The Evolution of Reproductive Seasonality in Mammals – ERS

Studying adaptation to seasonal cycles may generate important insights on the resilience of species to climate change, which disrupts environmental seasonality. However, our understanding of the evolution of reproductive seasonality is limited, and contrasts with our detailed knowledge of its physiological regulation. Strikingly, we are still unable to explain a great range of variation in reproductive seasonality (i.e., timing and width of the annual birth peak) across species, which is poorly predicted by environmental seasonality alone.

This gap is due in part to a general lack of a robust theoretical framework – and a shortage of associated predictions. The traditional focus on environmental seasonality has obscured the role of other determinants of reproductive seasonality, such as life history and sociality, which may profoundly alter the costs and benefits of reproductive seasonality. As a simple example, some mammals synchronize mid-lactation with the food peak, while others match it with weaning or conception, and the determinants of such variation are poorly understood. Moreover, life in groups exacerbates competition over food and reproduction, which may incite females to stagger births in some societies; but the effects of sociality on the evolution of reproductive seasonality have never been explored. Finally, field studies that have focussed on variation in breeding phenology are severely biased towards short-lived organisms (passerines and rodents) from temperate regions. This is problematic, because biodiversity is concentrated in the equatorial and tropical regions, and because long-lived organisms are expected to be particularly vulnerable to rapid environmental change.

The aim of this project is to develop and validate much-needed theory into the evolutionary causes and consequences of reproductive seasonality, focussing on mammals. Our working program combines (1) a theoretical study to predict the evolution and modalities of reproductive seasonality across a range of climates, life histories and societies; (2) a broad-scale comparative study to investigate the relationships between reproductive seasonality and a large set of ecological, life-history, and social traits in the evolutionary history of mammals; and (3) a population study to explore the ecological and social determinants of individual variation in reproductive seasonality and their fitness consequences in two wild social primates showing a striking contrast in breeding phenology. The brightly coloured mandrill (Mandrillus sphinx) lives in the equatorial forests of Gabon yet breeds seasonally, while its close cousin, the chacma baboon (Papio ursinus), lives in the highly seasonal savannas bordering the Namib Desert yet breeds year-round.

Pilot results of a comparative analysis including 365 mammal species suggest that life-history and sociality are both critical determinants of the evolution of reproductive seasonality, calling for in-depth analyses. We will ensure the development of theoretical and empirical parts in interaction to generate realistic models of predictive power for a wide taxonomic generality, as well as employ innovative methodology. This will include the latest developments in comparative analyses, which may allow us to decipher some causality links in the evolution of reproductive seasonality, as well as original, non-invasive approaches – including isotopic analyses and the measurement of thyroid hormone - to document seasonal variations in individual dietary intake in our population study.

The originality and strength of this project lie in its unique combination of theory and large-scale empirical tests within and across populations and species, its integration of multiple selective forces acting on seasonal breeding, its focus on slow-reproducing tropical mammals and the expertise of my team on the various approaches.