A decrease in the number of days with snow cover may lower survival rates for weasels with white winter coats, according to a study in Scientific Reports. Karol Zub and colleagues suggest that white-coated weasels are less camouflaged during shorter winters, which makes them more vulnerable to attacks by predators.

The weasel species Mustela nivalis has two subspecies with differently coloured winter coats, white and brown, which commonly coexist in the same habitat. Camouflaged weasels, with a coat colour matching their surroundings, are thought to experience reduced predation and increased survival chances.

Between 1997 and 2007 the authors captured and recorded 95 weasels with white winter coats and 23 with brown winter coats in Bialowieza Forest, Poland. They also studied weather data collected in the region over that time period and found that the mean number of days with permanent snow cover decreased from around 80 days to 40 days. This was mainly caused by the overall shortening of winter, with snow coverage disappearing earlier in more recent years. The researchers found that the proportion of weasels with white winter coats decreased correspondingly with the number of days with snow cover.

The authors also used models of white and brown weasels to test if coat colour affected detection by predators such as foxes, wolves and birds of prey. They found weasel models that were not well camouflaged were detected more often than those matching the background colour. The findings suggest that the decrease in white-coated weasels is due to an increase in detection by predators in a less snowy environment.