Group Size Predicts Social but Not Nonsocial Cognition in Lemurs

<div><p>The social intelligence hypothesis suggests that living in large social networks was the primary selective pressure for the evolution of complex cognition in primates. This hypothesis is supported by comparative studies demonstrating a positive relationship between social group size and relative brain size across primates. However, the relationship between brain size and cognition remains equivocal. Moreover, there have been no experimental studies directly testing the association between group size and cognition across primates. We tested the social intelligence hypothesis by comparing 6 primate species (total N = 96) characterized by different group sizes on two cognitive tasks. Here, we show that a species’ typical social group size predicts performance on cognitive measures of social cognition, but not a nonsocial measure of inhibitory control. We also show that a species’ mean brain size (in absolute or relative terms) does not predict performance on either task in these species. These data provide evidence for a relationship between group size and social cognition in primates, and reveal the potential for cognitive evolution without concomitant changes in brain size. Furthermore our results underscore the need for more empirical studies of animal cognition, which have the power to reveal species differences in cognition not detectable by proxy variables, such as brain size.</p></div

Performance in the cognitive tasks as a function of social group size and residual endocranial volume (ECV), a measure of relative brain size.

<p>We tested the hypotheses that social group size and relative brain size would predict species performance. A) As predicted by the social intelligence hypothesis, species characterized by larger social groups performed better in the social cognition task. Relative brain size did not explain species’ performance, and the slope of the relationship was negative. B) Group size did not predict performance on the non-social inhibitory control task. As in the social task, relative brain size was not a predictor of performance on the inhibitory control task.</p

Species’ performance on the social cognition and inhibitory control tasks.

<p>The tree structure at the bottom of the figure represents the phylogenetic relationships between the species. Error bars reflect the standard error of the mean.</p

The test trials for Experiment 1.

<p>Subjects were given the opportunity to pilfer food from one of two human competitors. In each condition, one of the competitors could see the food and the subject approaching, while the other could not because A) his back was turned B) he was oriented away from the food in profile, or C) a headband covered his eyes.</p

Scores (percent correct) from the social cognition task.

<p>Performance in each condition was compared to chance expectation (50%) using one-sample Wilcoxon tests to evaluate the hypothesis that subjects would attempt to steal the food that their competitor could not see.</p