Polecat body size and sex ratio change over time: Impact of invasive competitor or climate warming?

Change in body size is considered to be one of the animal responses to climate warming, although in many cases it is difficult to show whether it is evolutionary or a result of phenotypic plasticity. In this study, we analysed long-term changes in the body size and sex ratio of the European polecat Mustela putorius in relation to two factors: climate warming and competition with the invasive American mink Neovison vison. In the years 1959–2021, the average body mass of polecats inhabiting north-eastern Poland increased from 869 to 1109 g in males and from 449 to 690 g in females, whereas structural body size (the condylobasal length of the skull) increased from 64.4 to 68.0 mm in males and from 55.9 to 59.6 mm in females. The rates of these increases were different for both measures; in consequence, the index of body condition changed non-linearly over time with a high increase in last 30–40 years. The observed increase in polecat body mass and skull size correlated with the rise of mean winter and summer temperatures. We explain the recorded trends by easier access to amphibians (the staple food of polecats in the study area) in mild winters, which increases the survival rate of larger polecats and/or leads to a faster growth rate of subadults in warmer summers. Competition for food between the native polecat and invasive mink could play a role in the increase of polecat body size simultaneously with climate warming, but the timing of recorded changes suggests the second factor to be more important. Our study also confirmed a previous observation of increasing skewed sex-ratio towards polecat males, which, similarly to changes in polecat body size, could have resulted both from climate warming and competition with mink.