Ecological character displacement in Saharo-Arabian Vulpes: outfoxing Bergmann's rule

Red fox Vulpes vulpes, a widely distributed Holarctic carnivore, exhibits a temperature- and latitude-correlated size gradient in the Palaearctic region, but little change of size in the southern part of its range, in the Saharo-Arabian region, where it is sympatric with a smaller congener, Ruppell's sand fox V. ruppelli. The slope of the regression of red fox lower carnassial length against mean ambient temperature in the Saharo-Arabian region is very slight, and differs significantly from the slope in the allopatric zone. This deviation suggests ecological character displacement in the red fox in sympatry with its smaller congener. This hypothesis is enhanced by the existence of constant size ratios (1.18-1.21) between lower carnassial lengths of these 2 foxes throughout the Saharo-Arabian region, notwithstanding regional fluctuations in overall size. In Israel, where the still smaller Blanford's fox V. cana exists, the ratios between the means for both lower carnassial length and condylo-basal length of the 3 sympatric species are remarkably constant.