The swift fox (Vulpes velox) and the kit fox (Vulpes macrotis) are considerably the most similar species of canids, to such an extend that some scientists debate if they are subspecies of each other instead of different species. Currently, they are classified as separate species and exhibit morphological and genetic differences.
The morphological differences can be explained from the different geographical ranges the two species occupy. The swift foxes are separated from kit foxes by the Rocky mountains and the kit foxes occupy a range with a slightly warmer climate. To adapt to this climate, the species developed larger ears and weigh less for the most optimal thermoregulation.
These morphological differences are also visible when comparing the skulls of the two species. Due to the larger size of the ears, the opening of the external auditory meatus of the kit fox skull (right in the picture) are relatively bigger in comparison to the auditory bullae than the swift fox skull (left in the picture). The smaller body size of the kit fox, results in a slightly smaller skull with a narrower snout. The muzzle of the swift fox is visibly broader.
Hope this helps some people out identifying these two very similar species!
Figure: A lateral (above) and dorsal (below) comparison of the skull of a swift fox (shown left) and the skull of a kit fox (shown right). The skulls are from my personal collection.
Sources:
1. MacDonald, D.W. and Sillero-Zubiri, C. (2004). Biology and Conservation of Wild Canids. Oxford University Press, New York, 450 p.
2. Castelló, J.R. (2018). Canids of the World, Wolves, Wild Dogs, Foxes, Jackals, Coyotes, and Their Relatives. Princeton University Press, New Jersey, 331 p.
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