A comparison of the feeding biology of Mink Mustela vison and otter Lutra lutra

The feeding habits and prey selectivity of Mink Mustela vison and otters Lutra lutra were compared in two localities in Devon: a eutrophic lake and a moorland river, in which both species occurred and had access to the same prey populations. The effects of prey availability on the predators' diets were assessed by comparing prey consumed, as revealed by scat analysis, with estimates of prey abundance and size range. Otters specialized in fish at all times of year but showed seasonal variation in species taken. Selection for slow-moving fish and seasonal changes in behaviour of some fish species were the probable causes of this variation. Otters diversified more into non-fish food in summer, when fish availability was reduced. The main alternative prey in the lacustrine habitat was waterfowl, but in the riverine habitat, rabbits. Mink were more generalized carnivores, taking a variety of fish, waterside and terrestrial prey in all seasons. These three prey categories were taken to an almost equal extent in the lake but terrestrial prey dominated in the riverine habitat. Fish were taken most frequently in winter and birds and mammals in summer. Neither predator showed selection in respect of prey size. In each area, about one third of the otter and Mink diets was common to both species. Fish was the principal group of the shared component, and dietary overlap in respect of them was greatest in autumn and winter. In view of the dietary preferences of each predator, the existence of alternative prey items and limited degree of dietary overlap, it is considered unlikely that the two species competed for food to any extent. Other factors must therefore be responsible for the spread of feral Mink and the decline in otter populations in many parts of Britain.     

Competition between Eurasian otter Lutra lutra and American mink Mustela vison probed by niche shift

Interspecific competition is one of several constraints that might prevent an individual from maximising its energy intake. When an interspecific competitor is introduced, an individual is often forced to shift its diet according to the intensity of the competitive pressure. In this paper, we explore whether the introduced American mink ( Mustela vison Schreber) shifts its diet when the density of its potential competitor, the Eurasian otter ( Lutra lutra L.), is increased. We compared the diets of otter and mink at the same location but at two moments in time when the relative densities of these two species were different while controlling for the abundance of aquatic prey. Mink and otters are semi-aquatic mammals belonging to the same guild of mustelids and otters are expected to be the dominant competitor because they are larger and better at hunting underwater. The diets of otters and mink overlap to a great extent but while otters specialise mainly on aquatic prey, mink are able to exploit both aquatic and terrestrial prey. These observations prompted the hypothesis investigated in this work that at higher otter densities the diet of mink should change to include a higher proportion of terrestrial items. This hypothesis was supported by the data and at higher otter densities mink diet was observed to consist of a higher proportion of mammals and birds while fewer fish were present, although this pattern was present only in winter while no changes were observed in spring. Meanwhile the diet of otters remained basically unchanged. In the second part of the study, we investigated whether niche breadth and niche overlap between otter and mink changed at different otter densities. We found that niche overlap declined as the density of otters increased, in agreement with the prediction of habitat selection theory.     

Sexual dimorphism in the skull of minks Mustela vison, badgers Meles meles and otters Lutra lutra

Multiple-group principal component analysis and discriminant analysis were used to investigate the morphological differences between adult skulls of male and female minks, badgers and otters from Norway. The first principal component axis, calculated from the variance-covariance matrix of log-transformed data, was interpreted as a growth-free size axis in all three species, while the other components were interpreted as representing shape. Having largely separated size and shape variation, these two aspects of sexual dimorphism could be studied. The standardized component scores were subjected to an analysis of variance and discriminant analyses were performed on size-in and size-out data. Sexual dimorphism was disclosed on eight of the 12 components in minks and on seven of the 12 components in badgers and otters. In mink the multivariate differences were more due to size than to shape, whereas in badgers and otters most of the multivariate differences were due to shape, but the differences in size were also significant. The shape dimorphism was shown to be functionally related to jaw and neck muscles. The results were discussed in relation to recent theories to explain the evolutionary significance of sexual dimorphism in body size of mustelids. It was concluded that these theories do not fully explain the dimorphism found in the skulls of the moderately dimorphic badger and otter.     

