The stoat and the weasel are not rare, but they tend to keep out of the way. In some areas of Britain in winter, stoats become even more difficult to track down, especially if there is snow on the ground. This is because, in Scotland and parts of northern England, they turn white. This transformation, into what is called ermine, is shared with only one other British mammal, the mountain hare. For the stoat, turning white enables it to stalk its prey without being seen. The hare turns white to avoid being spotted by predators, including, of course, the stoat.
The shorter days of autumn cause Weasels and Stoats (right below) to lose their summer coat and grow a white winter coat. In northern populations the winter fur is white , whilst further south the color change may be incomplete or not at all. Also, more females go white than males. The tail remains black throughout the year.
Ermine was once highly sought after as fur, mainly for ceremonial costumes for aristocrats and royalty. The distinctive pattern of white with black spots was made by sewing together several skins, the black being the tip of the stoat's tail, the only part of the animal's fur not to turn white in winter.
The stoat's cousin, the weasel, is the world's smallest carnivore, some being less than 18cm (7ins) long. In Britain, weasels never turn white,but farther north, they do, and unlike the stoat, even the tip of the weasel's tail changes color.
They are found in a wide range of habitats, which include urban areas, lowland pasture and woodland, marshes and moors. Weasels hunt small rodents and their numbers depend on the abundance of their prey. Weasels have to eat one third of their body weight every 24 hours.
The weasel’s small size enables it to search through tunnels of mice and voles, which means it can hunt at any time of the day or year. They do not hibernate and can hunt even under deep snow. Only one litter, of 4-6 young, is born per season. Only one in 80-90 weasels survives to over two years old. They are small enough to be prey for predators like hawks, owls, foxes, cats and mink.
As climate change alters the weather patterns in the northern parts of their range, there is a possibility that this seasonal transformation will become less common. If snow becomes scarce, the advantage of being white will be lost, and these animals will remain their usual color throughout the year.
The Guardian, "Weatherwatch: Winter transformations" , by Stephen Moss, accessed December 9, 2010