Abstract <p>Differences between the strains of aggressive and tame American minks that arose as a result of selection for traits of defensive behavior (16–17 generations) were revealed based on the biomechanical indices of the mandible characterizing the mechanical potentials of the canine and carnassial teeth. The results are consistent with D.K. Belyaev’s theory of destabilizing selection: along with an increase in the variability of functions and the destabilization of the historically established system of their sexual differences (sexual dimorphism), new biomechanic features of the mandible were formed in the strain of tame minks. In contrast, the control strain of nonselected minks unaffected by selection retained significant sex differences in the biomechanical indices. Between the American minks from a Canadian natural population and the strains of aggressive and tame individuals, the differences in biomechanical indicators were less pronounced than between the strains themselves. Differences among the American mink, the European mink (<i>Mustela lutreola</i> L. 1758), and the Siberian weasel (<i>M. sibirica</i> Pallas 1773) in the biomechanical potentials of the canine and carnassial teeth of the mandible which reflect specializations in the genus <i>Neogale</i> and the specifics of the hunting behavior of the species were found. In the invasive American mink, the mechanical potential (MP) of the canine tooth prevails, compared to the MP of the carnassial tooth in both the European mink and the Siberian weasel, this trait being capable of ensuring the divergence of their trophic niches and contributing to the preservation of native species in areas of their sympatry with <i>N. vison</i>.</p>