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Aardwolves occur in east-central and southern Africa in regions of arid and semiarid grasslands. In South Africa, they feed almost exclusively on the termite Trinervitermes trinervoides during the warmer months of the year, whereas on cold winter nights these insects remain belowground, unavailable to aardwolves, At this time aardwolves shift their foraging efforts to a less abundant termite species, Hodotermes mossambicus. This species provides less food for aardwolves, making the wintertime potentially a period of energy stress for them.

In this study, we report the first estimates of field metabolism and water flux as determined by doubly labeled water for a myrmecophagous euthermic mammal. Specifically, we tested the hypotheses that a reduced basal metabolic rate and pulmocutaneous water loss lead to a conservative field metabolic rate (FMR) and water flux in free-living aardwolves during summer and winter. Further, we quantified the activity budget of aardwolves during summer and winter to construct a time-energy budget for them. We used knowledge of water flux in aardwolves and of the water content of their prey to construct a water budget during summer and winter, and to estimate the seasonal consumption of termites by aardwolves.

During summer (November-January), aardwolves that averaged 7768 g in body mass had a CO2 production of 112.5 L CO2/d, equivalent to an energy expenditure of 2891.2 kJ/d. During winter (June-July), when body mass averaged 8543 g, CO2 production averaged 71.8 L CO2/d, which translates to an FMR of 1844.8 kJ/d. Water flux equaled 615 mL/d in summer, but only 292 mL/d in winter, a decrease of 53.1%.

Aardwolves were active for an average of 8.77 h/d in summer, but only 4.05 h/d during winter. A time budget for active periods showed that aardwolves spent a significantly larger proportion of their active time feeding in summer than in winter (26.7 vs. 15.2%). In addition, males and females spent 12.6% of their time in winter in breeding activities such as courtship and copulation. For aardwolves in summer, our model estimated a DEE of 2370.5 kJ/d, whereas during winter it predicted a DEE of 2086 kJ/d. These values deviated from our estimates of FMR by -18% and +13.1%, respectively.

From data on water flux and the water content of termites, we estimated that aardwolves consumed 222445 Trinervitermes per night during the summer. Using previously obtained data on foraging, we predicted that aardwolves consume 833 termites per patch with an intake rate of 33 termites per second while feeding. During winter, aardwolves consumed 5867 Hodotermes and 2515 Trinervitermes per night. Our estimate of gross energy consumption was 5616 kJ/d in summer and 3022 kJ/d in winter Assuming an assimilation efficiency of 0.66, metabolizable energy intake would be 3707 kJ/d in summer and 1995 kJ/d in winter. These values exceed FMR measurements by +22% in summer and by +8.1% in winter. It appears that aardwolves were in positive energy balance during our measurements of FMR.