Contemporary Body Size Variation of Neotropical Rodents: Environmental and Genetic Effects

Body size is a pivotal ecological and evolutionary trait, as it can significantly impact both survival and reproductive success. To understand how human-mediated disturbances influence body size, we conducted a temporal analysis of body mass index (BMI) variations in 2788 individuals spanning six South American rodent species to describe their seasonal and yearly fluctuations between 2005 and 2009. Additionally, we used microsatellite genotyping to estimate genetic pedigrees for individuals from two of these species (Akodon azarae and Calomys musculinus). This enabled us to dissect the phenotypic variation of body size, offering insights into the evolutionary dynamics of that variation. We report significant increments of BMI across years in three species (A. azarae, Calomys venustus, and Oxymycterus rufus). In addition, we observed moderate and similar levels of narrow-sense heritability in A. azarae and C. musculinus, suggesting that part of the variation in this trait is attributable to additive genetic effects. Furthermore, the phenotypic variance, additive genetic variance, and evolvability of BMI were higher in C. musculinus when compared to A. azarae. These findings suggest that BMI in C. musculinus has the potential to exhibit a more rapid response to equivalent selection pressures than in A. azarae. The heritability and evolvability values also imply that the annual changes in BMI may be influenced, at least in part, by natural selection, probably in response to shifting environmental conditions within intensively managed agroecosystems. However, a long-term study is necessary to understand and predict the role of selection in the evolutionary dynamics of body size variation among rodents inhabiting agroecosystems.