A female from the first wild beaver colony in England for centuries has given birth to at least two young.
New footage shows the kits being helped through the water by their mother.
The images taken in Devon by local filmmaker Tom Buckley provide the first evidence of the new arrivals.
The Devon Wildlife Trust (DWT) said the slowly expanding population would help to provide an insight into their effect on the surrounding River Otter system in east Devon.
The Angling Trust warned that a population increase could have detrimental effects on other wildlife.
Mark Elliott, from the DWT, said: "We are thrilled that the beavers have bred. The baby kits appear fit and healthy … This tells us that the beavers are very much at home in this corner of Devon."
The two females were found to be pregnant when they were taken in to captivity to be tested for disease. It's not thought that the other female has yet had her kits.
Hunted to extinction
The colony of wild beavers was first spotted living on the River Otter in February 2014.
In January 2015, Natural England granted a licence to the Devon Wildlife Trust that allowed the beavers to remain on the river, as part of a pilot experiment.
This is the first time that an extinct mammal has been re-introduced to England. Other species, such as wild boar, have naturalised after escaping from enclosures.
Beavers were hunted to extinction in England and Wales for their valuable fur and glandular oil during the 12th Century and disappeared from the rest of the UK 400 years later.
Steve Hussey, from the DWT, said "We have no plans to tag them at the moment. If we do, it will just be for identification purposes. The less we disturb them the better.
"They are healthy beavers, so they will breed. But they are not like rodents, they only have one litter a year, and they take two to three years to reach sexual maturity. The river system in east Devon is nowhere near filled to capacity. "
Beavers are known as a "keystone species" because of the dramatic impact they have on their environment. They fell large and small trees, and create sophisticated dams that hold vast amounts of water. They use these lakes as "superhighways" to get to foraging areas.
Tom Buckley commented: "When I saw these new born baby beavers I was totally overwhelmed and I thought it was a miracle.
"One of the kits, however, seemed extremely unhappy to be out in the big wide world and as soon as its mother let it go, it rushed back to its burrow. Not surprising really - the world can be a very scary place."
Mark Owen, from the Angling Trust, said the fact the young beavers would not be tagged or tracked meant the trial lacked any "scientific credibility".
"There is an increasing prospect of a population explosion that could do considerable harm to other wildlife through the uncontrolled damming up of watercourses which can, among other things, prevent fish from reaching their spawning grounds," he said.
"This irresponsible programme should never have begun and it won't be long before the substantial sums spent in other European countries in dealing with problems caused by beavers will be required here in the UK."
But Friends of the Earth campaigner Alasdair Cameron said: "[Beavers] bring huge benefits to the countryside - boosting biodiversity and keeping the rivers clean - we're delighted that they are back and doing well."
The DWT is urging people not to search for the kits as they would be disturbed by noise and dogs.
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