Cranial shape variation in mink: Separating two highly similar species

Exploring fluctuating asymmetry in two recovering populations of the Eurasian otter

The Eurasian otter is a wide-ranging semi-aquatic mammal that underwent a significant population decline in the last century, leading to local extinctions, reduction and fragmentation of populations. The individuals of populations exposed to both external and internal stress may present the inability to produce a specific developmental outcome, generating developmental 'noise' (developmental instability (DI)). Factors contributing to DI include inbreeding depression, population bottlenecks, habitat loss and exposure to pollution. We analysed fluctuating asymmetry (FA) as a proxy of DI in two European otter populations that experienced a major decline in the 1990s. Using three-dimensional geometric morphometrics methods on skull samples of otters from the UK and Sweden, we compared the degree of FA both between populations before, during and after the decline. We found a higher FA in the UK populations compared with Sweden. The level of asymmetry differed significantly over time only in the UK population, where it was higher during the decline phase. FA in the UK populations can be attributed to the specific impact of polychlorinated biphenyls pollution that caused a bottleneck. More generally, our study suggests that habitat loss, pollution and limited gene flow may contribute to DI in declining otter populations, highlighting the need for continued investigation to identify and quantify the specific stressors behind this trend in local populations.

The relative performance of geometric morphometrics and linear-based methods in the taxonomic resolution of a mammalian species complex

Morphology-based taxonomic research frequently applies linear morphometrics (LMM) in skulls to quantify species distinctions. The choice of which measurements to collect generally relies on the expertise of the investigators or a set of standard measurements, but this practice may ignore less obvious or common discriminatory characteristics. In addition, taxonomic analyses often ignore the potential for subgroups of an otherwise cohesive population to differ in shape purely due to size differences (or allometry). Geometric morphometrics (GMM) is more complicated as an acquisition technique but can offer a more holistic characterization of shape and provides a rigorous toolkit for accounting for allometry. In this study, we used linear discriminant analysis (LDA) to assess the discriminatory performance of four published LMM protocols and a 3D GMM dataset for three clades of antechinus known to differ subtly in shape. We assessed discrimination of raw data (which are frequently used by taxonomists); data with isometry (i.e., overall size) removed; and data after allometric correction (i.e., with nonuniform effects of size removed). When we visualized the principal component analysis (PCA) plots, we found that group discrimination among raw data was high for LMM. However, LMM datasets may inflate PC variance accounted in the first two PCs, relative to GMM. GMM discriminated groups better after isometry and allometry were removed in both PCA and LDA. Although LMM can be a powerful tool to discriminate taxonomic groups, we show that there is substantial risk that this discrimination comes from variation in size, rather than shape. This suggests that taxonomic measurement protocols might benefit from GMM-based pilot studies, because this offers the option of differentiating allometric and nonallometric shape differences between species, which can then inform on the development of the easier-to-apply LMM protocols.

Mandible shape variation and feeding biomechanics in minks

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.

The incisive foramen as character in distinguishing morphologically similar species of mammals

An analysis of the morphology and variability of the size and shape of a key morphological structure in the rostral part of the skull—the incisive foramen—has been carried out. It is shown that incisive foramina are variable morphological structures, the features of which are group-specific (at the level of genera and families), and in some cases also species-specific. At both these levels, the shape and size of the incisive foramen have features that can serve as criteria for species identification by osteological patterns. Their location is important for diagnosis because these structures are preserved in most specimens that have suffered various kinds of damage (e.g. in fodder residues of carnivorous mammals or in owl pellets), and their placement in the anterior part of the bony palate as well as them being protected from the sides with rows of teeth makes these structures invulnerable to trauma-related variation. It is shown that there is a specific structure (size, location, and shape) of incisive foramina at the level of taxonomic groups of all ranks, from orders to species. The analysis was performed mainly on the examples of different groups of rodents as an order, represented by the largest number of pairs of close species. Examples with several different groups, in particular with different pairs of species of voles, mice, mole rats, ground squirrels, and others are considered. Examples with differences in close pairs of species in other groups (white-toothed shrews, polecats, roe deer, etc.) are also known. In all pairs of related species, a pattern was found, according to which species that are restricted to steppe ecosystems have the smallest incisive foramina, while species from wetland habitats have large ones. In many cases, groups of genera and families well differ in the shape and location of incisive foramina, and close pairs of species differ well in the size of these structures (primarily in length), although it is important to always consider the ontogenetic age of specimens: in young individuals, the incisive foramina are naturally small, similar to incisive foramina in other species, which are characterized by small incisive foramina in general. Based on the known data on the role of incisive foramina and the Jacobson organ in the life of mammals, hypotheses have been considered that may explain the differences in species and genera by the structure (size, location, and shape) of incisive foramina.