Cranial features of mainland and Commander Islands (Russia) Arctic foxes (Vulpes lagopus) reflect their diverging foraging strategies

Abstract

Populations of Arctic fox (Vulpes lagopus) in the Commander Islands, in the Russian Bering Sea, have been isolated since the Pleistocene and differ substantially in their cranial features from their mainland counterpart. Small rodents, the main prey of mainland Arctic foxes, are not found in the Commander Islands, where the main food source for Arctic foxes are large sea birds and marine mammals. Here we assessed whether differences in foraging strategy, particularly the size of available prey, could explain the observed differences in cranial features between mainland and island Arctic foxes. Because a large gape is necessary when foraging on large prey, we compared gape angles between islands and mainland in a sample of dry crania. We found an enlarged gape angle in both island populations. We also compared the rostrum to cranium length ratio and found it to be similar for the mainland and Bering Island Arctic foxes; however, a rostrum contraction was found in the Mednyi Island Arctic foxes. We show that cranial differences between mainland and Commander Islands fox populations could be explained by their different foraging ecology. Furthermore, the relative rostrum contraction in the Mednyi Island foxes provides further evidence for cranial resistance to deformation during biting. These results show the importance that distinct foraging strategies can have in Arctic fox divergent evolution, and, consequently, on future conservation plans for the two Commander Islands subspecies.