The landscape of senescence and lifespan across the tree of life

Background/Question/Methods

Some species live for a very long time, others have fleeting lives. Animals like elephants, whales and tortoises are comparatively long lived and in the plant kingdom bristlecone pines have life spans of around 5,000 years. At the other end of the scale, creatures such as the mayfly or annual plants live for just a day or a year. Underlying these life span estimates are the demographic trajectories of mortality and fertility that respectively capture the changing probability of death and number of offspring produced, with age. The canonical evolutionary theories of ageing suggest that senescence, a decline in fertility or an increase in mortality risk with age, is inevitable. Is it really?  I explore demographic data from the animal and plant kingdoms to question the inevitability of senescence and to highlight the need to examine a broad taxonomic range in order to understand the evolution of demographic traits.

Results/Conclusions

Demographic trajectories of fertility and mortality vary widely and can be classified in a continuum from strongly deteriorating with age, to negligible change, to improvement with age (negative senescence). Typical life span does not have a strong influence on where on this continuum a species lies: strongly senescent species can be short, or long-lived, as can negatively senescent species. However, there is good evidence that phylogenetic relatedness influences placement on the continuum, which suggests that primitive traits have an important role in determining aging patterns. The diversity of aging patterns contradicts theoretical predictions and suggests that extension to theory is needed. It is also obvious that high-quality demographic data are mainly available for a taxonomically biased set of species, which limits our understanding of the universality of aging patterns.