Spend less, live longer. The "thrifty aged" hypothesis.

Evolutionary biology has rejected any argument in favour of planned ageing. In this article, the metabolic changes that follow in ageing have been analysed, and in accordance with that, an evolutionary explanation for the senescence program has been proposed. Ageing is characterised by a set of metabolic features all of them designed to reduce energy expenditure and to promote energy storage. Otherwise, the undoubted beneficial effect to fitness that means to live longer could generate a conflict between parents and offspring for energy resources. The solution to that conflict is an adult economical individual, a "thrifty aged". The way to obtain this cheaper individual is through the genetically induced physiological changes that finally lead the process. In conclusion, it is proposed that ageing is the condition, imposed by natural selection, that let organisms to outlast the obligatory reproductive time.