Variation in actuarial senescence does not reflect life span variation across mammals

The concept of actuarial senescence (defined here as the increase in mortality hazards with age) is often confounded with life span duration, which obscures the relative role of age-dependent and age-independent processes in shaping the variation in life span. We use the opportunity afforded by the Species360 database, a collection of individual life span records in captivity, to analyze age-specific mortality patterns in relation to variation in life span. We report evidence of actuarial senescence across 96 mammal species. We identify the life stage (juvenile, prime-age, or senescent) that contributes the most to the observed variation in life span across species. Actuarial senescence only accounted for 35%–50% of the variance in life span across species, depending on the body mass category. We computed the sensitivity and elasticity of life span to five parameters that represent the three stages of the age-specific mortality curve—namely, the duration of the juvenile stage, the mean juvenile mortality, the prime-age (i.e., minimum) adult mortality, the age at the onset of actuarial senescence, and the rate of actuarial senescence. Next, we computed the between-species variance in these five parameters. Combining the two steps, we computed the relative contribution of each of the five parameters to the variance in life span across species. Variation in life span was increasingly driven by the intensity of actuarial senescence and decreasingly driven by prime-age adult mortality from small to large species because of changes in the elasticity of life span to these parameters, even if all the adult survival parameters consistently exhibited a canalization pattern of weaker variability among long-lived species than among short-lived ones. Our work unambiguously demonstrates that life span cannot be used to measure the strength of actuarial senescence, because a substantial and variable proportion of life span variation across mammals is not related to actuarial senescence metrics.精算衰老（actuarial senescence）的概念（本文将其定义为随年龄增长的死亡危险率升高）常与寿命时长相混淆，这掩盖了年龄依赖与年龄非依赖过程在塑造寿命差异中的相对作用。我们借助Species360数据库提供的研究契机——该数据库收录了圈养个体的寿命记录——来分析与寿命差异相关的年龄特异性死亡模式。我们报告了96种哺乳动物存在精算衰老的证据，并明确了对各物种间寿命差异贡献最大的生命阶段（幼年期、壮年或衰老期）。根据体重类别的不同，精算衰老仅能解释物种间35%~50%的寿命方差。我们计算了寿命对五个参数的敏感性与弹性，这五个参数代表了年龄特异性死亡曲线的三个阶段：幼年期时长、平均幼年期死亡率、壮年（即最低）成年死亡率、精算衰老起始年龄，以及精算衰老速率。随后，我们计算了这五个参数在物种间的方差。结合上述两步分析，我们得到了五个参数各自对物种间寿命方差的相对贡献。从小型到大型哺乳动物，寿命差异愈发由精算衰老的强度驱动，而壮年成年死亡率的驱动作用则逐渐减弱，这源于寿命对这些参数的弹性发生了变化——尽管所有成年存活参数均呈现出稳态化模式：长寿物种的变异性始终弱于短寿物种。我们的研究明确证实，无法通过寿命来衡量精算衰老的强度，因为哺乳动物中存在相当大且类群间存在差异的寿命变异，其与精算衰老指标并无关联。