Inter- and intraindividual variation in daily energy expenditure of the pouched mouse (Saccostomus campestris)

We measured the simultaneous energy expenditure of five male and six female pouched mice (Saccostomus campestris), over 3 consecutive days, using the doubly labelled water (DLW) technique. The aim was to assess the interindividual scaling of daily energy demands and also the repeatability of measures of daily energy expenditure when using doubly labelled water. 2. Individual mice were housed in cages, given free access to food and water and were kept in a natural photoperiod (February at 26⚬S) at a mean temperature of 26.5⚬C (daily range 25-28⚬C). 3. Males were significantly heavier than the females, as is typical in this species. Mean energy expenditure (Watts) (single-pool model) over 3 days, across individuals, was positively related to body mass (reduced major axis scaling exponent b=1.96) and to change 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 disproportionately costly for these animals to have high body masses. High cost may thus be a significant ecological factor limiting evolution of 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 energy 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-pool model to calculate energy expenditure changed this estimate to 83.5%. 5. Intraindividual variation in the daily energy expenditure was large. 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 budgets over the time-scale of single days. 6. Measures of energy expenditure of free-living animals using DLW are often made over 24-h periods. The ecological relevance of these measures may be questioned if the animals are not balancing their energy budgets over this time-scale.