Loss of developmental stability can lead to deviations from bilateral symmetry (i.e. Fluctuating Asymmetry -FA), and is thought to be caused by environmental and genetic factors associated with habitat loss and stress. Therefore, levels of FA might be a valuable tool to monitor wild populations if FA serves an indicator of exposure to stress due to impacts of habitat loss and fragmentation. In studies examining FA and habitat fragmentation, FA levels are often explained by loss of genetic variation, though few studies have addressed FA’s use as indicator of environmental impact. Here we investigated whether habitat loss, genetic variation and/or inbreeding affect the developmental instability in Brazilian Atlantic rainforest populations of a Neotropical water rat (Nectomys squamipes). We sampled individuals from eight forest remnants with different amounts of available habitat and assessed FA levels with geometric morphometric techniques using adult mandibles. We used observed heterozygosity (Ho) and inbreeding coefficient (Fis), from seven microsatellite markers, as a proxy of genetic variation at individual and population levels. Populations were not significantly different for shape or size FA levels. Furthermore inter-individual variation in both shape and size FA levels, as well as inter-populational differences in size FA levels, were best explained by chance. However, habitat availability was negatively associated with both inter-populational variance and average shape FA levels. This association was stronger in populations living in areas with less than 20% of habitat available, which presented higher variance and higher average of FA, suggesting that Nectomys squamipes might have a tolerance threshold to small availability of habitat. Our work was one of the first to use FA to address environmental stress caused by reduced habitat availability in small mammal populations from a Neotropical biome. We suggest that shape FA might serve as a conservation tool to monitor human impact on natural animal populations.

Loss of developmental stability can lead to deviations from bilateral symmetry (i.e. Fluctuating Asymmetry -FA), and is thought to be caused by environmental and genetic factors often associated with habitat loss and stress. Therefore, levels of FA might be a valuable tool to monitor wild populations if FA serves an indicator of exposure to stress due to impacts of habitat loss and fragmentation. In studies examining FA and habitat fragmentation, FA levels are often explained by loss of genetic variation, though few studies have addressed FA’s use as indicator of environmental impact. Here we investigated whether habitat loss, genetic variation and/or inbreeding affect the developmental instability in Brazilian Atlantic rainforest populations of a Neotropical forest-dwelling water rat (Nectomys squamipes). We sampled individuals from eight forest remnants with different amounts of available habitat and assessed FA levels with geometric morphometric techniques using adult mandible shape and size. We used observed heterozygosity and inbreeding coefficient (Fis), from seven microsatellite markers, as a proxy of genetic variation at individual and population levels. Populations were not significantly different for shape or size FA levels. Furthermore inter-individual variation in both shape and size FA levels, as well as inter-populational differences in size FA levels, were best explained by chance. However, habitat availability was negatively associated with both inter-populational variance and average shape FA levels. This association was stronger in populations living in areas with less than 20% of habitat available, which presented higher variance and higher average of FA, suggesting that Nectomys squamipes might have a tolerance threshold to small availability of habitat. Our work was one of the first to use FA to address environmental stress caused by reduced habitat availability in small mammal populations from a Neotropical biome. We suggest that shape FA might serve as a conservation tool to monitor human impact on natural animal populations.