Validation of heart rate and doubly labelled water as measures of metabolic rate during swimming in California sea lions

1. The measurement of energy expenditures in free-ranging animals is essential if we are to understand fully the interaction between a species and its environment. This study examined the validity of heart rate (fH) and doubly labelled water (DLW) as measures of field metabolic rate (FMR) in California Sea Lions (Zalophus californianus). 2. Oxygen consumption and CO2 production were measured over 24 h by direct respirometry in six juvenile sea lions. The respirometer consisted of a hood over a flume in which the sea lions were exercised to various levels for 15 min periods throughout each experiment. The exercise regime produced a mean metabolic rate which was 2.3 times the predicted basal metabolic rate (BMR) with mean maxima of 6.27 times the predicted BMR. 3. Simultaneously with direct respirometry, mean CO2 production was estimated using DLW and O2 consumption was estimated using fH, which had previously been calibrated against O2 consumption. 4. The mean\pmSD O2 consumptions from direct respirometry, fH and DLW were 11.80\pm2.40, 11.95\pm2.17 and 15.01\pm3.77 ml min-1 kg-1 respectively. Paired Student's t-tests showed no significant difference between O2 consumption by direct respirometry and the estimates from DLW and fH. DLW measurements ranged from -10% to +86% of the direct respirometry measurements (mean +36.4%) and fH measurements ranged from -28% to +23% of the direct respirometry measurements (mean +2.7%). 5. The range of estimated metabolic rates from fH was largely owing to individual differences in the slopes of the linear relationship between fH and O2 consumption. The range of metabolic rates from DLW could be partly attributed to the short duration of the experiments (24-25 h) but this was shown not to be the cause of the tendency to overestimate metabolic rate from DLW. It was concluded that both DLW and fH are valid methods for measuring FMR in California Sea Lions although it is possible that FMR could be overestimated when using DLW.