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Rodents are notable for their unique life history traits that render a fast-breeding capacity. Their short generation times allow local adaptations to appear, and several studies have tried to understand if—and how—geographical variables influence local adaptation in life history traits. We compared demographic and life history traits of the Neotropical cricetid Necromys lasiurus based on 7,400 museum specimens collected daily and simultaneously over 5 years at several sites in the Caatinga biogeographic domain subject to heterogeneous geographic and climatological variables. This sample, assembled by the National Plague Service between 1951 and 1955, includes skulls, skins, and individual specimen data currently deposited in the Museu Nacional (Rio de Janeiro, Brazil). Based on this material, we analyzed population fluctuation, breeding season, sexual size dimorphism, sexual maturity, and litter size. We also hypothesized that geographic variables (elevation, rainfall, temperature, longitude, and latitude) influence key life history traits, such as size at birth and litter size. Overall, N. lasiurus showed plastic reproductive patterns, with few traits shared among populations. Notably, there is strong seasonal variation in population fluctuation patterns, breeding events clumped in early dry season and old individuals distributed throughout all the year, as has been seen for populations in other biomes. Geography had no influence on size at birth but litter size was explained by the interaction between maternal weight and geographical variables. Plastic life history traits so intimately responsive to climatological variables likely contribute to N. lasiurus having one of the largest geographic ranges among Neotropical cricetids, as well as to its status as the most important plague reservoir in the Caatinga domain. [ABSTRACT FROM AUTHOR]