Monounsaturated Fatty Acids in Birds

23 The evolution of lifespan is a central question in evolutionary biology, begging the question why 24 there is so large variation among taxa. Specifically, a central quest is to unravel proximate causes of 25 ageing. Here we show that the degree of unsaturation of liver fatty acids predicts maximum lifespan 26 in 107 bird species. In these birds, the degree of fatty acid unsaturation is positively related to 27 maximum lifespan across species. This is due to a positive effect of monounsaturated fatty acid 28 content, while polyunsaturated fatty acid content negatively correlates with maximum lifespan. 29 Furthermore, fatty acid chain length unsuspectedly increases with maximum lifespan independently 30 of degree of unsaturation. These findings tune theories on the proximate causes of ageing while 31 providing evidence that the evolution of lifespan in birds occurs in association with fatty acid 32 profiles. This finding suggests that studies of proximate and ultimate questions may facilitate our 33 understanding of these central evolutionary questions. 34 35 36 37 38 39 40 41 42 43 44 45