Behaviour, temperature and terrain slope impact estimates of energy expenditure using oxygen and dynamic body acceleration

The energy used by animals is influenced by intrinsic (e.g. physiological) and extrinsic (e.g. environmental) factors. Accelerometers within biologging devices have proven useful for assessing energy expenditures and their behavioural context in free-ranging animals. However, certain assumptions are frequently made when acceleration is used as a proxy for energy expenditure, with factors, such as environmental variation (e.g. ambient temperature or slope of terrain), seldom accounted for. To determine the possible interactions between behaviour, energy expenditure and the environment (ambient temperature and terrain slope), the rate of oxygen consumption (V̇O2) was measured in pygmy goats (Capra hircus aegarus) using open-flow indirect calorimetry. The effect of temperature (9.7 to 31.5˚C) on resting energy expenditure was measured. The relationship between V̇O2 and dynamic body acceleration (DBA) was measured at different walking speeds (0.8 to 3.0 km.h-1) and on different inclines (0, +15˚, -15˚). The daily behaviour of individuals was measured in two enclosures: enclosure A (level terrain during summer) and enclosure B (sloped terrain during winter) and per diem energy expenditures of behaviours estimated using behaviour, DBA, temperature, terrain slope and V̇O2. During rest, energy expenditure increased below 22˚C and above 30.5˚C. V̇O2 (ml.min-1) increased with DBA when walking on the level. Walking uphill (+15˚) increased energetic costs three-fold, whereas walking downhill (-15˚) increased energetic costs by one third. Based on these results, although activity levels were higher in animals in enclosure A during summer, energy expenditure was found to be significantly higher in the sloped enclosure B in winter (means of enclosures A and B: 485.3 ± 103.6 kJ.day-1 and 744.5 ± 132.4 kJ.day-1). We show that it is essential to account for extrinsic factors when calculating animal energy budgets. Our estimates of the impacts of extrinsic factors should be applicable to other free ranging ungulates