Biology of the Polecat Mustela putorius: a literature review

The available literature on the biology of the European Polecat ( Mustela putorius ) is collated. Aspects covered are general biology and systematics, distribution, physical characteristics, variation, skeletal characteristics, habitat, general behaviour, diet and predatory behaviour, breeding, juvenile development, population structure, mortality, parasites and diseases, relations with man, and history in Britain. Notes on field signs are also included. Recent and current research on Polecats both in Great Britain and on the European continent are then reviewed and some new information is reported. An extended bibliography lists all major and many minor publications on M. putorius , together with selected references to feral Ferrets ( M. furo ) and Polecat-Ferrets ( M. putorius x M.furo ).     

Trophic status and feeding habits of the European Polecat Mustela putorius L. 1758

The trophic status of the European Polecat Mustela putorius was studied through a review of the diets of 18 different populations in Europe. Rodents represented the principal prey in 10 sites (55.5%) and were a prey of secondary importance in seven sites (38.8%). Anurans prevailed in three sites (17%) but constituted the second food category in eight others (44%). The ascendant hierarchical classification showed a relatively unvarying diet in Europe. The frequency of birds and invertebrates were important in the diversification of the diet, indicated by the value of the food niche index, whereas rabbits were more significant in southern regions. The constant incidence of predation on woodland rodents and amphibians gave a particular status to the Polecat and showed it to be a generalist feeder well adapted to the mid-European region.     

Helminth communities of the autochthonous mustelids Mustela lutreola and M. putorius and the introduced Mustela vison in south-western France

This study presents the first comprehensive helminthological data on three sympatric riparian mustelids (the European mink Mustela lutreola, the polecat M. putorius and the American mink M. vison) in south-western France. One hundred and twenty-four specimens (45 M. lutreola, 37 M. putorius and 42 M. vison) from eight French departments were analysed. Globally, 15 helminth species were detected: Troglotrema acutum, Pseudamphistomum truncatum, Euryhelmis squamula, Euparyphium melis and Ascocotyle sp. (Trematoda), Taenia tenuicollis (Cestoda), Eucoleus aerophilus, Pearsonema plica, Aonchotheca putorii, Strongyloides mustelorum, Molineus patens, Crenosoma melesi, Filaroides martis and Skrjabingylus nasicola (Nematoda) and larval stages of Centrorhynchus species (Acanthocephala). The autochthonous European mink harboured the highest species richness (13 species) followed by the polecat with 11 species. The introduced American mink presented the most depauperate helminth community (nine species). The prevalence and worm burden of most of the helminths found in M. putorius and M. lutreola were also higher than those of M. vison. Some characteristics of their helminth communities were compared to relatively nearby populations (Spain) and other very distant populations (Belarus). This comparison emphasized M. patens as the most frequent parasite in all of the analysed mustelid populations. It was possible to conclude that the invasive M. vison contributes to the maintenance of the life cycle of the pathogenic T. acutum and S. nasicola helminths, with possible implications for the conservation of the endangered European mink.     

The Pineal Gland of the Mink, Mustela vison: Light-, Fluorescence- and Electron Microscopical Studies

The pineal gland of normal and experimental female mink has been studied by light-, fluorescence- and electron microscopy. The general structure of the mink pineal is described. Two main cell types are recognized. One, termed pinealocyte, predominates in number. Though slight morphological differences (e.g. electron density of the cytoplasm and content of organelles) were observed, this study indicates that the pineal of mink only contains one single population of pinealocytes. The other, termed glial cell, inserted between the pinealocytes, is characterized by the presence of elongated processes, containing microfilaments. Different treatments (ovariectomy and LH—RH administration) and different endocrine states during the year induced morphological changes in the pinealocytes. A rich network of nerve fibres containing electron-dense granules (40–50 nm) is observed. Microspectrofluorometrically these fibres exhibit the spectral characteristics of cateholamines. All the pinealocytes show a yellow fluorescence. This cellular fluorophor shows the same microspectrofluorometric characteristics as does the fluorophor of serotonin. Occasionally, synaptic ribbons are observed in the perikaryon and the processes of the pinealocytes. A large number of cellular junctions between pinealocytes and endothelial cells is present. Their presumed function(s) are discussed. There is evidence of a blood-brain barrier within the mink pineal gland.     

