Introduction
The European polecat (Mustela putorius), a mustelid species also known as the common polecat, black polecat, and forest polecat, is currently recolonising Great Britain after nineteenth-century declines. This article delves into the dietary habits of the European polecat, examining its food preferences, adaptability, and the potential risks it faces due to fluctuations in prey populations and exposure to rodenticides. Understanding the polecat's diet is crucial for assessing its ecological flexibility and the factors influencing its recovery.
Generalist Predator with Regional Specializations
Polecats are typically described as generalist predators, consuming a wide variety of food items across their European range. Rodents and amphibians are common food items in all regions. They exhibit dietary diversity, with some evidence of regional specialization. For example, polecats specialize on rabbits in the Mediterranean and on amphibians in Switzerland and Poland. In Białowieża National Park, Poland, frogs comprised 60% and 90% of polecat total food biomass in summer and winter, respectively. Although polecats ate rodents, this was only when frogs were not available. In other localities, it has been found that impressions of specialisation by polecats simply reflect the local abundance of a given prey.
The Polecat's Diet in Great Britain: A Focus on Lagomorphs
In Great Britain, polecats predominantly eat lagomorphs. However, in periods when rabbit populations were severely reduced due to disease outbreaks, notably during the 1950s and 1960s due to myxomatosis, mammals comprised a much smaller proportion of polecat diet. There is also evidence of seasonal consumption of rodents (including brown rats Rattus norvegicus and field voles Microtus agrestis), particularly in the winter months. A flexible foraging strategy is thought to allow polecats to occupy and exploit diverse habitats, such as lowland, grassland, farmland, and riparian habitats, and may play an important role in enabling population persistence.
Dietary Analysis: 2012-2016
A dietary analysis of stomach contents from 99 polecats collected in Great Britain between 2012 and 2016 reveals that lagomorphs were the most abundant prey (66% frequency of occurrence, 95% confidence interval 53-74%), followed by other mammals (12%, 4-18%), amphibians (10%, 3-16%) and birds (7%, 1-13%). Diet varied seasonally; lagomorph occurrence was highest in spring and summer and lowest in autumn. This study aimed to explore dietary variation and niche breadth in polecats during the process of polecat population recovery and rabbit population variation. The findings were compared with earlier analyses of polecat diet in Britain in the 1960s, 1980s, and 1990s.
Methods of Analysis
Polecat carcasses, predominantly of animals killed on the road, were collected across Great Britain during the Vincent Wildlife Trust’s national polecat survey in 2012-2016. Date of collection and location were recorded. Animals were stored frozen until necropsy examination, which was carried out at National Museums Scotland. Stomach contents were collected from 99 polecats and refrozen prior to dietary analysis. Stomach contents were soaked in biological detergent for 24 h, rinsed through a 53-μm sieve, then stored in 70% ethanol. Identifiable macroscopic animal remains (undigested body parts, fragments of bone, feathers, fur, individual hair and insect remains) were separated from unidentifiable tissues. Ten samples were selected at random and were analysed for earthworm (Lumbricidae) chaetae and other microscopic remains. No identifiable microscopic remains were found. As there is no evidence from previous dietary studies to suggest that polecats eat earthworms, and since variation in microscopic remains do not relate to the primary processes of interest in this study, we considered only macroscopic remains in the remaining 90% of gut samples. Plant debris was considered to have been ingested when catching prey and was not included in diet composition quantification.
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Fur remains were identified using guard hair cuticle patterns after Teerink (1991). All loose hairs were collected and analysed. Cuticle patterns were examined under a microscope at ×40 magnification. Mammal remains were identified to species level, except for rabbits and brown hares Lepus europaeus, which were not separated and were classified as lagomorphs. Most bones were fragmented and unidentifiable, but those that were intact, together with teeth, were identified as insectivore, rodent, larger mammal or amphibian to species level using personal collections and appropriate keys. Bird remains were identified to order using Day (1966). Amphibians were determined by skin texture and, where possible, by webbing on feet. Fishes were identified by their bones and scales but were not identified to species, as fish were a rare item and not of primary interest.
