Can experiments on caloric restriction be reconciled with the disposable soma theory for the evolution of senescence?
A publication by Shanley and Kirkwood (2000) attempts to explain data on caloric restriction (CR) and life extension in the context of the Disposable Soma (DS) theory for the evolution of senescence. As the authors concede, this juxtaposition appears at first to offend intuition: According to the DS theory, senescence is the result of a tight budget for caloric energy, such that repair and maintenance functions are shortchanged; yet, in CR experiments, it is found that longevity decreases smoothly as the total caloric budget is increased. In the Shanley-Kirkwood model, an optimized allocation of resources causes energy to be diverted away from somatic maintenance at a greater rate than caloric intake increases, with the net result that more total energy is associated with less energy available for maintenance. In the present critique, the limitations of this model are detailed and its special assumptions reviewed. While the CR experiments find comparable life extension for males and females, measured relative to nonbreeding controls, the Shanley-Kirkwood model draws its energy budget from data on breeding females. In addition, the success in reproducing the observed relationship between feeding and longevity depends crucially on a mathematical relationship between food availability and the probability of reproductive success which may be difficult to justify.