The rate of living theory postulates that the faster an organism’s metabolism, the shorter its lifespan. The theory was originally created by Max Rubner in 1908 after his observation that larger animals outlived smaller ones, and that the larger animals had slower metabolisms. After its inception by Rubner, it was further expanded upon through the work of Raymond Pearl. Outlined in his book, The Rate of Living published in 1928, Pearl conducted a series of experiments in drosophila and cantaloupe seeds that corroborated Rubner’s initial observation that a slowing of metabolism increased lifespan. Further strength was given to these observations by the discovery of Max Kleiber’s law in 1932. Colloquially called the “mouse-to-elephant” curve, Kleiber’s conclusion was that basal metabolic rate could accurately be predicted by taking 3/4 the power of body weight. This conclusion was especially noteworthy because the inversion of its scaling exponent, between 0.2 and 0.33, was the scaling for lifespan and metabolic rate.
Mechanism
Mechanistic evidence was provided by Denham Harman's free radical theory of aging, created in the 1950s. This theory stated that organisms age over time due to the accumulation of damage from free radicals in the body. It also showed that metabolic processes, specifically the mitochondria, are prominent producers of free radicals. This provided a mechanistic link between Rubner's initial observations of decreased lifespan in conjunction with increased metabolism.
Current state of theory
Support for this theory has been bolstered by studies linking a lower basal metabolic rate to increased life expectancy. This has been proposed by some to be the key to why animals like the Giant Tortoise can live over 150 years. However, the ratio of resting metabolic rate to total daily energy expenditure can vary between 1.6 and 8.0 between species of mammals. Animals also vary in the degree of coupling between oxidative phosphorylation and ATP production, the amount of saturated fat in mitochondrial membranes, the amount of DNA repair, and many other factors that affect maximum life span. Furthermore, a number of species with high metabolic rate, like bats and birds, are long-lived. In a 2007 analysis it was shown that, when modern statistical methods for correcting for the effects of body size and phylogeny are employed, metabolic rate does not correlate with longevity in mammals or birds.
Disproving the Rate-of-Living theory
An article published in January 2020 demonstrates that there is no evidence for the Rate-of-Living theory across the tetrapod tree of life. Written by Gavin Stark, Daniel Pincheira-Donoso, and Shai Meiri, the article, explains why the Rate-of-Living theory no longer holds true. By using a data set that showed the lifespan of thousands of vertebrates and using the different ideas that the Rate-of-Living theory supports, they were able to come up with an experiment that was able to test how these ideas, such as metabolic rates relate to the lifespan of different vertebrates. The experiment results showed that the correlation between metabolic rate and the lifespans of animals especially for vertebrates doesn't hold true as originally thought. Other relationships that help to increase longevity such as colder climates for reptiles and amphibians were found. However, this relationship did not hold true for mammal species. The experiment also pointed out that reptiles and amphibians that are the same size as mammals and birds have relatively close lifespans to each other even though these species have significantly different metabolic rates. Overall the experiment proves that the rate of living theory doesn't support the lifespan of land vertebrates but instead proposes that lifespan is instead influenced by selection from external elements. In another article, Anne Bronikowski and David Vleck also explain how the Rate-of-living theory didn't quite follow their research either. By using the garter snake as the subject, they studied two different groups. One group had a shorter life span and fast growth which resulted in early reproduction, and the other group consisted of snakes with a longer lifespan, later reproductive rate, and later maturation. They wanted to determine if metabolism in fully grown and mature animals were different from the two different groups they were examining. It was discovered that the Rate-of-living theory initially, did agree with their metabolic results which demonstrated that the faster group would reach a larger mass, and had a higher metabolic rate compared to the longer lived group. The correlation between metabolism and lifespan the Rate-of-living theory created did prove true in this part of the experiment. However, Bronikowski and Vleck discovered the relationship between the Rate-of-living theory and their experiment isn't that straightforward. Between both groups, their isn't a large difference of their metabolic rates at birth. Resulting in the Rate-of-living theory inconsistent. It was also found that their wasn't a difference in either groups resting metabolic rate. Bronikowski and Vleck came to the conclusion that the reason for their results was because of increased oxygen consumption from the shorter lived group could have caused oxidative stress in the snakes resulting in a shorter lifespan.