Two methods were used to assess accuracy when identifying guard hair cuticle pattern. First, 10% of samples were randomly selected for a second blind analysis by the same analyst; the correspondence in the results was 100%. A third analysis, again blind, was carried out by a second researcher, this time on 10% of samples that contained hard parts and 20% of the samples that relied on hair identification. There was a 100% match between analysts for samples containing hard parts, an 86% correspondence for hair samples and a calculated Cohen’s kappa test of interrater agreement of 0.7, which is “substantial” according to Landis and Koch (1977). The level of overlap in identification indicated that identification of guard hair was sufficiently robust for inclusion in our data analysis.
All analyses were carried out in R (R Core Team 2011). Diet was summarised as the percentage frequency of occurrence (% FO), calculated as the number of each type of dietary item as a percentage of the total number of identifiable prey items. As our objective was to evaluate changes in polecat diet over time rather than assess differences in energetic requirements, frequency of occurrence was chosen as the best method for comparison as it was consistent with historical studies. Whilst frequency of occurrence may overestimate the importance of smaller food items in a carnivore’s diet, and caution should be used applying it in isolation to understand the effect of predators on prey populations, it is still an appropriate method for exploring a carnivore’s ecology. Polecats, in line with other small carnivores, usually only have one prey item per stomach. This means that the difference between frequency of occurrence calculated using total prey items or that using the number of stomachs is negligible (in this study, of the 99 polecat stomachs investigated and 79 that had identifiable contents, only three individual stomachs contained more than one item). As a result and for simplicity, we calculated frequency of occurrence per food item, expressed as a percentage of the number of occurrences of one food item of the total number of occurrences of all food items, to indicate the relative importance in diet. The frequency of occurrence matrix was replicated randomly 1000 times (bootstrapped with replacement 1000 times) to generate 95% confidence intervals following Reynolds and Aebischer (1991). Differences in FO of prey groups were compared using a chi-squared test.
Levins’ (1968) index of niche breadth was calculated following the formula:
Nb=1/∑pi2where pi is the proportion of records for each species in each group. The proportion of prey records for each group was bootstrapped with replacement 1000 times to generate 95% confidence intervals for Levins’ index.
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To analyse variation in the occurrence of lagomorphs in polecat diet in more depth using the 2010s data, a binomial logistic regression model of presence/absence was fitted to sex, season (where spring is March to May, summer is June to August, autumn is September to November and winter is December to February) and region (north, south, east and west based on British grid reference).
Fluctuations in Rabbit Populations and Dietary Adaptations
When rabbit populations in Great Britain crashed by up to 95% as a result of a myxomatosis epizootic in the 1950s, the diet composition and population dynamics of other mustelid carnivorans, such as stoats Mustela erminea and weasels Mustela nivalis, were affected. Rabbit populations recovered to their pre-myxomatosis levels by the 1990s, but since then, rabbit numbers have declined across Britain (England − 44%; Scotland − 82%; and Wales − 48%), possibly as the result of rabbit haemorrhagic disease (RHD), which has devastated rabbit populations across mainland Europe. Analysis of changes in rabbit records between 2011 and 2015 reveals spatial variation in rabbit declines, with the greatest reductions in central and southern England, along the Scottish borders and in north-east Scotland.
In Spain, generalist carnivorans (such as red fox Vulpes vulpes, badger Meles meles and genet Genetta genetta) reduced their consumption of rabbits in response to declines in rabbit populations following RHD outbreaks. In contrast, whilst rabbit consumption by Iberian lynx Lynx pardinus, which are near-obligate predators of rabbits, also reduced, lynx continued to preferentially select rabbits in spite of their reduced availability. Given that polecats in Britain are known to eat rodents and amphibians as well as rabbits, it is possible that reductions in rabbit populations would lead to polecats diversifying their diet.
Dietary niche breadth was greater in the 1960s, when rabbits were scarce, than in other decades, but did not differ between the 1990s and 2010s, indicating that diets have not diversified with recent rabbit declines. This may be because rabbit abundance is not yet low enough to cause dietary diversification or because polecats were collected in areas where rabbits were still abundant.
