Sex differences in lifespan have been labelled as one of the most robust features in biology. In human populations, women live consistently longer than men, a pattern that encompasses most mammalian species. However, when expanding both the taxonomic scope beyond mammals and the range of mortality metrics the female survival advantage over males is no longer the rule. Moreover, current evidence suggests that sex differences in actuarial ageing parameters (i.e. age at the onset of ageing and rate of ageing) are far from consistent across the tree of life. This chapter first reviews current knowledge of sex differences in mortality patterns across animals and appraises how these diverse patterns can be explained by the current evolutionary framework. It then emphasizes the relevance of going beyond the differences in mortality patterns by exploring how natural and sexual selection have shaped age- and sex-specific changes in reproductive performance and body mass across the tree of life, and by identifying some possible biological pathways modulating ageing in a sex-specific way. Finally, it highlights how evolutionary theories can be relevant to understand the widespread differences in causes of death between sexes, offering a complementary approach to gain a comprehensive understanding of the evolution of sex differences in health and ageing, with likely biomedical implications.

The dream of eternal youth and immortality has always fascinated human societies. Even today, this quest is the source of major financial investments, particularly for the development of anti-ageing drugs. To unravel the mysteries of longevity, scientists have long been observing and quantifying the lifespan of animals. These decades of extensive comparative biology research have documented the extreme diversity of lifespan on Earth and identified key ecological and life history factors driving this diversity and, more recently, molecular pathways that might modulate it. However, the maximum lifespan of a species is far from being an accurate representation of a species’ ageing trajectory, both biologically and demographically. For a given species, the changes in mortality risk over the life course can be complex, and the ageing process is much more accurately described by ageing parameters, such as the age of onset of actuarial senescence and the rate of actuarial senescence. This chapter argues that current research in the comparative biology of ageing should now focus on the diversity of actuarial senescence patterns documented across the tree of life, as well as the species-specific causes of death, to identify key genetic and physiological determinants associated with delayed actuarial senescence or low actuarial senescence rate. Just a few years ago, such research projects would have seemed unrealistic, but the recent development of omics tools, coupled with the increased availability of demographic data for a wide range of species with contrasting life histories, lifestyles and habitats make such exciting comparative analyses now achievable and full of promise.

This opening chapter provides an overview of the future societal and subsequenl scientific challenges associated with population ageing. More specifically, it emphasizes how the field of biodemography constitutes a relevant framework for future research programmes aiming to address questions of paramount importance regarding both the causes (e.g. evolutionary, mechanistics) and consequences (demographic, medical) of the ageing process. Finally, this chapter details the book contents.