Bats possess an exceptional maximum lifespan in relation to their body size when compared to other similar sized mammals. Consequently, bats are excellent models in which to investigate the mechanism that underlies the process of ageing. The oxidative stress theory proposes that species with a high maximum lifespan possess a larger quantity of membrane lipids resistant to peroxidation, compared to species with a lower maximum lifespan. As fatty acid composition of livers, hearts and skeletal muscles may potentially offer a better explanation for the long maximum lifespan of bats, monounsaturated fatty acids, polyunsaturated fatty acids and saturated fatty acids of four bat species (Nyctalus noctula, Vespertilio murinus, Hypsugo savii and Pipistrellus pipistrellus) were measured. When compared to other similar sized mammals with lower maximum lifespans such as Mus musculus (normal house mouse), bat membranes were seen to contain a higher amount of monounsaturated fatty acids and a lower amount of polyunsaturated fatty acids, thus conforming to the membrane pacemaker hypothesis. Docosahexaenoic acid (22:6 n-3) is a frequently investigated polyunsaturated fatty acid in literature and is supposedly prone to oxidation, so it is an important fatty acid to compare with. This study confirms the expected correlation of monounsaturated fatty acids with maximum lifespan of four bat species (Nyctalus noctula, Vespertilio murinus, Hypsugo savii, Pipistrellus pipistrellus) and other similar sized mammals. For example, docosahexaenoic acid has got a negative impact on maximum lifespan, whilst monounsaturated fatty acids instead show a positive influence. Within Pipistrellus pipistrellus, docosahexaenoic acid (22:6 n-3) values together with the Peroxidation Index (PI) were higher than expected with no plausible explanation. Due to the fact that Heterocephalus glaber (naked mole rat), a similar sized mammal (~35 g), also possesses an exceptional maximum lifespan (> 28.3 years), a special focus was given to comparing these two species. Although docosahexaenoic acid (22:6 n-3) liver mitochondria levels within Heterocephalus glaber were remarkably lower when compared to those of bats, comparisons still confirm the oxidative stress theory of declining polyunsaturated fatty acid values with increasing maximum lifespan. The same hypothesis applies for the peroxidation index. Overall, membrane composition is undoubtedly a determining factor influencing maximum lifespan.