Allow Dr John Jackson to introduce you to the fat or edible dormouse, Glis glis. If you have not yet met one in your woodlands, you may do in the future.

THIS must be one of the few mammals that is normally politely called by its latin or scientific name by local residents in the Chilterns, where ‘Glis’ and tales about them are almost a part of regional folklore now.

To look at, Glis are not dissimilar to a two-thirds-sized grey squirrel, but they are not close relatives. However, like the North American grey squirrel, this animal is not native to these islands and is classed as an alien invader.

Unlike the grey squirrel, Glis is nocturnal, lives mainly up in the tree canopy and hibernates – sleeping or entering torpor for around seven months from around October to the following May.

Given enough food, it can increase in weight several fold over the summer, with an adult tipping the scales at almost 300 g by the end of a good mast year.


As its common name of the ‘fat’ or ‘edible’ dormouse implies, the gastronomic value of this creature is well established. The ancient Romans kept these rodents in large terracotta pots called dolia or gliraria, fattening them up on nuts in the autumn.

When plump, the animals either entered hibernation of their own accord or the pots could be immersed in cold water to induce winter slumbers – a clever technique for storing convenience food for a tasty snack in the days before deep freezers.

However, it seems it was not the Roman colonisers who successfully imported Glis to these shores.


The Rothschilds were and are a wealthy international banking dynasty.

For his coming-of-age present in 1889, Walter was given his very own natural history museum building on the edge of Tring Park in north-west Hertfordshire.

The museum survives as such to this day, open to the public and functioning as an outlier to the main Natural History Museum in London, housing its ornithological section. And across the road, Tring Park (130 ha) itself is managed by the Woodland Trust as one of its flagship sites.

Over time, Walter Rothschild amassed a remarkable collection of both dead museum specimens and a menagerie of the weird and wonderful – giant tortoises, zebras, kiwis, emus and other species from far and wide which had the run of Tring Park.

The late Victorians had a craze for importing exotics like grey squirrels from North America. For some unknown reason (perhaps not to feel left out), around 1902 Walter introduced a handful of Glis glis – a nocturnal, tree-living rodent found in woodlands across much of central Europe – into Tring Park.

By the 1930s, the descendants were breeding and well established in Hertfordshire and neighbouring counties and also recorded up in Shropshire and Warwickshire – possibly hitching a lift there from the Chilterns on timber lorries, hidden in the holes in tree trunks.

The 2018 Mammal Society’s study indicated the edible dormouse has an approximate population of 23,000 in the UK – double what was estimated in 1995.

This rodent is now well entrenched across the well-wooded Chilterns AONB and further afield – and spreading both naturally and with a little assistance from their friends. They have leap-frogged some formidable physical barriers such as the M4 and a helping human hand is suspected.


Under EU legislation such as the Berne Convention, Glis glis is classed as an endangered animal so enjoys legal protection in its native continental European home. It figures on the international Red List of threatened species too. At the same time, it is an exotic, invasive alien here in Britain.

So that presents a bit of a legal conundrum in the UK, where this creature can be a real chronic pest problem in forests and even more so in homes and other buildings, yet enjoys protection under European treaties.

Officially, it should only be trapped and dispatched here under licence by pest controllers.

Although not entirely within the law, citizens who live-trap or corner Glis in their homes do not always have the heart to dispatch them when faced with a pair of dark flashing eyes – so they indulge in a bit of illicit translocation, ferrying their captives further up the road to release them in some other wood to become somebody else’s problem.

Forestry Journal:


When the ideal conditions of a warm, dry summer and a beech mast year coincide, edible dormice proliferate – a baby boom is on – often followed by a bust year when very few are around.

Sometimes they gnaw the bark off from smaller conifers several metres up the trunk and can cause real economic losses in younger plantations, but broadleaves are not immune.

Debarking in early summer tends to be sporadic, serious and unpredictable and can cause death, infection and subsequent snappage of the pole-stage soft and hardwoods.

