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The life cycles of bats are constrained by availability of resources such as food and shelter. Disruptions limiting otherwise naturally regulated access to these resources ultimately affect the fitness of bat populations and may eventually lead to local extinctions. We sampled bats before and after an Amazonian mega-dam water filling and retrieved data on their reproductive phenology, testing if and how sex ratio, seasonal abundance, and reproductive patterns respond to impacts of the newly operating hydroelectric dam. We analyzed data from 3,298 bat captures of 49 species. Females outnumbered males, previously and after the dam implementation, and we observed seasonal variations in the abundance of bats, which was lower after the implementation of the dam for most species. All species had at least 1 reproductive peak of pregnancy and/or lactation beginning at a variable time across seasons. Most species and functional ensembles were reproductively bimodal, with exceptions being the polymodal Artibeus obscurus and unimodal Phylostomus elongatus. Operations of the hydroelectric dam affected the abundance of both sexes equally, and effects were stronger in the rainy season, altering the timing of reproduction for females, delaying and shortening pregnancy and lactation. These changes expressed in female reproductive activity may be related to the shortfall of resources because of loss of irreplaceable vegetation caused by the dam filling. As we cannot predict the roles of variation of reproductive patterns observed after the filling in the long run we reinforce the importance of long-term, continuous monitoring and management to safeguard the reestablishment of reproductive patterns of females in order to repopulate and keep bat populations healthy in remaining areas along following dam construction. [ABSTRACT FROM AUTHOR]