On the evolution of growth and senescence /

Abstract

Consistent associations between growth and senescence are seen throughout nature. Whilst a larger size correlates positively with lifespan between species, this relationship is reversed within a species so that the smallest members tend to be the longest‐lived. Indeterminate growth ‐ i.e. growth that continues post‐maturity ‐ is a strong predictor for an especially slow rate of ageing. A number of interventions which alter the rate of growth, especially at a point early in development, have been shown to have enduring effects on later growth and lifespan. This thesis provides a theoretical examination of why relationships such as these may have evolved. Two dynamic programming models are here presented. Both consider associations between growth and longevity within a species and ask whether these are compatible with idea of a trade‐off between somatic maintenance and other fitness‐enhancing functions as predicted by the disposable soma theory. The first reproduces the sexual dimorphism in longevity and in body size seen baboons; it predicts that males should ‘choose’ a faster rate of ageing and a greater investment in growth than females. The second suggests that a faster rate of ageing may be an optimal response to low food availability in early life in humans. A critical appraisal is also given to two recent theories of the evolution of ageing which rely explicitly on differences in body size and/or growth to explain differences in lifespan: the hyperfunction theory and the heat dissipation limit theory. What these can teach us about the evolution of senescence and whether they can provide plausible challenges to the disposable soma theory is considered.