Morpho-anatomical and physiological characteristics

  The type of body of the European mink is intermediate between the type of body of the L. lutra and the M. putorius – the silhouette is slender, the torso is elongated, the limbs are short, and the tail is not very fluffy and its length is slightly less than half of the body length. The body posture is more compact compared to other representatives of the Mustelidae family. The head is slightly flattened (which makes it easy to swim and dive), with wide and short ears, set on the neck almost as thick as the head. The size of the ears is about 1.5-2.4 cm. The length of the body of the adult European mink (without tail) is 25.0-40.0 cm ♀ and 28.4-44.0 cm ♂, the tail length is 10.5-18.0 cm ♀ and 11.7-19, 0 cm ♂ and the body weight is 370.0-740.0 g ♀ and 450.0-1250.0 g ♂.

The limbs of the M. lutreola are five-fingered and have sharp claws, with the posterior legs being semi-plantigrade. The length of the anterior legs is about 7.5-8.5 cm, and of the posterior legs is about 10,0-11,5 cm. Fingers are connected with the hairy, well-developed interdigital web, which in the posterior legs reaches the third finger member. The digital tips, and metacarpal/metatarsal tips are bright and visible (uncovered with hair) in both in summer and in winter.

The coat is glossy, from ginger-brown, through dark chestnut, to dark brown; the down hair is gray. The fur on the limbs and the tail is darker than the fur on the back, while the brightest color is marked on the underside of the body. The overall tone of the fur color, despite the abovementioned differences, is homogeneous.

The coat of the European mink consists of the guard hair, the bristle hair and the down hair. The guard hairs have relatively well developed core (about 60.5% thickness of the hair), present almost on the entire length of the stem. In the bristle hairs the core is thinner (about 46.7% thickness of the hair) and less pronounced. In the down hairs the thickness is the smallest (about 33.3% of the thickness of the hair), although it occurs over the entire length of the stem. The characteristics of the hair of the M. lutreola, with sketches and microphotographs, is included in the study by Debrot et al..

Molting occurs twice a year – the spring molt from March to May spring, and the autumn molt from August to November. The summer hair is shorter, less thick and brighter, whole the winter hair is denser and gloss. The difference between the summer and the winter coat is much less marked than in the case of other representatives of the Mustela genus. The coat bears distinct features of adaptation to the water-way lifestyle, such as: very thick down hair (much thicker than the hair than other Mustela, although not as thick as the hair of the European otter) and the shape of cover hair – a very wide, flattened mid-section. The proper adult coat appears after the first fall molting.

There is description of a specific form of the M. lutreola, occurring in Silesia in historical times, in which the white coloration at the end of the muzzle was reduced or did not occur at all. The white spot, or spots, of varying shapes and sizes often occur on the fur covering the ventral part of the neck and the chest. The white spots on the breast occur in approximately 50% of individuals in the North European and East European populations, in 10% of individuals from the French population, and only about 2% of animals from Northern Spain. The white-colored spots, their number and shape, are individually specific and can serve to individual identification.

The skin of the European mink is relatively thick, measuring approximately 1.70 mm on the ventral side and approximately about 1.48 mm on the dorsal side; the dermis (corium) measures 1.68 mm (ventral side) and 1.45 mm (dorsal side) respectively. The epidermis and the corneum layer of the epidermis are thicker on the dorsal side than on the ventral side, with thicknesses of 34 μm (dorsal epidermis) and 18 μm (dorsal corneum), and 22 μm (ventral epidermis) and 11 μm (ventral corneum) respectively. The Malpighian layer (the so-called living epidermis) comprises of 1-3 layers of cells, and the papilary layer of the dermis is much thicker than the reticular layer, measuring 1.06 mm (papilary) and 0.38 mm (reticular) respectively on the back, and 1.13 mm (papilary) and 0.55 mm (reticular) on the chest and abdomen. Horizontal clusters of collagen fibers occur mainly in the dermis and are characterized by relatively loose plexus. The fat cells occur both in the papillary and reticular layers. The sebaceous glands are single-patched, oval-shaped and elongated and measure approximately 112 × 224 μm in the skin of the ventral side and 40 × 242 μm in the skin on the dorsal side. The sweat glands are sparsely scattered and have a sausage-like and twisted shape and their diameter is about 28 μm. In the skin of the European mink there are no arrector pili muscles.

