The evolution of non-seasonal breeding in primates

Reproductive seasonality offsets the energetic costs of reproduction by synchronizing births with peak resources and is traditionally expected to increase with latitude and environmental seasonality. However, life-history and behavioural strategies may also buffer energetic shortages and reduce dependency on environmental cycles. Here, we propose and test an integrative framework integrating climatic, life-history and behavioural factors using high-resolution measures of reproductive seasonality for 132 wild primate populations from 94 species. As expected, reproductive seasonality declines at lower latitudes, in less seasonal and in less predictable environments, even after controlling for productivity. It also decreases in species that spread reproductive costs by extending developmental periods, or when a higher infant mortality urges females to resume fertility shortly after loss. Unexpectedly, reproductive seasonality increases with environmental productivity and is not reduced by cognitive (foraging innovations), social (allomaternal care), or ecological (dietary breadth) buffering. Broader diets even enhance seasonality. These findings suggest that reproductive seasonality emerges from opportunity more than from constraints in productive environments, where females exploit abundant resources to accelerate their reproductive pace. Together, our results shed light on the diverse selective pressures shaping primate reproductive seasonality, including climate, life-history pace, and infanticide risk, and help to explain why humans reproduce year-round.