I started catching a few while keep wrong up in Perthshire it intrigued me so I done some snooping the growth of winter fur is indeed caused by the decrease of light in the day but the ermine process is triggered by cold conditions for the coat to turn the temp needs to be below a certain temp consistently I can't remember the temp sorry. There was also reasearch that an ermine was also hereditary as some mustelids would not produce white fur while others of the same species would in the same conditions. I think where both wrong and right that if you stuck one in a shoe box it would not turn white and if you stuck one in the fridge it would also probably not turn white I think it's a combination of all of the above with a bit of good old fashion adapt to the enviroment or starve