Assessing aging patterns for comparative analyses of mortality curves: Going beyond the use of maximum longevity

An increasing number of studies have investigated the diversity of actuarial senescence (i.e. the increase of mortality with age) and mortality patterns in the wild. Most of these studies used maximum longevity as a metric, and only some of them were based on the analysis of mortality or survival curves. However, maximum longevity is not a reliable metric to assess mortality patterns. Two types of metrics can be distinguished: the metrics of pace that include biological times and describe how long individuals live and the metrics of shape that are dimensionless and describe the form of mortality patterns for a given pace. We review the use of pace metrics in comparative analyses of mortality curves and actuarial senescence performed so far, with a special focus on longevity metrics. We demonstrate that multiple statistical issues are associated with the use of maximum longevity, although this metric is the most commonly used to analyse actuarial senescence patterns in research fields such as genomics of ageing. We thus argue for using alternative metrics of longevity. We then propose different metrics of shape and point out the relevance of using several metrics in future comparative analyses of mortality. In particular, two shape metrics can be easily computed from observed distributions of ages at death. We illustrate our approach based on both pace and shape metrics by performing a comparative analysis of actuarial senescence and mortality across 30 species of mammals. The results strongly support the relevance of using shape metrics based on the distribution of the ages at death to assess reliably patterns of mortality. In particular, we found that the pace and the shape of ageing, although statistically independent, could be associated in mammals. We conclude that the metrics defined from the distribution of ages at death provide a complementary approach to mortality or survival curve analyses and, by offering straightforward standardization, provide a promising tool for future comparative analyses of actuarial senescence and mortality across the tree of life. A free Plain Language Summary can be found within the Supporting Information of this article.