Sexual dimorphism is a fascinating evolutionary phenomenon to explore in depth, particularly within Carnivora and the family Mustelidae. Previous findings show high instances of dimorphism within these taxanomic groupings (Gittleman 1997), and in some instances the degree of dimorphism may be a short term responsive adaptation to environmental conditions (Powell 1997). It is then especially relevant to examine dimorphism in depth for populations living in temperate ecosystems likely to undergo drastic alteration due to climate change in the near future. This study on variability in dimorphism for Minnesotan Mustelids; with sexual selection and ecological pressure focuses on 4 select species found in Minnesota, namely Mustela erminea, M. frenata, Neovison vison and Procyon lotor serving as a control, with at least 5 specimens of each sex sampled for each species. Delving deeper into a geographically specific genus can reveal more information about the evolutionary and phylogenetic history for dimorphism in this area, and help to illuminate possibilities about the future of these populations as they face new challenges. The two main hypotheses for driving dimorphism are ecological niche separation and sexual selection (Gittleman 1997). In this study, differences in canine and carnassial teeth were compared, to find which of these driving forces was more prominent historically for the populations living in Minnesota; with dimorphic canines pointing to sexual selection and dimorphic carnassials indicating dietary specialization between the sexes. Many significant dentition and skull differences were found for half of the species examined.