Determining the drivers of intraspecific variation in cognitive performance is crucial for understanding how cognition evolves, but experimental studies are scarce. Several correlational studies have supported the hypothesis that living in larger groups enhances cognitive performance. However, empirical explanations of whether and how living in large groups causally affects individual cognitive performance remain unclear. Thus, in this study, red junglefowl chicks, Gallus gallus, were raised in artificially created smaller and larger groups of ecologically relevant sizes, before assaying their performance in tests of inhibitory control, discrimination learning and reversal learning. Group size did not affect reversal learning performance However, compared to chicks from larger groups, chicks from smaller groups tended to perform worse in the inhibitory control task, and performed better in the discrimination learning task. Group size also affected resampling in a reversal learning task depending on sex, where females in smaller groups and males in larger groups tended to be faster than males in smaller groups. In addition, stress, locomotor activity, neophobia and boldness were explored as potential covariates of group size, which could affect cognitive performance. Of these covariates, only locomotor activity differed between the two group sizes, where chicks in smaller groups were more active. More active chicks also learnt the discrimination task faster. Thus, locomotor activity could be a mechanism by which group size affects discrimination learning. Taken together, our results indicate that group size can causally affect aspects of cognitive performance, and that these effects may be sex specific. Moreover, our results for locomotor activity suggest that the effects of group size on cognitive performance may not be specifically related to differences in cognitive demand between group sizes. However, further studies are needed to disentangle how social dynamics influence individual differences in cognitive performance, and thus, cognitive evolution.
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