Defecation, apparent absorption efficiency, and the importance of water obtained in the food for water balance in captive brown long‐eared (Plecotus auritus) and Daubenton's (Myotis daubentoni) bats

Mean apparent dry mass absorption efficiency by P. auritus fed on mealworms was 0.853 (S.D.=0.4, n=43). Mean apparent energy absorption efficiency by P. auritus and M. daubentoni fed on mealworms was 0.90 (S.D.=0.007, n=8). The mean energy content of mealworms was 28.6 kJ.g^‐1 dry mass (S.D.=1.1, n=8), and that of faeces was 19.5 kJ.g^‐1 (S.D.=0.7, n=8) in P. auritus and 19.9 kJ.g^‐1 (S.D.=1.3, n=8) in M. daubentoni. Water content of mealworms was 61.1% wet mass (S.D.=1.4, n=173); water content of faeces was 73.3% in P. auritus (S.D.=6.8, n=76) and 72.3% (S.D.=7.0, n=42) in M. daubentoni. Oven‐dried mealworms consisted of <1% ash, <1% carbohydrate, 31% lipid and 39% protein. We suggest that the dry mass unaccounted for (28%) represented chitin, of which 59% was apparently absorbed during digestion. Apparent absorption by mass of both lipid and protein by P. auritus fed on mealworms was greater than 90%. Cumulative post‐prandial defecation was sigmoidal in both bat species with 50% (by mass) of faeces being voided within 4 h and 95% within 12 h of feeding. On the basis of previous measurements, using doubly‐labelled water, of daily energy expenditure and water flux in free‐living P. auritus, we predict that water intake via the food, as free water and as potential metabolic water, represents 20 to 40% of total daily water flux for P. auritus during lactation in the wild.