In most mammals, reproductive cost differs between males and females in timing and duration because of the different sex-specific energy allocation strategies to maximize fitness. The differences in reproductive strategy adopted by sexes may result in differences in seasonal variation of body mass. Here seasonal variations in body mass are discussed for two species of vespertilionid bats: Daubenton's bat (Myotis daubentonii) and Savi's pipistrelle (Hypsugo savii). Both species were observed to have a seasonal variability in body conditions, which was sex dependent. In late spring and for a given forearm length, females were heavier than males, but differences were small in late summer. Furthermore, female'’s body mass decreased during late spring and summer likely to support the energy requirement for lactation, while male's mass did not change (H. savii) or slightly increase (M. daubentonii) over the same period. On the contrary male M. daubentonii depleted body fat reserves during early autumn, likely because of the energy expenditure to increase mating opportunities. Our results suggest that seasonal changes in body condition in hibernating bats may reflect the differences in reproductive strategies between sexes.
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