Sexual Dimorphism in the Skull of the Feral American Mink (Mustela vison Schreber)

Sexual dimorphism in the skull of the feral American mink is discussed, based on 15 measurements of 53 adults from southern Norway. The differences in means are highly significant. Variation is found to be greatest in males. Sexual dimorphism is thought to be an adaptation for different reproductive roles of the sexes in a polygynous mating system with lack of male parental care.     

Modelling the benefits of American Mink Mustela vison management options for terns in west Scotland

American Mink Mustela vison is a semi-aquatic predator that has invaded the west coast of Scotland and many of its associated islands. We developed a GIS model of their potential range based on their dispersal abilities and habitat use, which revealed that most islands in west Scotland are accessible to Mink, and that these host a large proportion of the region's Common Sterna hirundo and Arctic Terns S. paradisaea . Mink are predators on tern eggs and chicks, and statistical modelling of long-term productivity data demonstrated that unprotected sites within their range have an average productivity of 0.33 chicks per pair, whereas that at sites where Mink were trapped was 253% higher. We assessed the benefits of current Mink control projects for terns in the Western Isles and the remainder of west Scotland using a population modelling approach. This showed that both projects delivered considerable benefits for Common Terns, because a large proportion of their numbers were within the area of the control programmes and in sites that would be accessible to Mink if no control were in operation. For Arctic Terns, the benefits were less clear, as a larger proportion of their numbers were outside the control areas, and many of these were in sites isolated from, or unsuitable for, Mink. We discuss the implications of these findings for future strategic planning of Mink management in west Scotland.     

Comparative Enzymologic Study of Catalytic Properties of Blood Serum Cholinesterase of the American Mink Mustela vison

Comparative enzymologic study of catalytic properties of cholinesterase (ChE) in blood serum of the American mink Mustela vison Schr. has revealed several peculiarities of this enzyme. First, using the method of substrate–inhibitor analysis, homogeneity of the ChE preparation has been established, i.e. only one ChE has been found in mink serum. Second, the rate of acetylcholine hydrolysis was higher than of thiocholine substrates, among which propionylthiocholine was hydrolyzed at the highest rate. Third, propionylthiocholine had the highest V/K _M value that reflects to a degree affinity of the substrate to enzyme. Fourth, the phenomenon of substrate inhibition, which is not inherent for mammalian serum cholinesterases, is revealed and kinetically analyzed. Fifth, study of inhibitory specificity has not revealed differences of the mink serum ChE from other serum ChE.     

An assessment of British farmers' and gamekeepers' experiences, attitudes and practices in relation to the European Polecat Mustela putorius

Postal questionnaires were distributed to farmers and gamekeepers within the Polecat’s Mustela putorius main range in Britain. Only 11% of responding farmers had ever experienced damage by Polecats; 28% regarded the species as a threat to livestock. Conversely 53% of farmers believed Polecats control Rabbits Oryctolagus cuniculus, and 39% believed they control rodents on the farm. Two-thirds of responding gamekeepers had experienced Polecat predation of penned game, with the commonest access routes via ‘pop-holes’ and beneath the perimeter wire. Most gamekeepers (68%) regarded the Polecat as a minor pest, but ranked it as a less serious threat to game than predators such as the Fox Vulpes vulpes, Feral Cat Felis catus, Stoat Mustela erminea, corvids and Mink Mustela vison. Majorities of both farmers and gamekeepers would be concerned about an increase in the numbers of Polecats, and most wished to be free to control the species. Trapping was regarded as the main defence against Polecat predation of game; 91% of gamekeepers had trapped Polecats over the preceding 5 years. A minority of farmers carried out Polecat control; this activity was more prevalent on farms near the fringe of the species’ range. Pest-control practices likely to impact accidentally or indirectly upon Polecats, such as rodenticide use, fumigation (‘gassing’) of Rabbit burrows and ferreting, were also more prevalent on farms towards the fringe of the species’ range. These findings are discussed in the light of the Polecat’s status as a Scheduled species recovering its range in Britain. In anticipation of the species’ further spread into areas where game shooting is prevalent, recommendations are made regarding the need to improve game husbandry and to modify trapping practice. In particular, the night-time closure of pop-holes and the effective exclusion of Polecats from tunnel traps are suggested as a means of promoting greater tolerance of Polecats and compliance with the law.     