Rodenticides and Secondary Exposure
Rodents are thought to be the major route by which polecats are exposed to second-generation anticoagulant rodenticides (SGARs) in Britain. Secondary exposure of polecats to SGARs increased 1.7-fold between 1993 and 2016, and the most recent study indicated that 79% of polecats had been exposed. It may be that this increase has been a result of an increase in the proportion of rodents in polecat diet. Secondary exposure to SGARs may be lethal in sufficient concentration, or lead to a range of sub-lethal effects. Increased rates of secondary exposure to SGARs have not prevented polecat expansion over the same time period, but whether or not polecat abundance or the rate of population expansion have been affected by SGARs exposure is unknown.
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Rodents did not increase in diet between the 1990s and 2010s and still occur with < 10% frequency, indicating that rodents need not contribute much to diet to expose polecats to rodenticides.
Sex-Based Dietary Differences
Between-sex dietary differences have been observed in some mustelids. Studies of polecat diet in Britain have previously found that female polecats tend to eat fewer rabbits and more birds than male polecats, though these differences were not statistically significant. It is possible that as rabbit abundance has declined, increased competition for available rabbits may have led to more pronounced dietary differences between male and female polecat diets.
Historical Persecution and Conservation Status
In Britain, the European polecat was regarded as a serious poultry predator prior to the introduction of wire netting, therefore eliminating it was considered the only option to protect stock. The polecat may be the best example of a species for which the level of killing really did make a difference to the population.
The European polecat is afforded both national and European protection; it is listed on Schedule 6 of the Wildlife and Countryside Act 1981 and Regulation 41 of the Conservation (Natural Habitats, &c.) Regulations 1994 and is listed on Annex V of the Habitats Directive.
Physical Characteristics and Subspecies
The appearance of the European polecat is typical of members of the genus Mustela, though it is generally more compact in conformation and, although short-legged, has a less elongated body than the European mink or steppe polecat. The tail is short, about one-third its body length. The eyes are small, with dark brown irises. The hind toes are long and partially webbed, with weakly curved 4 mm-long, nonretractable claws. The front claws are strongly curved, partially retractable, and measure 6 mm in length. The feet are moderately long and more robust than in other members of the genus.
The polecat's skull is relatively coarse and massive, more so than the mink's, with a strong, but short and broad facial region and strongly developed projections. In comparison to other similarly sized mustelids, the polecat's teeth are very strong, large and massive in relation to skull size. Sexual dimorphism in the skull is apparent in the lighter, narrower skull of the female, which also has weaker projections. The polecat's running gait is not as complex and twisting as that of the mink or stoat, and it is not as fast as the mountain weasel (solongoi), stoat or least weasel, as it can be outrun by a conditioned man.
The dimensions of the European polecat vary greatly. The species does not conform to Bergmann's rule, with the pattern of size variation seeming to follow a trend of size increase along an east-west axis. Males measure 350-460 mm (14-18 in) in body length and females are 290-394 mm (11-16 in). The tail measures 115-167 mm in males and 84-150 mm in females. Adult males in middle Europe weigh 1,000-1,500 grams (35-53 oz) and females 650-815 grams (23-29 oz).
Several subspecies of the European polecat are recognized, including:
- Common polecat (M. p. putorius)
- Welsh polecat (M. p. anglius)
- Mediterranean polecat (M. p. aureolus)
- †Scottish polecat (M. p. caledoniae)
- Domestic ferret (M. p. furo)
- Middle Russian polecat (M. p. mosquensis)
- Carpathian polecat (M. p. rothschildi)
Fur and Coloration
The winter fur of the European polecat is brownish black or blackish brown, the intensity of which is determined by the colour of the long guard hairs. On the back and flanks, the dark tone is brightened by bright whitish-yellowish, sometimes yellowish-greyish underfur which shows through. The lightly coloured underfur is not equally visible on different parts of the body. On the back and hindquarters, the underfur is almost completely covered by the dark guard hairs. On the flanks, though, the lightening is well defined, and contrasts sharply with the general tone of the back. The throat, lower neck, chest and abdomen are black or blackish brown. The limbs are pure black or black with brown tints, while the tail is black or blackish brown, completely lacking light underfur. The area around and between the eyes is black-brown, with a longitudinal stripe of similar colour along the top of the nose. The ears are dark brown and edged with white. The summer fur is short, sparse and coarse. It is greyer, duller and lacking in the lustre of the winter fur.
