The relationship between foot size of wild canids and regional snow conditions: evidence for selection against a high footload?

Abstract

Mammals occupying northern areas may experience reduced fitness through the deleterious effects of snow on locomotion, energy expenditure, and food acquisition. Accordingly, in areas of deep and soft snow, selection may favour mammals possessing disproportionally large feet and lower footload (body mass/foot surface area). We collected carcasses of coyote Canis latrans (n = 472), red fox Vulpes vulpes (n = 199), arctic fox Alopex lagopus (n = 52), grey fox Urocyon cinereoargenteus (n = 17), and wolf Canis lupus (n = 14) and compared body mass, total foot area, difference in surface area between front and hind feet, and footload, among species and populations. All measured morphological attributes differed significantly among canid species; grey fox had the smallest feet, whereas arctic fox had both the lightest mass and lowest footload of all canids. For all species, adult males tended to be heavier and have larger feet than did adult females, and for most species adults were heavier and larger than were juveniles. Foot area in red foxes increased significantly with latitude, with populations found north of 48 °N (i.e. approximate latitude where snow may become limiting) averaging 12% larger feet than those found farther south. For coyotes, body mass increased with latitude, with populations found north of 48 °N averaging 26% heavier mass than those occurring farther south. Coyote foot area also increased with latitude, with populations found north of 48 °N averaging 25% larger feet than those found farther south. When indices of snow severity (mean annual snowfall, mean number of days with snowcover) were considered, foot area for red foxes was correlated with annual snowfall as well as latitude, whereas coyote morphology failed to correlate with any snow-related variables. These results suggest that snow may have contributed to selection for foot size in some wild canids (i.e. arctic fox, red fox), but such selective forces were probably relatively weak and inconsistent across species.