The rate of living theory postulates that the faster an organism’s metabolism, the shorter its lifespan. First proposed by Max Rubner in 1908, the theory was based on his observation that smaller animals had faster metabolisms and shorter lifespans compared to larger animals with slower metabolisms. The theory gained further credibility through the work of Raymond Pearl, who conducted experiments on drosophila and cantaloupe seeds, which supported Rubners initial observation. Pearls findings were later published in his book, The Rate of Living, in 1928, in which he expounded upon Rubner's theory and demonstrated a causal relationship between the slowing of metabolism and an increase in lifespan. 
The theory gained additional credibility with the discovery of Max Kleibers law in 1932. Kleiber found that an organisms basal metabolic rate could be predicted by taking 3/4 the power of the organism's body weight. This finding was noteworthy because the inversion of the scaling exponent, between 0.2 and 0.33, also demonstrated the scaling for both lifespan and metabolic rate, and was colloquially called the "mouse-to-elephant" curve.