An integrative view of senescence in nature

Senescence-the decline in age-specific contribution to fitness with increasing age-has been widely investigated in evolutionary ecology. A tremendous amount of detailed empirical analyses have now revealed the widespread occurrence of demographic senescence (i.e. both actuarial and reproductive senescence) and have started to identify factors (e.g. environmental conditions) that modulate its timing and intensity, both within and across species. In this special feature, we have built on this flourishing work to highlight several axes of research that would benefit from more integrative and multidisciplinary approaches. Several contributions compiled in this special feature emphasize that our understanding of senescence remains taxonomically limited, mostly focused on birds and mammals, and is therefore not representative of the biological diversity displayed across the tree of life. In line with this observation, the influence of some peculiar lifestyles (e.g. involving sociality or modularity) on the evolution of senescence is yet to be deciphered. Understanding of the diversity of senescence patterns across and within species and among traits will necessitate the establishment of new metrics as a golden standard to fully account for age-specific changes recorded in individuals' performance. This is illustrated with the specific case of actuarial senescence. This special feature also highlights that the diversity of biological samples collected from wild plants and animals, along with accurate demographic data, is expanding. The fast development of new molecular tools now offers a unique opportunity to launch research programmes at the interface of physiology, health and ageing in non-model organisms. We argue that while these different research axes constitute key avenues of investigations for the coming years, they are only the tip of the iceberg. To appreciate the full complexity of the senescence process in nature, from its evolutionary causes to its demographic consequences, we also need a better understanding of the role played by both environmental conditions and gene-environment interactions, of constraints, and of senescence, an improved assessment of the influence of individual heterogeneity, and the consideration of transgenerational effects when quantifying the fitness consequences of senescence.