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 w...