My PhD project aims at understanding the ecology and evolution of sibling rivalry in social primates. Sibling competition has lasting consequences on offspring development, behaviour and fitness. However, most studies have focused on competition between same-aged broodmates or littermates. Less is known on sibling competition in species which produce one offspring at a time, like humans. I will collect behavioural data from juveniles, and use long-term data from the Tsaobis Baboon Project and the Mandrillus Project to investigate the dynamics and consequences of sibling competition in chacma baboons and in mandrills.