I HAVE a rare treasure in my garden - a white stoat. For most of the year and throughout most of the British Isles, stoats are a beautiful chestnut colour with a creamy white belly. But in some winters a few stoats moult their chestnut coat and replace it with white fur. Only the black tips of the tail remain. This transformation is thought to take place so that the stoat can camouflage itself against winter white landscapes.

But only those stoats with an inherited gene actually turn white. In these animals the colour change is understood to be triggered by a “switch” in the stoat’s brain which reacts to cold temperatures and reduced daylight. This switch controls the amount of melanin the stoat produces. As well as providing excellent camouflage in snow, white fur is also supposed to conserve heat. But a stoat’s snow-white coat, known as ermine, is something of a mixed blessing since in years where there is no snowfall these animals stand out like a sore thumb.

In fact throughout history their beautiful winter white coats have courted danger since for centuries they were used to trim ceremonial coats. When George VI was crowned in 1937 about 50,000 stoat pelts were imported for his robe and crown. The Queen wore this same ermine-trimmed robe and crown at her own coronation, the black tips of the stoat tails punctuating the edge at regular intervals.

Here in the UK white stoats are usually found in Scotland or mountainous regions. Even in these colder climates not all stoats turn white. In Yorkshire white stoats are rare. But two winters ago wildlife surveillance cameras in my garden captured a stoat with a half-white tail. This was Bandita, a female stoat whose story became famous after she featured on a BBC Natural World documentary in October. Her transition from chestnut-brown took between five to six weeks. It was fascinating to watch the process. First a band of white appeared above her tail, just above its black tip. Then flecks of white appeared at her muzzle and then her paws turned. The five digits, already pale in colour, became even paler and the whiteness spread until each paw was completely white. Then then the colour spread up her legs to her belly.

This was followed by white fur on the tips of her ears and then the white band at her belly wrapped itself around her back. Her back was the last part of her coat to moult and as it did she went through a patchy “pie bald” stage. Eventually her entire body was a pure white except for a band of chestnut fur around her eyes which made her look as though it was wearing a mask and was the origin of her name, Bandita.

I have read that stoats can turn white very fast and that when it happens it is known as a “catastrophic moult”. But for Bandita the process was more gradual. It was interesting to see that the following year it followed the same pattern as it had the year before, with the first flecks of white appearing just above the black tip of her tail.

This year we have had barely any snow and Bandita’s transition to ermine has been much slower, taking 41 days to gradually go from chestnut to a mottled-brown until all that is now left is a ripple of brown dots stretching down her back like a cape.

Interestingly Bandita’s brother did not turn as white as she did in the first year and Bandita’s kits, those that have remained in the garden in winter time have only ever developed a fringe of white around their muzzles, despite being very pale in colouring when they were tiny.

This week a pure white stoat was spotted on the valley opposite my home and I wondered if this could be one of Bandita’s latest litter. I have heard the older residents of Thixendale talk about how white stoats were once much more common in this remote Yorkshire Wolds valley and I whether climate change has had an effect on the need for stoats to make the change. Either way, Bandita is a very rare and precious thing and I intend to treasure her time in my garden for as long as I can.