The Common or Hazel Dormouse spends at least half its life asleep
The public has been urged to scour woods for half-eaten hazelnuts to help trace one of the UK's rarest mammals.
The People's Trust for Endangered Species (PTES) has launched its third Great Nut Hunt to establish where the endangered dormouse can be found.
Nutshells with a neat round hole chewed on one side is the best indication of the nocturnal animals' presence.
Numbers of the once widespread species have fallen by 39% since 1992, making it vulnerable to extinction, PTES said.
The trust's chief executive Jill Nelson said: "Nationally they have disappeared from more than half of their historic range due to the loss and fragmentation of their habitat, particularly hedgerows, as well as their sensitivity to climate change.
"In addition to being so rare, the small dormouse is an elusive, nocturnal, arboreal, hibernating mammal, making it a difficult creature to spot."
To encourage people to join them in searching for dormouse habitats, the conservation charity commissioned 20 silver and one gold-plated nuts and hidden them in suitable sites across England and Wales.
Dormouse expert Dr Pat Morris will oversee the identification of tens of thousands of nuts the trust hopes the public will send.
THE ENDANGERED DORMOUSE
Numbers have fallen by 39% since 1992 - making the species vulnerable to extinction
There are 800 known dormouse sites in England and Wales
Of those, 500 were discovered in the previous two PTES nut hunts
Thousands of volunteers took part in the 1993 and 2001 hunts, sending in hazel nuts from more than 2,000 sites
The species has been reintroduced into 16 sites in 12 English counties over the last 16 years
He said: "Dormice open these nuts by making a neat round hole on one side, leaving characteristic tooth marks around the edge of the hole and providing a reliable method for identification.
"We can make use of these nibbled nuts to gather data about the presence or absence of dormice and improve knowledge about the distribution of the species as well as the general health of our woodlands and hedgerows."
Poul Christensen, acting chairman of the government's conservation body Natural England, said the survey would help ensure the long-term survival of one of the UK's best-loved mammals.
He said: "The Great Nut Hunt is the ideal way for people of all ages to have fun discovering more about dormice and how they live, but it can also really make a difference to dormouse conservation."
The hunt marks the 21st anniversary of the national dormouse monitoring programme for the dormouse, which was immortalised by Lewis Carroll in Alice's Adventures in Wonderland.
BBC environment correspondent Sarah Mukherjee said the mammal, which is just a few inches long, spends at least half its life asleep, emerging in late April to feed and breed for a few months.
Members of the public who want to take part in the survey should:
• Find some local woods or large overgrown hedgerows and ask permission from the owner
• Look for hazel trees or shrubs and search underneath for nuts
• Collect chewed nuts, recording the amount of time you spent searching and the number of people who searched
• Sort the nuts using the identification guide, fill in the survey form and send the nuts to PTES