Is coat-colour polymorphism in Eurasian red squirrels (Sciurus vulgaris L.) adaptive?

Abstract

Coat-colour polymorphism, the relative frequencies of red, brown and black fur-morphs, was examined in nine populations of the Eurasian red squirrel (Sciurus vulgaris L.) in northern Italy. The proportions of the three different coat-colours differed between populations. Red morphs were most common in mixed woodlands of the Upper Po-plain, but rare in populations from the Alps. The highest frequencies of black morphs were found in subalpine conifer forests in the western Alps (Gran Paradiso) and in montane mixed conifer forests in the central Alps (Valtellina). Coat-colour did not affect juvenile survival, and there were no significant differences in mean foot length or body mass among subadults and adults of different colour-morphs. However, inter-population variation in the frequency of black morphs was positively correlated with the proportion of spruce (and/or fir) in the forest and with the density of trees. We suggest that the combination of a denser and more cryptic fur in black morphs gives them a selective advantage over other coat colour morphs in wet, dense spruce-fir forests of the Italian Alps, which could explain the within- and between population variation in the relative frequencies of coat colour phenotypes observed in this study.