Animals that live in areas that usually have a lot of snow in the winter often change their coats to match their surroundings. Some weasels have evolved so that in the autumn they moult their brown summer coat and change to a white version. In spring they reverse the process and return to the brown version.
When there is deep snow, weasels of both colours tend to hunt underneath it, which, after all, is where the small rodents are, so the predation rates for both species are reduced. In periods of winter without snow, however, white weasels are much more conspicuous than brown ones. In recent years, because of climate change, there have been more days without snow, and the date when snow disappears in the spring has become progressively earlier.
The question for the future seems to be how long M. n. nivalis as a subspecies can survive, given the milder, largely snow-free winters that are becoming more frequent as climate change takes hold. One of the important factors in this is likely to be its ability to change the time when it moults its coat, both in the autumn and the spring. Though the weasels are already showing signs of moulting into their brown summer coats earlier in the spring, they have not yet changed the time of their autumn moult. This is thought to be because autumn temperatures have remained relatively stable while springs have been getting warmer.
Another problem for the weasel is that, even when it can shed its winter coat earlier, that will not be helpful in the increasing periods during the winter when there is no snow. Since moulting and growing a new coat is a serious business, chopping and changing during the season is not possible. In addition to ambient temperature, moulting and therefore coat colour change is triggered by day length, the change in which affects the weasel’s hormonal state. So starting a new moult is only possible during spring and autumn when the days reach the appropriate length as well as the right temperature.
The potential loss of this beautiful and unique subspecies of the least weasel because of anthropogenic climate change will be one more crime to be laid at the door of humanity.