The geographical distribution of red and arctic foxes differ. The hypothesis that this difference results directly from their relative adaptations to extreme cold is evaluated and dismissed. An alternative hypothesis is developed from considerations of body size and biogeographic effects on productivity. This suggests that the northern limit of the red fox's geographic range is determined directly by resource availability (and thus ultimately by climate), whereas the southern limit of the arctic fox's range is determined, through interspecific competition, by the distribution and abundance of the red fox. Predictions from this hypothesis are fulfilled, particularly by data on fur harvests. The argument is extended to other pairs of similar canid species, with the conclusion that equivalent interactions between body size, secondary productivity and direct competition have general relevance to their geographical distributions.