Why might total metabolism be a useful way to compare life spans among species?

Comparing life spans across species using units of time can be misleading because metabolic rates vary significantly due to environmental factors, temperature, and periods of dormancy like hibernation or torpor.12

Total metabolism is considered a potentially more adequate measure because it accounts for the "rate of living" theory, which suggests that aging may depend on the expenditure of a fixed amount of vital energy. By measuring total lifetime energy expenditure rather than just chronological time, researchers can better compare species that have vastly different activity levels or environmental exposures. For example, while some small mammals like bats have long chronological life spans, their lifetime energy expenditures are comparable to other small mammals because they spend significant time in deep torpor, which drastically reduces their metabolic rate.12

However, it is important to note that while total metabolism offers a compelling analytical framework, the comprehensive data required to apply this metric across diverse species are currently almost entirely lacking.2