In monotonic mammals, most individuals experience the birth of a younger sibling. This period can induce losses of maternal care and can be physiologically, energetically and emotionally challenging for the older sibling, but has rarely been studied in wild primates. We used behavioral data collected from a natural population of mandrills to investigate changes in maternal caregiving and the mother-younger relationship throughout the transition to siblinghood (TTS), comparing juveniles that had recently experienced the birth of a younger sibling to juveniles that had not. We found that TTS was associated with an abrupt cessation of the weaning process for the juvenile and a decrease in maternal affiliation. Juvenile reactions were sex-specific, with males associating less with their mothers, while females tended to groom their mothers more often after the birth of their sibling. Despite the substantial loss of maternal care, juveniles showed no increase in conflict or anxiety-related behaviors. This study helps to explain why short birth intervals are often a risk to juvenile survival in monotonic primates. Our results contrast with existing studies and underline the importance of examining TTS in species and populations with different life histories and ecologies.