Pearl is regarded as one of founders of biogerontology. In 1908 Max Rubner observed that mammals of different size and longevity had equal mass specific metabolic output. Partly based on the observation that the longevity of fruit flies varies inversely with ambient temperature, Pearl (like Rubner) also asserted that maximum life span is inversely proportional to basal metabolic rate. Pearl accepted Alexis Carrel's erroneous ideas that normal somatic cells don't age, and that aging must therefore be due to dysfunction at the body level. Pearl speculated that lifespan was limited by vital cell components that were depleted or damaged more rapidly in animals with faster metabolisms. Denham Harman's free-radical theory of aging later provided a plausible causal mechanism for Pearl's hypothesis.