Where are the Stoats in Ermine

By all accounts it has been a cold winter, with temperatures across the county dipping to their lowest level for many years. Under such freezing conditions you might expect there to be a few records of Stoats in ermine, the white winter fur that has long been valued as regalia for nobility and the judiciary. However, here in Norfolk the record of a Stoat in ermine continues to be a very rare occurrence. As we shall see, there is a good reason for this.

The white winter coat of a Stoat is a feature of northerly populations, the brown fur replaced with white during the autumn moult and only the black tip to the tail retaining its colour. In temperate latitudes, such as our own with its variable climate, the moult into ermine is most frequently seen in Scotland, Wales and Northern England. For example, in the extreme northeast of Scotland, some 90% of adult Stoats turn white in winter. Further south, there are occasional reports of individuals in ermine, together with others which may appear mottled.

Temperature at the time of moult has an influence on whether or not an individual Stoat will go into ermine. Research has demonstrated that there is some form of chemical switch in the Stoat’s brain, triggered by temperature, which determines whether or not the new coat is white or brown. The switch determines whether or not melanin is produced in the growing fur and, in its absence, the fur is devoid of colour (i.e. it is white). Since fur on different parts of the body is replaced at different stages of the autumn moult, and because the switch changes position (on or off) with changes in temperature, it is possible for some tracts of the coat to emerge brown and others white. It is this that gives rise to the mottled individuals that are sometimes seen. Here, in Norfolk, the temperatures at the time of moult are normally sufficiently warm enough to just produce brown coloured winter coats, and so Stoats in ermine remain rare.

However, it is not quite as simple as this, because there is also a genetic component to the transition into ermine. Clever experiments, one translocating a Stoat from the Alps to England, and another keeping a Stoat from a northerly latitude in a warm room, illustrated that temperature does not act alone. More recent studies have confirmed this and have demonstrated that there is a hereditary basis to coat colour that is also sex-linked. This results in females being more frequently found in ermine than males. Having a white coat is advantageous in cold, snow-covered latitudes, providing both protection from predators and better insulation. Further south, a brown coat is advantageous.