The skull of the M. lutreola is delicate, flattened in the dorsal-ventral plane, especially in the forefront part. The neurocranium is not very elongated, the zygomatic arches are relatively long and moderately wide apart, while the viscerocranium is quite elongated. The postorbotal constriction, a typical feature of the Carnivora is poorly expressed and at the length of the narrowest point is equal to the interorbital segment or slightly longer. The mammary epithelium is also poorly expressed, and the cranial can is small, swollen, relatively short and narrow (ratio of their length to width is about 1: 1.7) and widely spaced. The occipital comb is clearly visible in M. lutreola skull, while the sagittal comb is slightly weaker. The skulls of juveniles are characterized by a less developed viscerocranium and more expanded neurocranium. The general structure of the skulls exhibits significantly lower level of predatory specialization than in the case of the European polecat or the American mink.

The condylobasal length of the skull is 5.6-6.8 cm, while the jugal length is 2.9-4.0 cm. The postorbital constriction is shortened and poorly expressed. The sagittal crest that runs along the parietal bones, and the lambdoid crest that surrounds the back of the occiput are also poorly expressed, with their distinctness depending on the sex and increasing with age. The bulla tymphanica is almond-shaped bulla and small. In many respects the skull of the European mink shows intermediate features between the European polecat and the Siberian weasel.
The European Mink has 34 teeth, and its tooth pattern, typical for the representatives of the Mustela genus, is (I3 C1 P3 M1)/(I3 C1 P3 M2). The species has the teeth of cutting-and-crushing type. The canine teeth and the three-root carnassial teeth (P3 in the upper jaw and M1 in the mandible) are much developed. The inner surface of the main tip of the lower carnassial tooth (M1) is smooth, without the vertical edge. The second upper premolar (P2) does not penetrate into the cavity in the frontal part of the crown of the carnassial tooth (P3), only touching the front edge of its crown. The lower carnassial tooth (M1) lacks one of the nodules in the inner sequence – the so-called metaconid. The first premolar tooth of the jaw and the mandible has one root. The tongue is smooth.

Around the anus, under the tail, there are very well developed paired odor glands (anal glands). Liquid secretion they produce performs signaling and defensive functions. As far as the olfactory communication is concerned, the secretion of anal glands plays a major role in individual identification and, most probably, in the selection of partners during the mating season and the recognition of mating readiness. Defensive (repelling) character of the secretion of anal glands was reported in the earliest literature references of the species. The irritated or frightened European mink may spray the secretion of the gland in the defensive response. The glandular secretion consists of 12 chemical compounds. The concentration and composition of the secretions show significant individual variations, independent of sex. The only sex-related differences occur in the reproductive period when the secretion of anal glands of males and females have significantly different as far as concentration of 3 components is concerned. In the period immediately preceding the breeding season, males of reproductive age produce more volatile components of the secretions, while females do it both at the time of the mating season and in the period immediately preceding it. The composition of the mixture of odorants is similar to the composition of the secretion produced by the species closely related with the European species, such as the European polecat and the Siberian weasel, though its stinky nature being weaker than in the case of the M. putorius.

The average liver weight is approximately 29.2 g (females) and approximately 39.9 g (males); the heart – approx. 5.0 and approx. 6.0 g, the lungs – approx. 7.5 g and approx. 10.5 g, the brain – approx. 7.9 and approx. 9.0 g, the pancreas – approx. 2.6 g and approx. 2.4 g, the kidney (left) – approx. 2.5 g and approx. 3.4 g. The European mink, like the other representatives of the Mustelidae family, does not have a cecum.

