The decade-long research led by staff from Chester Zoo and Natural Resources Wales has observed behaviour never seen in the UK before.
The researchers studied a group of hazel dormice in Denbighshire as part of the project.
It found some dormice breed before they are one year old.
The research also established that some produce two litters in a year, which was known behaviour in continental European mice but had never been observed in the UK before.
The study is the longest and largest of its kind in the UK.
Chester Zoo's biodiversity officer Sarah Bird said: "Promiscuity in females had never been recorded before; and it will increase genetic diversity within litters, helping population survival.
"Equally important are the other breeding behaviours we have seen.
"It is likely that those years when females breed twice, and the young of the year also breed, is when populations can really increase.
"These behaviours are likely to be critical for healthy sustainable dormouse populations in the UK."
Hazel dormice are protected under European law.
The numbers of dormice and the range of the species have both been in decline since recording of the animals and their habitat began in 1990.
However the Denbighshire site at Bontuchel, near Ruthin, showed a large, stable population of dormice.
"Our survey site in north Wales is on the very north western edge of their European range and, from the very start, we've been surprised by how many hazel dormice are there," Ms Bird explained.
"That's why it's a very important project and a very important site for study, to understand why this woodland is so good for dormice."

The plan to restrict the promotion of foods high in fat, salt and sugar to under 16s is being consulted on by the Committee of Advertising Practice.
It would see the existing ban on TV ads extended to all media, including online outlets, the industry regulator said.
Current limits on using celebrities and characters to sell food to children could be relaxed for healthier foods.
About one in five children in the England is classed as obese by the end of primary school, according to the Health and Social Care Information Centre.
The self-regulatory body, CAP, which sets rules for UK advertising, said the new rules could make a "meaningful contribution to tackling this important health issue".
Currently, food and drink deemed unhealthy can be advertised to children in non-broadcast media, unlike television where strict regulation prohibits it through content and scheduling restrictions.
Ofcom research suggests that 96% of 12 to 15-year-olds spent more time online than watching TV last year.
The advertising industry needed to make sure its regulations reflected changing media habits among young people, CAP said.
Evidence showed advertising had a "modest" effect on children's food preferences, it said.
However, factors such as parental influence, opportunities for physical exercise and education playing a greater role in solutions to childhood obesity, it argued.
Chairman James Best said: "Advertising is just one small factor in a very complex equation but we believe we can play a positive part in addressing an urgent societal challenge.
"In proposing new rules, our aim is to strike the right balance between protecting children and enabling businesses to continue advertising their products responsibly."
Alex Neill, of consumer watchdog Which?, said a rule change was "long overdue", and vital to tackling childhood obesity.
But Children's Food Campaign co-ordinator Malcolm Clark said the proposals had "too many gaps in the detail of the consultation and scope for the rules to be weakened".
The regulator had "missed obvious opportunities to make the UK a world leader in putting the protection of children's health above food and advertising industry profits," he said.
The consultation closes on 22 July.
Female dormice which have offspring with more than one father in the same litter can help ensure their survival, a 10-year study has found.
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A complete ban on advertising unhealthy foods to children is being considered as a way to reduce childhood obesity.