If you want a long life, make sure to senesce

Background/Question/Methods

At first sight, evolution should act forcefully against senescence, the process of deterioration with age. A mutation that has the sole effect to increase life expectancy has an evolutionary edge. Thus, it is hard to see why evolution would tolerate senescence. Because all other things being equal life expectancy is longer without senescence, evolutionary explanations for the phenomenon have been found in the trade-off of life expectancy with reproduction (disposable soma theory), and in the phenomenon that late-life events face less stringent Darwinian selection (mutation accumulation and antagonistic pleiotropy). These ideas do not preclude other possibilities.

A third possible explanation is based on the fact that senescent organisms can simply live longer than non-senescent organisms. A constant but high mortality rate can result in a shorter life than a mortality rate that increases with age from a very low initial level. There are several processes that could cause mortality to start off low, but to increase during the course of life. These processes have in common that they improve early performance because the organism is better at coping with its environment, while this state is unmaintainable.

In this presentation we 1) introduce this alternative line of reasoning, 2) discuss a number of processes that could lead to the envisaged trade-off, and 3) present a simple and general model to understand what mortality trajectory is optimal for an organism given a certain environment and physiological constraints.

Results/Conclusions

Our results show that the optimal solution always fully depends on the combination of the organism’s state and the environment. Neither the environment, nor the physiological constraint alone gives information about the optimal solution.

The results of our model contribute to understanding life histories in which high age-independent mortality is combined with absence of senescence (e.g. in case of hydra). Ecologists are in an excellent position to chart environmental challenges and constraints, while other disciplines, e.g. biophysics, biochemistry and physiology, are able to contribute knowledge about the processes that impose physiological constraints. The theory can provide ecologists with new hypotheses about the system they study.