Timing is everything in selection on life-history driven by extrinsic mortality

Abstract

The central crux of modern life history theory is that extrinsic mortality compounds across life to drive reduced investment in longevity. This hypothesis has been subject to extensive laboratory and ecological testing. However, such studies often fail to support this hypothesis, and the range and inconsistency of the results has proven difficult to explain. The effect of extrinsic mortality on longevity is not as straightforward as increased hazard drives increased senescence. The shape of extrinsic mortality conditions over life shapes selection as significantly as the strength, and altering the age-structure of extrinsic mortality conditions can produce a wide range of life history phenotypes. We construct a theoretical model that shows that under typical laboratory extrinsic mortality interventions (ie a single application of mortality at a specified life stage) optimum phenotype is highly sensitive to the exact timing of the condition used. This framework provides an explanation for experimental results that do not accord with the predictions of modern life history theory and presents an opportunity to further investigate how more ecologically relevant age-structured mortality conditions could be used to solve perennial problems in ageing biology.