High levels of isotope elimination improve precision and allow individual-based measurements of metabolic rates in animals using the doubly labeled water method

Doubly labeled water (DLW) can be used to measure energy expenditure in free-ranging animals, but questions have been raised about its accuracy in different species or contexts. We investigated whether differences in the extent of isotope elimination affects the precision and accuracy of the DLW method, which can vary according to the experimental design or metabolic rate of the species. Estimated total energy expenditure by the DLW method (TEE(dlw)) was compared with actual total energy expenditure simultaneously measured via respirometry (TEE(resp)) in streaked shearwaters Calonectris leucomelas, a pelagic seabird. Subjects were divided into three groups with different experimental conditions: at rest on the ground for 24 h (Group A) or for 48 h (Group B), and at rest on the water for 24 h (Group C). TEE(dlw) in Group A matched TEE(resp), whereas there was an overestimation of TEE(dlw) in both Groups B and C compared with TEE(resp). However, compared with Group A, TEE(dlw) in Groups B and C had reduced the isotopic analytical variability and thus higher precision. The best regression model (TEE(dlw) = 1.37 TEE(resp) − 14.12) showed a high correlation (R(2) = 0.82) between TEE(dlw) and TEE(resp) and allows a correction factor for field metabolic rates in streaked shearwaters. Our results demonstrate that the commonly made assumption that the DLW method is not appropriate for individual-based estimates may be incorrect in certain circumstances. Although a correction factor may be necessary when using the DLW method to estimate metabolic rate, greater levels of isotope eliminations provides DLW estimates with high precision, which can adequately represent relative individual estimates. Nevertheless, the DLW method, should be used with caution when characterizing interspecies difference of energy expenditures.