INTERINDIVIDUAL AND INTRAINDIVIDUAL VARIATION IN DAILY ENERGY-EXPENDITURE OF THE POUCHED MOUSE (SACCOSTOMUS-CAMPESTRIS)

1. We measured the simultaneous energy expenditure of five male and si x female pouched mice (Saccostomus campestris), over 3 consecutive day s, using the doubly labelled water (DLW) technique. The aim was to ass ess the interindividual scaling of daily energy demands and also the r epeatability of measures of daily energy expenditure when using doubly labelled water. 2. Individual mice were housed in cages, given free a ccess to food and water and were kept in a natural photoperiod (Februa ry at 26-degrees-S) at a mean temperature of 26.5-degrees-C (daily ran ge 25-28-degrees-C). 3. Males were significantly heavier than the fema les, as is typical in this species. Mean energy expenditure (Watts) (s ingle-pool model) over 3 days, across individuals, was positively rela ted to body mass (reduced major axis scaling exponent b=1.96) and to c hange in body mass over the 3 days. Together these variables explained 75% of the variation in energy expenditure. Sex was not a significant factor influencing energy expenditure, once the mass effects had been removed. The high exponent in this relationship means that it is disp roportionately costly for these animals to have high body masses. High cost may thus be a significant ecological factor limiting evolution o f body size in this species. Use of the two-pool model did not change the exponents. 4. Changing mass by 1 g involved a change in daily ener gy expenditure of 7.1 kJ. The maximum theoretical energy content of 1 g of tissue was 39 kJ, hence the maximum conversion efficiency during tissue deposition or withdrawal was 82% [(39-7.1)/39]. Using the two-p ool model to calculate energy expenditure changed this estimate to 83. 5%. 5. Intraindividual variation in the daily energy expenditure was l arge. The average coefficient of variation across individuals was 24.5 % (SD = 7.9%, n = 9). This variation was not an artefact of errors in the isotopic analysis, but probably reflected imbalance in energy budg ets over the time-scale of single days. 6. Measures of energy expendit ure of free-living animals using DLW are often made over 24-h periods. The ecological relevance of these measures may be questioned if the a nimals are not balancing their energy budgets over this time-scale.