My PhD is about the social and ecological determinants of individual variations of reproductive seasonality and their consequences for the fitness of both mothers and offspring. My model species are two populations of wild primates: chacma baboons in Namibia (Tsaobis Baboon Project) and mandrills in Gabon (Mandrillus Project). This study includes long-term data based analysis, along with morphological and behavioural data collected during two field seasons at Tsaobis.