The body temperature of the adult European mink ranges from 36.2 to 38.4°C, with a pulse rate of 210 to 300 beats per minute and a respiratory rate of 40-65 breaths per minute. The most developed senses in this species are hearing and smell, while the sight is relatively weak.

Ahmed et al. described the amino acid sequence of the hemoglobin (EC 4.1.1.39) of the M. lutreola. The determined length of the α chain is 141 amino acids and the β chain consists of 146 amino acids. The amino acid sequence of the α chain and the β chain of the European mink and the European polecat have one (α) and four (β) different amino acids, in the case of the European badger (Meles meles) – eight (α) and five (β) different amino acids; in the case of honey badger (Mellivora capensis) – nine (α) and one (β) different amio acids; in the case of the Eurasian otter – two (α) and two (β) different amino acids. In the case of the ferret, the differences occurred only in the chain β and include four amino acids.

Studies on the inhibitory effect of alkaloids on monoamine oxidase (MAO, EC 1.4.3.4) in the European mink suggest existence of several MAO isoforms in this species.

The selected haematological and biochemical indicators of blood of the European mink (source: Foundation LUTROLA 2006)

n – number of examined animals

The sexual dimorphism is also manifested in the proportions of parts of the body, and, for example, the length of the tail of the male usually ranges from 35 to 52% of the length of the entire body, while in females – 37 to 45%. Another manifestations of the sexual dimorphism include differences in size and mass of bodies – the males are approximately 16-25% larger and approximately 30-40% heavier than the females. The value of the sexual dimorphism indicator, calculated as a logarithm of the ratio of body weight of the males to body weight of the females, is 0.17 (for the north-eastern population) and 0.22 (for the south-eastern population) and is one of the lowest in the Mustela genus. For comparison, the value of the sexual dimorphism indicator in the M. putorius is 0.33, in the M. eversmani – 0.31, and in M. sibirica – 0.23. Palazón et al. determined the sex-specific function for the Body Condition Index (BCI) of the European mink, which for the females is BCI♀ = body mass /2,38 × total body length2,11, and for the males BCI♂ = body mass /3,20 × total body length2.22.

The sexual dimorphism is manifested in the skull structure, both in terms of size and shape. Values of all craniometric measurements are larger for the males. As for the differences is shape of the skull, the males have relatively higher neurocranium, broader zygomatic arches, more prominent viscerocranium, wider bulla tymphanica and higher mandible. Females and bony ridges are less pronounced in females. Skulls of the males are generally larger and heavier than skulls of the females. Abramov and Tumanov conducted a detailed study of the sexual dimorphism in the skull of the M. lutreola, demonstrating on the basis of the discriminatory analysis, the measurement of zygomatic breadth and the interorbital breadth allowed to distinguish between the sexes with a 96.5% probability.

The females have 3-5 pairs of nipples. The length of the baculum of the males is approximately 3.5-4.6 cm. The baculum has a structure typical of the Mustela genus, i.e. it is bent in the form of the hook, however the groove on the lower side of the bone is wider than that of the polecats and the Siberian weasel, and the tip of the bone is bent in the form of a spoon. On the abdominal side, the part of the terminal of the penis bone tilts slightly to the right.

For practical reasons – mainly because of the protective means conducted for the European mink, as the species critically endangered with extinction – it is very important to distinguish it from the closely evolutionary related European polecat and the form the American mink manifesting close ecological similarity. In large part, the habitats of these species in Europe overlap. This coexistence is problematic in that it can lead to erroneously positive observations of the presence of the European mink based on the detection of traces and marks of foraging, during the monitoring of its (natural and reintroduced) population. It also brings the risk of accidental victimization of the European mink in result of eradication measures taken against the N. vison, in the case of erroneously negative observations of the presence of European mink in a given area. The proper identification of the European mink is also important because in the past, even in the twentieth century, it was quite common to erroneously recognize it or identify it with the American mink, especially in popular scientific literature.

