The short-beaked echidna (Tachyglossus aculeatus) is an exceptionally long-living mammal having a maximal lifespan of -50 years. This is about four times that predicted from its body mass and, consequently, its longevity quotient is ∼4.This longevity quotient is similar to two other exceptionally long-living mammalian species; the naked mole-rat (Heterocephalus glaber) and Homo sapiens. In recent times, the types of fats that make up cellular membrane have been implicated in the determination of a species' maximum lifespan. This modification of the oxidative stress theory of aging, which has been called the membrane pacemaker theory of aging, derives from the fact that polyunsaturated fats are very susceptible to lipid peroxidation whereas monounsaturated fats are resistant to peroxidation. As a test of the theory we measured the fatty acid composition of membrane lipids isolated from tissues of echidnas.We found that, as In the other long-living mammals, echidna membranes are more monounsaturated and less polyunsaturated than would be predicted from their body size and that the peroxidation Index of their membrane lipids is what their maximum longevity would predict.