Mosaic metabolic ageing: Basal and standard metabolic rate age in opposite directions and and independent of environmental quality, sex or lifespan in a passerine

O_LICrucial to our understanding of the ageing process is identifying how traits change with age, which variables alter their ageing process and whether these traits associate with lifespan. C_LIO_LIWe here investigated metabolic ageing in zebra finches. We longitudinally monitored 407 individuals during six years and collected 3213 measurements of two independent mass-adjusted metabolic traits: basal metabolic rate (BMRm) at thermoneutral temperatures and standard metabolic rate (SMRm), which is the same as BMRm but at ambient temperatures below thermoneutrality. C_LIO_LIBMRm decreased linearly with age, consistent with earlier reports. In contrast, SMRm increased linearly with age. To the best of our knowledge, this is the first quantification of SMRm ageing, and thereby of the contrast between SMRm and BMRm ageing. C_LIO_LINeither metabolic rate nor metabolic ageing rate were associated with individual lifespan. Moreover, experimental manipulations of environmental quality that decreased BMRm and SMRm and shortened lifespan with 6 months (12%) did not affect the ageing of either metabolic trait. Females lived 2 months (4%) shorter than males, but none of the metabolic traits showed sex-specific differences at any age. C_LIO_LIOur finding that ageing patterns of metabolic rate vary depending on the ambient temperature illustrates the importance of studying ageing in an ecologically realistic setting. C_LIO_LIOur results add to the mounting evidence that within an organism ageing is an asynchronous process. C_LI