In this context, the non-invasive methods of recognition of the M. lutreola, such as the previously described genetic test for the nuclear microsatellite sequence Mel08, or nested-PCR-RFLP techniques for the mitochondrial D-loop fragment are of a special importance. Another method of a very significant discriminatory power is also the analysis of the microscopic structure (light microscopy) of the guard hairs – the pattern of the scales makes it possible to distinguish the European polecat from the European mink and the American mink, while the structure of the hair root makes it possible to distinguish the last two species. The superiority of the method based on the structure of the guard hair over the techniques of molecular genetics lies in its simplicity and low cost of execution, as well as the possibility of performing the very informative analysis based on only one mature guard hair (to genetic analyses it is necessary to collect at least 10 hair follicles). The superiority of the methods of the molecular identification over the methods described below lies in comparative character of the latter and the fact that they involve comparing the characteristics of the analysed biological material, the traces or the marks of foraging with the comparative material and the differential identification based on that comparison.

Identification of the European mink in the field is also based on direct recognition of the traces of the marks of foraging, but because of their high similarity with the traces of the marks of foraging of the M. putorius and the N. vison it may be difficult and unreliable. The traces of the European mink are characterized by imprints of five digital tips, often with a distinct imprint of interdigital webbing, short imprints of claws (shorter than of the M. putorius) and elongated imprint of the interdigital tip and have a length of approximately 3.5-4.0 cm for the anterior legs and 3, 6-6.5 cm for the posterior legs. The imprints of the digital tips and interdigital tips are usually higher in comparison to the European polecat and the American mink. The traces of the anterior and the posterior legs are set in a characteristic pattern, slightly sloping (less oblique than in the case of the N. vison) and reflecting the leaping motion of the European mink. The imprints of the paws appear in regular clusters of two or four, with quadruple traces occurring less frequently than in the case of the American mink, and the pattern of traces is more regular in this respect (on average, approximately 80% of the route is travelled in one pattern of claw traces) than in the case of the European polecat (on average, about 40-60% of double traces in the route); at the same time, in the context of maintaining homogeneous and non-chaotic pattern of traces on the paths, the traces of the M. putorius are more regular. The distance between the imprints of the anterior and the posterior paws (the size of leaps) is between 25 and 45 cm, while the spacing between the traces of the left and right paws is 8 cm. The probability of finding the traces of the European mink is the highest along the edge of watercourses and water reservoirs. In winter, the presence of mud – or other material on the edges of the water reservoir, which contaminates the fur of the animal during leaving the water – on the snow is a helpful mark trace of the presence of mink related to the semiaquatic lifestyle of the species. Such characteristic “dirty traces” are left on the snow or coastal flora, also in result of the wiping (drying) of the fur by the European mink after leaving the water.

The faeces of the European mink are solid, their length is approximately 5-8 cm in and width 5-8 mm, they are spirally twisted, sharpened at one end and brown or black in colour. The faeces of the European mink are usually more cylindrical than the faeces of the European polecat. The feeding grounds of the European mink are characterized by the remains of the prey; this species collects and hides the prey, which is then used in the periods when the prey is less accessible, however such feeding grounds do no differ from the feeding grounds of the European polecat and the American mink and without the presence of distinct traces they are difficult to distinguish. Using automatic camera trappings is highly reliable method of detecting the presence of the European mink in the field, especially in conjunction with trace identification.

Identification based on the bone material of the European mink (fossil remains or post mortem skeleton), in the context of its distinction from the European polecat and the American mink, is the easiest with the use of the craniometric methods. Youngman proved that the highest capacity differentiation between the M. lutreola and the M. putorius is offered by the comparison of maximal width of occipital condyle and the width between the processus postorbalis and the comparison of the condylobasal length and the palatial length (PPL) and the width of the occipital bone. For distinguishing the M. lutreola from the N. vison, it is best to use the sum of the lingual length and the central length of the upper molar M1 (LLM1) and the previously defined PPL value.