1. 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.2. 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.3. 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.4. 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.
K E Y W O R D Sageing, comparative analysis, demography, longevity, senescence 66 | Functional Ecology RONGET aNd GaILLaRd.
Senescence:A general process involving the decrease of individual performance with increasing age.Actuarial senescence: The increase of mortality with age.
Rate of actuarial senescence:The magnitude of increase of mortality rate with age.Onset of actuarial senescence: The age when actuarial senescence begins.Longevity: A metric measuring the lifetime of a given species, population or individual.Age-specific mortality rate: The probability to die at each age.Hazard of mortality (also called force of mortality µ): The instantaneous rate of mortality usually preferred over mortality rate in demographic analysis.
Cumulative survival:The percentage of remaining individuals at each age from birth (100%) to one year beyond the oldest individual (0%).
Gompertz model of mortality:A model commonly used that describes the mortality ...