Cranial shape variation in mink: Separating two highly similar species

Abstract

European and American minks (Mustela lutreola and Neovison vison, respectively) are very similar in their ecology, behavior, and morphology. However, the American mink is a generalist predator and seems to adapt better to anthropized environments, allowing it to outcompete the European mink in areas where it has been introduced, threatening the survival of the native species. To assess whether morphological differences may be contributing to the success of the American mink relative to the European mink, we analyzed shape variation in the cranium of both species using 3D geometric morphometrics. A set of 38 landmarks and 107 semilandmarks was used to study shape variation between and within species, and to assess how differences in size factored into that variation. Sexual dimorphism in both size and shape was also studied. Significant differences between species were found in cranial shape, but not in size. Relative to American mink, European mink have a shorter facial region with a rounder forehead and wider orbits, a longer neurocranium with less developed crests and processes, and an antero-medially placed tympanic bullae with an anteriorly expanded cranial border. Within species, size-related sexual dimorphism is highly significant, but sexual dimorphism in shape is only significant in American mink, not in European mink. Additionally, two trends common to both species were discovered, one related to allometric changes and another to sexual size dimorphism. Shape changes related to increasing size can be subdivided into two, probably related, groups: increased muscle force and growth. The first group somewhat parallels the differences between both mink species, while the second group of traits includes an anterodorsal expansion of the face, and the neurocranium shifting from a globous shape in small individuals to a dorsoventrally flattened ellipse in the largest ones. Finally, the sexual dimorphism trend, while also accounting for differences in muscle force, seems to be related to the observed dietary differences between males and females. Overall, differences between species and sexes, and shape changes with increasing size, seem to mainly relate to differences in masticatory-muscle volume and therefore muscle force and bite force, which, in turn, relate to a wider range of potential prey (bigger prey, tougher shells). Thus, muscle force (and dietary range) would be larger in American mink than in European mink, in males than in females, and in larger individuals than in smaller ones.

Keywords: American mink; European mink; cranium; geometric morphometrics; sexual dimorphism; shape variation.

FIGURE 1

Cranial anatomy in minks. European mink MNHN 1986‐459 in dorsal (a), ventral (b), and lateral views (c). American mink MNHN 2005‐647 in dorsal (d), ventral (e), and lateral views (f). See Table S2 for additional information on both specimens. Scale bar is 3 cm long. Red lines represent the outline of the auditory bulla, while the small boxes provide a closer look at the pterygoid processes, both traits can be used to identify each species. Some of the anatomical features mentioned throughout the text are illustrated here using the following abbreviations: C, canine; fm, foramen magnum; Ir, incisor row; itf, infratemporal fossa; M1, molar 1; mp, mastoid process; nc, nuchal crest; orb, orbit; P2–P4, premolars 2–4; pal, palate; poc, postorbital constriction (marked by arrows); pop, postorbital process; ptp, pterygoid process; sc, sagittal crest; tym, tympanic bulla; zyg, zygomatic arch [Colour figure can be viewed at wileyonlinelibrary.com]

FIGURE 2

Landmark configuration. (a) lateral view; (b) caudal view; (c) cranial view; (d) ventral view; (e) dorsal view. Red dots represent landmarks (as defined in Table 2), blue dots represent semilandmarks along curves, and green dots illustrate semilandmark patches on surfaces [Colour figure can be viewed at wileyonlinelibrary.com]

FIGURE 3

Principal component (PC) analyses of the whole sample (a, b) and the adult subsample (c, d). The percentage of total variance explained by each PC is given in parentheses. European mink are represented in blue circles, American mink in pink squares, European polecats in green diamonds, and hybrids in orange triangles. In panels (a, b) adults are represented by full symbols, and juveniles with open symbols. In panels (c, d) adult specimens of known sex are represented by large symbols (females: open; male: full), while small symbols are used for those with unknown sex. Minimum convex polygons for males (darker shades) and females (lighter shades) of each species are drawn on panel (c) [Colour figure can be viewed at wileyonlinelibrary.com]

FIGURE 4

Principal component (PC) analysis of the adult mink subsample. The percentage of total variance explained by each PC is given in parentheses. European mink are represented in blue circles, American mink in pink squares, while large open symbols represent females, large full symbols males, and small full symbols unsexed specimens. The wireframes illustrate the shape of the specimen with lowest and highest score for each PC in lateral and dorsal view. The arrow in panel (a) signals that specimen size increases along PC2 (orange for small, red for large). Minimum convex polygons for males (darker shades) and females (lighter shades) are drawn for each species (b), and pooling both species (c) [Colour figure can be viewed at wileyonlinelibrary.com]

FIGURE 5

Phenotypic trajectory analysis for species and sex in adult mink cranial shape (a) and PC2 and PC3 combined (b). To visualize the trajectories, multivariate data are summarized using a PCA on the fitted values, and the two first PCs are shown for each analysis (with percentage of total variance explained in parentheses). Symbols as in Figure 4, with mean scores for each group in black [Colour figure can be viewed at wileyonlinelibrary.com]