When Senescence Is Optimal and When It Is Not

A simple model is developed that captures the main features of life reproduction, growth and maintenance. These features are determined by a single state variable, vitality. The resulting optimal life-history strategies are classified with respect to the course of their characteristic age-trajectory of mortality. It is shown that the range of optimal life histories is wide. The main factors that determine whether an optimal lifehistory follows a non-senescent strategy or a senescent strategy are the costs of growth and maintenance. Of almost equal importance are the costs of reproduction. Mortality conditions may have a strong influence on the boundary between non-senescence and senescence if the costs of maintenance and reproduction are relatively low. If the costs are too high, even reduction of intrinsic or extrinsic mortality to zero cannot shift a senescent strategy to a non-senescent one. Efficient systems of reproduction and growth and, to some extent, low mortality conditions lead to non-senescent life-history strategies.