitive evolution has been explained at the proximate level by shifts in absolute and relative brain volume and at the ultimate level by differences in social and dietary complexity. However, no study has integrated the experimental and phylogenetic approach at the scale required to rigorously test these explanations. Instead, previous research has largely relied on various measures of brain size as proxies for cognitive abilities. We experimentally evaluated these major evolutionary explanations by quantitatively comparing the cognitive performance of 567 individuals representing 36 species on two problem-solving tasks measuring self-control. Phylogenetic analysis revealed that absolute brain volume best predicted performance across species and accounted for considerably more variance than brain volume controlling for body mass. This result corroborates recent advances in evolutionary neurobiology and illustrates the cognitive consequences of cortical reorganization through increases in brain volume. Within primates, dietary breadth but not social group size was a strong predictor of species differences in self-control. Our results implicate robust evolutionary relationships between dietary breadth, absolute brain volume, and self-control. These findings provide a significant first step toward quantifying the primate cognitive phenome and explaining the process of cognitive evolution.