In contrast to birds, the contribution of body reserves to sustain reproductive activities of migratory bats has not being examined. We used C stable isotope analysis to track the importance of nutrients stored in body tissues of the lesser long-nosed bat (Leptonycteris yerbabuenae) in Central Mexico. The bat migrates seasonally between areas dominated by vegetation types with contrasting C stable isotope values: in spring–summer, it forages in cactus forests before commuting to dry and wet forests, where mating and births occur. We collected breath and whole blood from nonreproductive individuals in a cactus forest in spring–mid-summer, from mating individuals in an evergreen forest in mid-late summer, and from lactating females in winter in a tropical deciduous forest. We also collected hair and milk from lactating females and several tissues from naturally aborted fetus in late autumn. We tested the hypothesis that nutrient reserves accumulated in cactus forest contribute to the maintenance of adults when they commute to their reproductive grounds, to the construction of offspring tissues during pregnancy, and to the production of milk. The importance of energy stores accumulated in cactus forests to fuel oxidative metabolism was marginal for mating males but it was high for some mating females. Nutrient stores accumulated in cactus forests contributed to ∼50% of synthesis of fetus tissues but their contribution for milk production was negligible. Female lesser long-nosed bats can be described as capital-income breeders in relation to the development of offspring during gestation and as incomer breeders in relation to lactation.