Resting and daily energy expenditures during reproduction are adjusted in opposite directions in free-living birds

1. Reproduction is energetically expensive, and daily energy expenditure (DEE) often peaks during the period of rearing young. The “potentiation” hypothesis predicts that high DEE needs to be sustained by a corresponding up-regulation of metabolic machinery, thus a concomitant increase of the resting metabolic rate (RMR) is expected. Alternatively, the “compensation” hypothesis predicts that DEE and RMR are regulated independently and animals may maintain low RMR to maximize the energy available for reproduction. This might particularly be the case if DEE was limited, e.g. by extrinsic food supply or intrinsic physiological factors. 2. We tested these hypotheses in free-living seabirds by manipulating their energy demands (experimentally reduced or increased brood size) and supplies (providing supplemental food), and simultaneously measuring their DEE and RMR (by the doubly-labeled water method and an indirect hormonal proxy, respectively). 3. In support of the ‘compensation’ hypothesis...