Plastic but repeatable: rapid adjustments of mitochondrial function and density during reproduction in a wild bird species

Most of the energy fluxes supporting animal performance flow through mitochondria. Hence, inter-individual differences in performance might be rooted in inter-individual variations in mitochondrial function and density. Furthermore, because the energy required by an individual often changes across life stages, mitochondrial function and density are also expected to show within-individual variation ( i.e. plasticity). No study so far has repeatedly measured mitochondrial function and density in the same individuals to simultaneously test for within-individual repeatability and plasticity of mitochondrial traits. Here, we repeatedly measured mitochondrial DNA copy number (a proxy of density) and respiration rates from blood cells of female pied flycatchers ( Ficedula hypoleuca ) at the incubation and chick-rearing stages. Mitochondrial density and respiration rates were all repeatable ( R = [0.45-0.80]), indicating high within-individual consistency in mitochondrial traits across life-history stages. Mitochondrial traits were also plastic, showing a quick ( i.e. 10 days) down-regulation from incubation to chick-rearing in mitochondrial density, respiratory activity, and cellular regulation by endogenous substrates and/or ATP demand. These downregulations were partially compensated by an increase in mitochondrial efficiency at the chick-rearing stage. Therefore, our study provides clear evidence for both short-term plasticity and high within-individual consistency in mitochondrial function and density during reproduction in a wild bird species.