The Gastrointestinal Bacteria of Mink (Mustela vison L): Influence of Age and Diet

Total numbers of aerotolerant and anaerobic bacteria, and densities of Enterobacteriaceae, lactobacilli, staphylococci, salmonella and shigella, and campylobacteria were enumerated in the contents of the stomach, small intestine (and the associated mucosa), and colon of mink beginning at 2 weeks of age to adulthood, and in adults that were fed diets with different levels and types of fiber or food deprived. Highest densities of all bacterial groups were found in the colon at all ages (up to 108 cfu per g for total anaerobes), but were 2–4 orders of magnitude lower than those of other mammals. When all regions were pooled, significant age-related increases (p<0.05) were detected for anaerobes, aerobes, and staphylococci, and these coincided with the dietary shift at weaning. Enterobacteriaceae did not vary with age. Lactobacilli were never common isolates, but were detected more often after weaning, particularly in adults fed diets containing the 2 sources of fiber. Campylobacteria were detected only at 2 weeks of age, and salmonella and shigella were not isolated from any of the experimental mink. Total bacterial densities, the relative proportions of the bacterial groups, and age- and diet-related effects differ from those known for other mammals, which may be related to the carnivorous diet and rapid movement of digesta through the GIT.     

Invasive predators and the conservation of island birds: the case of American Mink Mustela vison and terns Sterna spp. in the Western Isles,Scotland

Capsule Colonies were larger and breeding success lower in mink-inhabited areas.

Aims To examine the impact of mink on dense aggregations of ground-breeding seabirds on islands previously isolated from mammalian predators.

Methods We compare 1990–93 tern breeding data with records before and after mink arrival.

Results In the early 1990s, terns showed no preference for mink-free islands. The breeding behaviour and success of tern colonies in mink-inhabited areas did not significantly differ from that in mink-free areas. However, colonies were larger and breeding success lower in mink-inhabited areas compared to mink-free areas, trends which might reflect longer term mink impact.

Conclusions As mink spread south there seems to be a gradual increase in tern colony size affecting Lewis, then Harris and, recently, the Sound of Harris.     

Cytodifferentiation in the Pars Distalis of the Pituitary Gland in the Fetal and Juvenile Mink,Mustela vison

Hypophysial tissue of mink (Mustela vison) embryos, newborn young and kits aged 1 day to 27 weeks was examined by light and electron microscopy. Among the embryos five different ontogenetic stages were distinguished. It appears that the primordium of the pituitary gland differentiates in early embryonic life. Several types of glandular cells containing variable amounts of granular material were recognizable by the time of birth. The role of the granular cells and the origin of the lumina in the pars distalis are speculated upon (in the Discussion).     

Fine Structure of the Pars Distalis of the Pituitary Gland in the Female Mink,Mustela vison

Pituitary glands of intact and experimental adult females of mink, Mustela vison, were examined by electron microscopy. Conventional methods involving removal of endocrine glands (ovaries and adrenals), administration of radioactive isotope, ¹³¹I, blocking agents (thiouracil and metopirone) and hormones (thyroxine, hydrocortisone, thyrotropin and luteinizing hormone releasing hormones) were employed. Five categories of granular cells were distinguished both by their ultrastructural characteristics and qualitative changes throughout the year and following different treatments. The cell types are described and their functions discussed. From conventional electron microscopical studies it proved difficult to draw any satisfactory conclusions about the gonadotropic cells. Further investigation by means of immunocytochemistry and radioimmunoassay techniques is required to determine, whether the presumptive gonadotropic cell type produces both FSH and ICSH or only one of these hormones. Morphologically two types of agranular cells were identified. Their morphological inter-relationship and function are discussed briefly.     

Cytological and Experimental Studies on the Pars Distalis of the Mink Mustela vison,a Light Microscopic Study

The pars distalis of adult minks were examined by light microscopy. Seasonal variations in cytology and the effect of photostimulation and various drugs were studied. Six types of glandular cells were identified by their structural and staining qualities. To each has been tentatively assigned an endocrine function. Problems in the interpretation of experiments involving administration of drugs are discussed. Discussion is also devoted to the difficulties in demonstrating the mink prolactin cells.