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

1. Crucial to our understanding of the ageing process is identifying how
traits change with age, which variables alter their ageing process and how
these traits associate with fitness. 2. Here we investigated metabolic
ageing in outdoor-living captive zebra finches experiencing foraging
costs. We longitudinally monitored 407 individuals over 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), measured as BMRm but at ambient
temperatures below thermoneutrality. 3. We define mosaic or asynchronous
ageing as the difference in standardized absolute ageing rates between
traits, and we estimate the degree of asynchrony using the
within-individual correlation of change in trait values with age. 4. BMRm
decreased linearly with age, consistent with earlier reports. In contrast,
SMRm increased linearly with age. The absolute standardized change with
age was significantly faster for BMRm compared to SMRm, and the
within-individual correlation of age related change was negligible. To the
best of our knowledge, this is the first quantification of SMRm ageing,
and the finding that SMRm and BMRm age in opposite directions. 5. Neither
metabolic rate nor metabolic ageing rate were associated with variation in
lifespan between individuals. Moreover, experimental manipulations of
environmental quality that decreased BMRm and SMRm and shortened lifespan
by 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. 6. Our findings
indicate, in contrast to the current view, that baseline energy
requirements increase with age, because animals do not generally live in
thermoneutral conditions, and illustrate the importance of studying the
ageing phenotype in an ecologically realistic setting.