Polecats were found in two major phenotypes a typic one and a dark fur one with no black mask. Colour mutations include albinos, leucists, isabellinists, xanthochromists, amelanists, and erythrists. In typical erythristic individuals, the underfur is usually bright reddish. The guard hairs on the trunk are bright reddish or reddish brown. Black guard hairs are absent on the lower body and head. In some rare cases, the guard hairs are so light, they are almost indistinguishable from the pale-yellow underfur. These individuals are called "amelanistic".
Behavior and Reproduction
European polecats use several den sites distributed throughout their home ranges and are often most active around rabbit warrens. Some European polecats use farm buildings or haystacks as daytime resting sites in winter. Occasionally, European polecats use abandoned European badger or red fox burrows.
Like other mustelids, the polecat is usually a silent animal, though it will growl fiercely when angered, and squeak when distressed. The European polecat is a seasonal breeder, with no courtship rituals. During the mating season, the male grabs the female by the neck and drags her about to stimulate ovulation, then copulates for up to an hour. The species is polygamous, with each male polecat mating with several females. The gestation period lasts 40-43 days, with litters usually being born in May-early June. Each litter typically consists of five to ten kits. At birth, the kits weigh 9-10 g (0.32-0.35 oz) and measure 55-70 mm (2.2-2.8 in) in body length; they are blind and deaf. At the age of one week, the kits are covered in silky, white fur, which is replaced with a cinnamon brown-greyish woolly coat at the age of 3-4 wk. Weaning begins at three weeks of age, while the permanent dentition erupts after 7-8 wk.
Hunting and Diet Composition
The European polecat's diet consists of mouse-like rodents, followed by amphibians and birds. Its most frequent prey item in the former Soviet Union is the common vole and rarely the red-backed vole. In large river floodlands, water vole are common prey. In spring and winter, amphibians (especially grass frogs and green toads) become important food items. Selective predation on male frogs by the polecat decreases the occurrence of polyandry in frog populations. However, because amphibians have little calorific value, the polecat never grows fat on them, no matter how many it consumes. In Central Europe, the diet in winter months is dominated by birds including quail, grey partridges, grouse, chickens, pigeons and passerines.
The European polecat hunts its prey by stalking it and seizing it with its canine teeth, killing the animal with a bite to the neck. This killing method is instinctive, but perfected with practice.
Hybridization
In some parts of Britain, the abandonment of domestic ferrets has led to ferret-polecat crossbreeds living in the wild. The European polecat can hybridise with the European mink, producing offspring termed khor'-tumak by Russian furriers and khonorik by fanciers. Such hybridisation is very rare in the wild, and typically only occurs where the European mink population is in decline. A polecat-mink hybrid has a poorly defined facial mask, yellow fur on the ears, grey-yellow underfur and long, dark brown guard hairs. It is fairly large, with a male attaining the peak sizes known for European polecats weighing 1,120-1,746 g (2 lb 7+1⁄2 oz - 3 lb 13+9⁄16 oz) and measuring 41-47 cm (16-18+1⁄2 in) in length; a female is much larger than a female European minks weighing 742 g (26+3⁄16 oz) and measuring 37 cm (14+1⁄2 in) in length. The majority of polecat-mink hybrids have skulls bearing greater similarities to those of polecats than to minks; they can swim well and burrow for food like polecats, but are difficult to tame and breed, as males are sterile.
The first captive polecat-mink hybrid was created in 1978 for fur, but breeding of these hybrids declined as European mink populations decreased. Studies on the behavioural ecology of free-ranging polecat-mink hybrids in the upper reaches of the Lovat River indicate that hybrids stray from aquatic habitats more readily than minks, and tolerate both parent species entering their territories, though the hybrid's larger size, especially the male's deters intrusion. During summer, the diets of wild polecat-mink hybrids are more similar to those of minks than to the polecats, as they feed predominantly on frogs.
The European polecat can also hybridise with the Asian steppe polecat or the domestic ferret to produce fertile offspring. European-steppe polecat hybrids are very rare, despite their sympatry in several areas.
Distribution
The European polecat is widespread in the western Palaearctic to the Urals in the Russian Federation, though it is absent from Ireland, northern Scandinavia, and much of the Balkans and eastern Adriatic coast. It occurs only marginally in northern Greece. It is found in Morocco in the Rif Mountains, from sea level to 2,400 m (7,900 ft).