Energetics of Pregnancy and Lactation in Freeranging Little Brown Bats (Myotis lucifugus)

We combined field measurements of metabolic rate, made with doubly labeled water, with data from our previous studies to examine reproductive energetics in 24 female little brown bats (Myotis lucifugus). Including estimates of tissue or milk production, M. lucifugus required an average of 33.7 kJ d⁻¹ of assimilated energy in pregnancy compared to 41.3 kJ d⁻¹ during lactation. Predicted insect consumption was 5.5 g d⁻¹ for a 9-gpregnant female and 6.7g d⁻¹ for a 79-g lactating female. About 2% of total energy assimilated during pregnancy was stored as new tissue, whereas lactating females exported 32% as milk. Estimated assimilated energy demand on the first day of lactation was 33.8 kJ d⁻¹ and increased to 60.3 kJ d⁻¹ at peak lactation. By subtracting laboratory measurements of roosting costs from observed metabolized energy expenditure, we calculated that foraging flight by 9-g pregnant M. lucifugus required 4.46 kJ h⁻¹; this was 13% less than allometric predictions. Foraging fight accounted for the largest proportion of the daily metabolized energy budget during pregnancy (61%) and lactation (66%). The large amount of energy devoted to foraging by this aerial-feeding bat may partially explain the low proportion of energy it allocates to tissue production and milk export.