Discriminative Reactions to Behaviour and Social Attributes of Non‐Mother Individuals in Wild Infant Japanese Macaques on Yakushima ( <scp> <i>Macaca fuscata yakui</i> </scp> )

ABSTRACT Mammalian infants exhibit context‐dependent behavioural reactions in social interactions. Despite extensive research on these discriminative reactions, studies have lacked diversity in infant behaviours and have reported inconsistent findings. To address these gaps, we investigated how non‐human primate infants respond to non‐mother individuals, predicting fewer positive and more negative reactions to rough behaviours, and fewer negative reactions toward familiar individuals, such as close kin and maternal frequent grooming partners, as well as toward those ranked higher than their mothers. We tested these predictions using all‐occurrence sampling of infant handling, recording infant reactions during the first 16 weeks of life in wild Japanese macaques ( Macaca fuscata yakui ). We found that infants exhibited fewer positive and more negative reactions, except for leaving reactions, toward rough handling. They exhibited more frequent crying when handled by close kin; showed more positive reactions and fewer leaving and crying reactions toward individuals who frequently groomed with their mothers; and displayed more twitching and crying reactions when roughly handled by individuals ranked higher than their mothers. Younger infants exhibited more twitching, and female infants displayed more crying. Higher scores of maternal rejectiveness were linked to fewer leaving reactions in infants. These findings indicate that non‐human primates acquire behavioural flexibility early in development through active engagement in social contexts. Such behavioural development may contribute to ensuring safety, promoting social bonding, conserving energy during social interactions and maintaining maternal relationships.