Reproductive seasonality is a major adaptation to seasonal cycles and varies substantially among organisms. This variation, which was long thought to reflect a simple latitudinal gradient, remains poorly understood for many species, in part due to a lacunary theoretical framework. Because seasonal cycles are increasingly disrupted by climate change, a better understanding of the ecology of reproductive seasonality could generate important insights on how climate change may affect biodiversity. The goal of this study was to investigate the drivers of evolutionary transitions towards reproductive seasonality using a realistic agent-based optimisation model simulating the life cycle of a female yellow baboon, who typically breeds year-round. Specifically, we tested the influence of three ecological traits (environmental seasonality, productivity and unpredictability) and three life-history traits (daily reproductive energy expenditure, reproductive cycle length and infant extrinsic mortality) on the intensity of reproductive seasonality. To do so, we simulated diverse reproductive phenology strategies (from non-seasonal to highly seasonal), assessed which were optimal and computed, for the set of optimal strategies, the intensity of reproductive seasonality. We then induced variation in each trait of interest and examined how it affected the intensity of reproductive seasonality. We found significant effects of all three environmental traits: high reproductive seasonality was favoured by high environmental seasonality, low environmental productivity and low unpredictability. It was further, and most strongly, favoured by high daily reproductive energy expenditure. In contrast, there was no significant effect of reproductive cycle length and infant extrinsic mortality. Our modelling approach successfully disentangled the effects of environmental seasonality, productivity and unpredictability on the intensity of reproductive seasonality, which likely all contribute to generate the well-known association between latitude and reproductive seasonality. Our results further highlight the critical importance of life history pace on the evolution of reproductive seasonality. Overall, this study contributes a powerful theoretical framework and modelling tool that may apply across the life-history space, as well as sheds new light on the emergence and maintenance of non-seasonal breeding in slow-living species, including humans.
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 nonseasonal 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.
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