Mandible shape variation and feeding biomechanics in minks

Abstract

European and American minks are very similar in ecology, behavior and morphology. Both species hunt terrestrial vertebrates and aquatic prey, but the American mink is a more generalist predator which, among other factors, allows it to outcompete the European mink in areas where it has been introduced. We used 3D geometric morphometrics and estimates of muscle mechanical advantage to assess the degree of variation in mandibular morphology, and to determine whether such variation reflects dietary differences between the two species. The three main axes of variation represented interspecific differences, a common allometric trajectory between species and sexes, and the interspecific effect of sexual size dimorphism, with males having overall stronger bites than females. Differences in mandible shape and biomechanical parameters suggest that American minks are better equipped for preying on terrestrial vertebrates, while the features seen in European mink could be related to tougher prey, fish capture, or both. Additionally, within each species, the larger specimens of each sex present indicators of a higher percentage of terrestrial prey in their diet. These results indicate a low potential dietary overlap between both species, suggesting that factors other than prey competition may have a role in the decline of the European mink.

Figure 1

Sexual dimorphism in mandible size and jaw biomechanics. Boxplots for centroid size (A) and PC2 scores (B) by species and sex, and for the MAs of the anterior temporalis (C) and the deep masseter (D) by species, of the posterior temporalis by sex (E) and of the superficial masseter by species and sex (F). Bold line denotes the median (quartile 2; Q2), while the box represents interquartile range (IQR: Q1 to Q3) with whiskers extending 1.5 times IQR. An asterisk on the top left corner of a box indicates that the mean for that group is significantly different from all other asterisks in the panel. In all plots by species and sex, all groups not different from each other are placed within a yellow box. Abbreviations: F, female; M, male; Mlu, European mink; Nvi, American mink.

Figure 2

Principal component analyses of mandible shape variation in minks. (A) PC1 vs PC2, (B) PC1 vs PC3, (C) PC2 vs PC3. The percentage of total variance explained by each principal component is given in parentheses. Key: blue circles, European mink (Mlu); pink squares, American mink (Nvi); solid symbols, males; open symbols, females. Minimum convex polygons for males (darker shades) and females (lighter shades) of each species are drawn on panel (B). The wireframes on the right illustrate the shape variation along each PC from the lowest (PC1: blue/Mlu; PC2: yellow/small; PC3: light grey/female) to the highest score (PC1: pink/Nvi; PC2: red/large; PC3: dark grey/male).

Figure 3

Mandible anatomy in minks (A–D) and landmark configuration used (E–J). European mink MNHN 1991–350 in lateral (A) and medial views (B). American mink NMS M454/67 in lateral (C) and medial views (D). Scale bar = 3 cm. See Table S3for additional information on both specimens. The landmark configuration is shown on the 3D mesh for the American mink above, in lateral (E), medial (F), cranial (G), dorsal (H), ventral (I), and caudal views (J). Red dots represent landmarks (as defined in Table 4), while blue dots represent semilandmarks along curves. Black lines in (E) represent the out-levers used in the biomechanical analyses, while grey lines represent the in-levers. Abbreviations: madm, lever arm of the deep masseter; masm, lever arm of the superficial masseter; mata, lever arm of the anterior temporalis; matp, lever arm of the posterior temporalis; out.c, out-lever at the canine; out.m, out-lever at the carnassial.