Ecological Character Displacement in Saharo-Arabian Vulpes: Outfoxing Bergmann's Rule

The red fox, 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. 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 two foxes throughout the Saharo-Arabian region, notwithstanding regional fluctuations in overall size. Moreover, in Israel, where the still smaller Blanford's fox exists, the ratios between the means for both lower carnassial length and condylo-basal length of the three sympatric species are remarkably constant. Condylo-basal length and canine diameter ratios between the red fox and Ruppell's sand fox in the different localities studied, while displaying the same general pattern, are considerably less regular than ratios of lower carnassial lengths. We suggest that the length of the carnassials relates directly to the feeding habits of these species, and is therefore the suitable morphological character to use when studying ecological character displacement in this group. The length and shape of the skull is influenced by other functions, as well as by autecological factors. Correlating the size of the red fox with various latitude correlated variables (e.g., ambient temperature) should be accompanied by investigation of its size relationship with congeners.