When and why senescence evolves

Background/Question/Methods

Demographic senescence, the decay in fertility and survival rates with age, remains one of the most striking phenomena in ecology and evolution.  Evolutionary theories of senescence make stringent predictions about the shape of these patterns. Resting on Hamilton’s influential results, fertility and survival rates across adult ages are supposed to inevitably decrease due to the action of selective forces for all iteroparous species across the tree of life. Ecologists have found good evidence for senescence in many taxa, and in many cases have cited this as support for Hamilton’s claim of ubiquitous senescence. However, the evolutionary theories of senescence demand more than simply its detection. We will outline examples of mismatches between theory and data and go on to highlight initiatives seeking to reconcile the discrepancy.

Results/Conclusions

Age-trajectories of numerous plant and animal species are at odds with current evolutionary theories of senescence. Fertility patterns in vertebrates for example, are generally hump-shaped, and there are several groups where fertility and survival rates do not decrease from reproductive maturity onwards, as Hamilton suggested. Some species seem to escape a decrease in vital rates. For example, the desert tortoise Gopherus agassizii and the plant Borderea pyrenaica experience rising survival and fertility throughout adult life, and young specimens of the freshwater polyp Hydra vulgaris face just the same mortality and fertility as older ones. Part of the reason, why such a grave mismatch between theory and data could remain unnoticed over many decades, lies in the diverging notions (e.g. physiological vs. demographic senescence), definitions and measures of senescence across disciplines. The newly developed concepts of “pace” (the timescale of life as measured e.g. by life-expectancy) and “shape” (the direction and magnitude of change in vital rates as measured e.g. by percentage change over the life course) enable us to extract a more exact comparative picture of the variability of senescence patterns across the tree of life. These concepts have been nourished by powerful new statistical methods (BaSTA and IPMpack-software packages) and two comprehensive demographic datasets, DATLife (Demography Across the Tree of Life) and COMPADRE II (COMPArative Demographic REsearch), which collectively contain demographic data on hundreds of species. The emerging natural question is then: why and how do organisms escape senescence? To answer this question, mutual understanding and agreement about concepts and measures of senescence are crucial.