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Monitoring polecat populations using roadkill

Updated Thursday 27th September 2012

Are polecat populations growing? One way to check is to survey numbers of polecats live and dead on the roads

European polecat, Mustela putorius Creative commons image Icon 'Velvet' / Peter Trimming / CC BY 2.0 under Creative-Commons license A young polecat The European polecat, Mustela putorius, is classified in the same family as ferrets, stoats and weasels. the Mustelidae. Adult females have a body length of about 32-39 cm and the males about 33-45 cm. Adult males weigh about 0.8-1.5 kg and adult females about 1.5 kg.

The coat comprises a thick basal layer of beige fur and a longer dark brown fur. The tail is short and black and facial fur is striped dark brown and beige. The dark brown ears are bordered with beige fur.

The life (and death) of a polecat

Each polecat has a large territory, where they range to find food and build their dens.

Polecats build their dens in cavities underneath barns and tree roots or in sloping stream banks. Mating occurs between March and May and the female gives birth to four to five kits in May or June. After about four weeks of lactation the kits are weaned and move on to food caught or collected by their mother.

Thereafter the kits spend about four months of play and learning with their mother, before dispersing.

It is at this point the inexperienced young polecats are vulnerable to being hit by traffic on busy roads.

Polecats forage at night, taking rabbits, eggs, birds, amphibians, fish, carrion, and rodents, especially rats. Live prey are killed by a bite to the neck.

Polecats were persecuted vigorously, because they were perceived to steal from farms and shooting estates, taking eggs, chickens and game birds. The defence strategy of polecats also contributed to their unpopularity. A frightened polecat will stand on its forepaws and release a foul smelling secretion from its posterior scent glands.

Polecat populations in the UK

By the end of the 19th century, polecats were extinct in England and restricted to a small area around Aberdovey in Mid Wales.

However, since the early 1990s and probably earlier, polecats have spread out of Wales to Scotland and English regions. There were also releases of captive bred polecats in the 1980s and 1990s.

Now polecats are protected and have the status of a priority species in the UK Biodiversity action plan. There is more awareness too of the beneficial roles of polecats in catching rodents and rabbits.

The re-colonisation of English regions by polecats has been tracked by the Vincent Wildlife Trust (VWT) since the early 1990s. A number of surveys monitored the polecat population by live trapping and collection of road casualties.

Polecat surveys in the 2000s

The surveys carried out in 2004-2006 and 2009-2010 involved volunteers who recorded both live and road killed polecats while driving.

John Messenger, from the VWT, reported that in 2009, recorders surveyed a total of 108,641 km of road during one month (15th September to 15th October). Polecats were found in seven English regions. A total of 101 dead polecats, three live polecats, 19 dead mink and one live mink were recorded.

The data, including total km of road surveyed, and numbers of roadkill and live polecats, was converted to an abundance index. The survey results indicate that the polecat population had increased in most English regions since the 2004-2006 surveys.

Polecats were most abundant in Wales and the West Midlands. Polecats had also increased in other English regions: North West England, East of England, Southeast England and Yorkshire and Humberside.

Inevitably the data collected are affected by human behaviour. Volunteers will drive more miles if there’s a good chance of seeing a polecat—and the numbers of people interested in volunteering vary from region to region.

The mileages driven in regions with apparent low polecat populations were lower—they include North East England, East Midlands, South West England and London.

More recent surveys

Recorders for the polecat survey in 2010 surveyed 62,197 km of road and recorded a total of 39 dead polecats and three live polecats. The relatively small amount of data collected by the lower number of recorders in 2010 was a concern.

Furthermore, as John Messenger pointed out, those polecats hit by cars tended to be inexperienced young animals that had been born in spring 2010. So fluctuations in the numbers of polecats observed as roadkill may reflect the numbers of young produced in the spring.

Nevertheless, the surveys have shown that from the 1990s, polecats have spread into many English regions from Wales and they are breeding successfully.

Polecat diet

Creative commons image Icon '8 out of 10 polecats prefer ...' / Peter Trimming / CC BY 2.0 under Creative-Commons license A polecat being fed Roadkill polecats also provide opportunities to investigate the polecats’ diet by analysis of stomach contents. Analyses of polecat stomach contents by the VWT revealed that 85% of the remains of the stomach contents were rabbits.

Johnny Birks, from the VWT, observed that that radio-tagged polecats on the Kemerton Estate on Bredon Hill spent half their time in rabbit warrens, where they slept and hunted the rabbits.

This is typical behaviour for predators who will take advantage of an easy way to catch prey. In autumn and early winter when rat numbers peak, polecats hunt rats in farmyards and barns.

Control methods for reducing the rat population comprise putting down poison bait—the poisons are anticoagulants that cause internal bleeding, leading to death. Polecats that feed on rats in farmyards ingest the poison when eating rats that have a body burden of anticoagulant rodenticide.

Analyses of liver samples from roadkill polecats were used to investigate the extent of poisonous substances such as rodenticides and insecticides.

A study in 2003 analysed the livers of 100 road-killed polecats and discovered residues of anticoagulant rodenticides in 40% of the polecats. The commonest residues were the second generation anticoagulant rodenticides, difenacoum and bromdiolene.

The concern was that polecats may accumulate lethal amounts of the rodenticides and die. However, the data showing the spread of polecats in England suggested that despite their ingestion of rats that had eaten poison bait, polecats had not been prevented from expanding their range.

Polecat populations: looking ahead

As predators, polecats are an important part of the ecosystem. Their spread from their previous stronghold in Wales into English and Scottish regions has been successful.

Polecats are vulnerable on our busy roads and their liking for rats puts them at risk of accumulating a body burden of rat poison.

Nevertheless, the good news is that our polecats are breeding successfully and continuing to spread.

This article is part of our Saving Species: academic insights section, where we bring you more stories about biodiversity and conservation from our expert academics.

 

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