![]() However, comparative data across multiple populations of nonhuman animals are rarely available, making it difficult to unveil the forces underlying mortality differences within versus between species. Understanding the biological constraints on ageing requires mortality data for multiple populations of nonhuman species, as well as for humans. The tight positive linear relationship between life expectancy ( e 0) and lifespan equality ( \(\varepsilon\) 0) across a large range of human populations indicates strong but poorly understood constraints underlying variation in human mortality 3, 13. For instance, if deaths are evenly distributed across age classes or show multiple modes, the result is high lifespan variation and low lifespan equality, while if deaths are concentrated at the tail-end of the lifespan distribution (as in most developed nations), the result is low lifespan variance and high lifespan equality. The distribution of ages at death tells us whether deaths are evenly distributed across the range of observed lifespans, or are concentrated around certain ages. Lifespan equality is highly correlated with other measures of the distribution of ages at death, such as the coefficient of variation and the Gini coefficient, often used to measure economic inequality 13. ![]() While life expectancy at birth (a measure of the ‘pace’ of mortality 15) describes the average lifespan in a population, lifespan equality (a measure of the ‘shape’ of mortality 15) describes the spread in the distribution of ages at death in a population (see also 16, 17). Although no consensus has been reached about the invariant rate of ageing hypothesis, further evidence that biological constraints may shape human ageing comes from the remarkably consistent relationship between life expectancy at birth ( e 0) and lifespan equality ( \(\varepsilon\) 0) in a diverse set of human populations 3, 13, 14. Understanding the nature and extent of biological constraints on the rate of ageing and other aspects of age-specific mortality patterns is critical for identifying possible targets of intervention to extend human lifespans, and for understanding the evolutionary forces that have shaped lifespans within and across species. On the other hand, across multiple mammal species, measurable differences in the rate of ageing have been documented between populations in different environments (e.g. Furthermore, Bronikowski and colleagues 11 observed greater variation in initial adult mortality than in the rate of ageing across several populations of baboons. ![]() Several studies have documented a strong phylogenetic signal in the rate of ageing across multiple species of birds and mammals, suggesting biological constraints and little within-species variance in this rate 9, 10. This ‘invariant rate of ageing’ hypothesis has received mixed support. Indeed, researchers have long hypothesised that the rate of ageing is relatively fixed within species, not only in humans but also other animals 7, 8, 9. These correlations suggest that ageing evolves in concert with a suite of other traits, which may produce constraints on the rate of ageing within species. Consequently, the nature of biological constraints on ageing is a central problem in the health sciences and, because of its implications for demographic patterns, is also of long-standing interest in ecology and evolutionary biology.Īcross species, rates of ageing are strongly correlated with other aspects of the life history-pre-adult mortality, age at first reproduction, birth rate, metabolic rate and generation time-as well as with morphological traits such as body size and growth rate 5, 6. Further substantial extensions of human longevity will depend on whether it is possible to slow the rate of ageing or otherwise reduce late life mortality. These gains have resulted from shifting the majority of deaths from early to later and later ages, with no evidence of slowing the rate at which mortality increases with age (i.e. The maximum human life expectancy has increased since the mid-1800s by ~3 months per year 1. ![]() Nature Communications volume 12, Article number: 3666 ( 2021) The long lives of primates and the ‘invariant rate of ageing’ hypothesis
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