Abstract
Reproductive seasonality is a major adaptation to seasonal cycles that consists in a clustering of reproductive events in order to synchronize the associated energetic costs with the most productive season of the year. This strategy is widespread, especially in mammals where females face high breeding costs, and it varies widely across species, from strongly seasonal to non-seasonal. Yet we are still unable to predict variations in reproductive seasonality between species solely on the basis of the seasonality of their environment. This situation reflects an insufficient attention given to other potential determinants of reproductive seasonality, such as life history or sociality, and more generally the absence of a global theoretical framework to explain the diversity of strategies observed. In this study, I propose such a theoretical framework, defined by eleven hypotheses, on the ecological, life history and social determinants of reproductive seasonality, that I further test through three complementary methodological approaches: (i) a populational study, (ii) a theoretical modelling study and (iii) a comparative analysis. We found support for nine of these eleven hypotheses, even though some of these findings may be taxon-specific. In particular, we show that the intensity of reproductive seasonality declines as species live in less seasonal (1), more productive (2) and more unpredictable environments (3), spread their reproductive effort in time (4), buffer annual food shortage through foraging innovations (cognitive buffer) (5) or a broader dietary range (ecological buffer) (6), suffer from high infant mortality relatively to adults (7) and experience acute reproductive competition among females (8), with rank-related variations (9). These results show the relevance of this new theoretical framework that offer a comprehensive synthesis to understand the complexity of the determinants of reproductive seasonality in mammals, thereby improving our predictive potential with regard to their resilience to climate change. They also shed new light on the emergence and maintenance of non-seasonal breeding in slow-living species, with implications in ecology, life-history, behavioural ecology and anthropology.