Energy budgets of lactating and non-reproductive Brown Long-Eared Bats (Plecotus auritus) suggest females use compensation in lactation

1. The energy budgets of lactating and non-reproductive female Brown Long-Eared Bats fed primarily on noctuid moths (approximate to 27.2 kJ g(-1)) were constructed and compared in flight enclosures in captivity. 2. The average dry food consumption of non-reproductive individuals was 1.8 g bat(-1) day(-1) (gross energy intake = 48 kJ day(-1)). The average food consumption throughout days 10-35 of lactation was 2.0 g bat(-1) day(-1) (gross energy intake = 53 kJ day(-1)). Lactating females obtained six times more energy from increased food consumption than from mobilization of fat stores, compared with non-reproductive bats. 3. Milk export, calculated using the difference in water turnover between lactating and non-reproductive bats (measured using H-2 turnover) averaged 2.6 ml bat(-1) day(-1) (22.9 kJ day(-1)). This was similar to the average milk intake of sucklings estimated from H-3 turnover (22.9 kJ day(-1)). 4. Energy available for respiration from food and mobilization of fat stores was 18.2 kJ day(-1) for lactating females compared with 36.8 kJ day(-1) for non-reproductive females. In comparison, respiratory daily energy expenditure (DEE) of lactating and non-reproductive bats, measured by doubly labelled water (DLW), was 21.3 kJ day(-1) and 23.6 kJ day(-1), respectively. Hence, there was a discrepancy between respiratory DEE (measured by DLW) and net available energy estimates for non-reproductive bats but not for lactating bats. 5. Respiratory DEE for lactating bats was equal to or less than that of non-reproductive females, suggesting they used compensatory mechanisms in their energy budgets in lactation.