Interaction between organ mass and citrate synthase activity as an indicator of tissue maximal oxidative capacity in breeding European Starlings: implications for metabolic rate and organ mass relationships

1. Numerous studies have suggested that basal metabolic rate (BMR) is related to variation in mass of specific ‘energy-expensive’ organs such as heart and kidney. 2. However, there are inconsistencies among studies with regard to those organs that show positive relationships with BMR, potentially because such studies assume constant organ metabolic intensity or capacity (e.g. oxidative activity). 3. This paper investigates metabolic adjustments (citrate synthase activity) in four key organs (pectoral muscle, heart, kidney and liver) in free-living reproductive female European Starlings ( Sturnus vulgaris ) throughout the breeding season to determine if the cost of egg production results in readjustments in organ mass-specific enzyme activity. 4. Citrate synthase (CS) activity varied in relation to breeding stage and/or year, but this variation was not consistent among different organs. For some organs, total enzymatic activity was directly related to changes in organ mass in relation to breeding stage. For others, increased organ mass was associated with a decrease in mass-specific CS activity, resulting in no net change in total organ oxidative capacity among breeding stages. 5. Even though the liver is actively involved in yolk precursor production during egg formation, no evidence was found for up-regulation of mass-specific oxidative enzyme activity in this organ. 6. Metabolic adjustments at the organ level may occur independently from organ mass changes and this confounding effect may be responsible for part of the inconsistencies found between previous studies in terms of organs mass–RMR relationships.