It was first documented in the early 1960s in Wendover Woods by the Forestry Commission. Of 14,000 trees surveyed, 14.6 per cent had been gnawed by Glis, representing a 25 per cent loss in revenue. In some stands, 70 per cent of the stems had suffered.

The modus operandi is normally to gnaw off the bark in neat windows or sometimes in spirals in early summer to get at the rising sap, and a trained eye can distinguish that from grey squirrel work.

Glis also stand accused of being eco-vandals, eating hole-nesting birds and their eggs up to woodpecker size and displacing or devouring the much smaller native hazel dormouse and tree-roosting bats.

Forestry Journal:


But it is when these creatures enter houses or properties that conflict is at its height, especially when the animals take up residence for the winter in attics, cavity walls and sheds, rather than going underground to pass the winter as they would in wilder woods in Central Europe.

Once indoors, the patter of tiny feet in the night in cavity walls, the attic or the airing cupboard disturbs the human residents. These creatures also fall into water tanks, raid the larder and have caused house fires when chewing through electricity cables. The nuisance factor is very real.

Forestry Journal:


For the past 20 years, a team of volunteers has studied Glis in the Royal Forestry Society’s 72 ha Hockeridge and Pancake Woods in the Chilterns, astride the Bucks/Herts border. That is probably the longest continuous running research project on small mammals in the British Isles, encouraged to start when I was the RFS CEO.

As well as living in natural cavities in tree trunks, these creatures will use specially designed wooden nest boxes people put up trees to live and breed in. These are based on a design from Italy to trap the animals in areas where pine nuts are gathered commercially.

The 200 plus boxes in the RFS woods are checked monthly over the summer for occupants by a former FC researcher and a band of volunteers. If an endoscope search shows anyone is at home, the box is brought down to earth and the inhabitant(s) tipped into a plastic bag and then handled carefully, examined, recorded and their individual microchip read or one inserted under the skin in the same way a vet does to pets.

Over 2,000 have been checked and marked to date and some in the study have been around for more than 10 years – remarkably long for a small rodent. 

The project is revealing some of the secrets of this little-known woodland dweller and has also developed a cheap and effective tube trap from a modified tree shelter for use outdoors if called for in the future.

Forestry Journal:


Some fear this furry interloper could one day become the next or equivalent to another grey squirrel pest in UK forestry. So far this species has not colonised so extensively or rapidly as greys.

Maybe only time will tell, but this animal appears well established and here to stay.

Points to ponder include:

  • Has this nocturnal arboreal animal been overlooked and is it more widespread than reported? 
  • Could a warming, more Continental climate benefit it, or will that also adversely affect the Chiltern beechwoods that provide the Glis with both flowers to bring them into breeding condition after their winter fast and then mast to fatten them up to overwinter?
  • Are the potential losses they cause in forestry even worth worrying about or is damage merely aesthetic?
  • Will the current legislation change when the UK leaves the EU?
  • Will HS2 and associated line-side tree planting offer a new conduit for Glis to extend off the Chilterns up to Birmingham in record time?
  • They tend to attack younger late-thicket to pole-stage trees – will expanded woodland creation provide a welcome new home?
  • Habitat fragmentation has slowed dispersal so far but will schemes to link up woodlands to create corridors have a boomerang effect and assist this alien’s spread?
  • And the real impact of Glis preying on woodland birds, native dormice and bats is being studied in the RFS Chiltern woods – what will it reveal?
  • Will this alien have untold consequences for woodland ecology and forestry that are not evident yet?
  • Should we try to eradicate this exotic species while that may still be possible as was done with coypu and musk rat?
  • Abroad, this rodent causes losses in fruit trees and nuts grown commercially. What might happen if Glis colonised areas in the UK such as old cider orchards?
  • Will COVID-19 mean this project is suspended so cannot forge ahead in 2020 or beyond?

Only time will tell.

Forestry Journal remains dedicated to bringing you all the latest news and views from across our industry, plus up-to-date information on the impacts of COVID-19.

Please support us by subscribing to our print edition, delivered direct to your door, from as little at £69 for 1 year – or consider a digital subscription from just £1 for 3 months.

To arrange, follow this link:

Thanks – and stay safe.