Metabolic costs of growth in free-living Garter Snakes and the energy budgets of ectotherms

1, The metabolic or respiratory cost of growth (R-G) is the increase in met abolic late of a growing animal, and it represents chemical potential energ y expended in support of net biosynthesis but not deposited as new tissue. 2, Two statistical methods (multiple non-linear regression and analysis of regression residuals) were used to calculate R-G from data (n = 68) from a doubly labelled water study of free-ranging Garter Snakes (Thamnophis sirta lis fitchi) in northern California. 3. The sample-wise ('ecological') cost of growth was 2.07 kJ per gram of ne t growth (equivalent to 8.63 kJ g(-1) dry tissue); reanalysis of a subset o f efficient growers yielded a more conservative 'physiological' estimate of 1.67 kJ g(-1). 4. Our empirical estimate of R-G, among the first reported for squamate rep tiles and free-living animals of any kind, compares closely with published, laboratory-derived values for ectotherms. 5, The metabolic costs of growth accounted for an average of 30% of total f ield metabolic rates for these snakes, which were growing at a mean rate of 3% of body mass per day. However, our method probably underestimated the t otal ecological cost of growth for large animals, because potential growth costs that covary with body size were not included. 6. Distinction between conceptual and empirical energy budgets clarifies re lationships among body size, metabolic rates, and the physiological and eco logical costs of growth.