Within-individual changes in mitochondrial DNA copy number across the life course and links to individual performance

Ageing is characterized by complex biological processes reflected in cellular and molecular changes. Mitochondria, which are crucial for energy production and cellular homeostasis, are particularly vulnerable to age-related deterioration. The number of copies of mitochondrial DNA (mtDNAcn) varies across and within tissues in response to energetic activity and mtDNA integrity, and is related to health and physical performance. Age-related changes in mtDNAcn can be difficult to study due to differential survival of phenotypes into older ages, but studies of changes in mtDNAcn within individuals are very limited. In this study, we investigated changes in red blood cell mtDNAcn across the life course within individual zebra finches (Taeniopygia guttata), a well-established avian model, from the nestling stage into old age. Our findings revealed a pronounced decline in relative mtDNAcn during post-natal development, followed by comparative stability throughout adulthood. This pattern was remarkably consistent among individuals. We found no significant relationship between variation in mtDNAcn and growth during the nestling period. However, based on measurements of disturbed take-off speed in late adulthood, we found that individuals with higher physical performance at that stage had higher relative mtDNAcn, suggesting a link between variation in individual bioenergetics and biological state.