Evolutionary determinants of non-seasonal breeding in wild chacma baboons

ABSTRACT Animal reproductive phenology varies from strongly seasonal to non-seasonal, sometimes among closely related or sympatric species. While the extent of reproductive seasonality is often attributed to environmental seasonality, this fails to explain many cases of non-seasonal breeding in seasonal environments. We investigated the evolutionary determinants of non-seasonal breeding in a wild primate, the chacma baboon ( Papio ursinus ), living in a seasonal environment with high climatic unpredictability. We tested three hypotheses proposing that non-seasonal breeding has evolved in response to (1) climatic unpredictability, (2) reproductive competition between females favouring birth asynchrony, and (3) individual, rank-dependent variations in optimal reproductive timing. We found strong support for an effect of reproductive asynchrony modulated by rank: (i) birth synchrony is costly to subordinate females, lengthening their interbirth intervals, and (ii) females delay their reproductive timings (fertility periods and conceptions) according to other females in the group to stagger conceptions. These results indicate that reproductive competition generates reproductive asynchrony, weakening the intensity of reproductive seasonality at the population level. This study emphasizes the importance of sociality in mediating the evolution of reproductive phenology in gregarious organisms, a result of broad significance for understanding key demographic parameters driving population responses to increasing climatic fluctuations.