This, in turn, derives from Proto-Indo-European *puḱ- 'thick-haired; tail'. The latter clade has been separated from all other red fox populations since the last glacial maximum, and may possess unique ecological or physiological adaptations. In addition, no evidence is seen of interbreeding of eastern red foxes in California with the montane Sierra Nevada red fox V. necator or other populations in the Intermountain West (between the Rocky Mountains to the east and the Cascade and Sierra Nevada ranges to the west. A large subspecies, the colour along its spine is light, dull yellowish-reddish with a strongly developed white ripple and greyish longitudinal stripes on the anterior side of the limbs. However, relative to dimensions, red foxes are much lighter than similarly sized dogs of the genus Canis.
Compare to the Hindi pū̃ch 'tail', Tocharian B päkā 'tail; chowrie', and Lithuanian paustìs 'fur'. Also, introduced eastern red foxes have colonized southern California, the San Joaquin Valley, and San Francisco Bay Area, but appear to have mixed with the Sacramento Valley red fox V. Substantial gene pool mixing between different subspecies is known; British red foxes have crossbred extensively with foxes imported from Germany, France, Belgium, Sardinia, and possibly Siberia and Scandinavia. Their limb bones, for example, weigh 30 percent less per unit area of bone than expected for similarly sized dogs.
Apart from its large size, the red fox is distinguished from other fox species by its ability to adapt quickly to new environments.
The bushy tail also forms the basis for the fox's Welsh name, llwynog, literally 'bushy', from llwyn 'bush'. A large subspecies measuring 70–90 cm in length and weighing 5–10 kg, the maximum length of the skull for males is 163.2 mm. They display significant individual, sexual, age and geographical variation in size.
Likewise, Portuguese: from rabo 'tail', Lithuanian uodẽgis from uodegà 'tail', and Ojibwa waagosh from waa, which refers to the up and down "bounce" or flickering of an animal or its tail. The fur is bright red with a strongly developed whitish and yellow ripple on the lower back. On average, adults measure 35–50 cm (14–20 in) high at the shoulder and 45–90 cm (18–35 in) in body length with tails measuring 30–55.5 cm (11.8–21.9 in).
Gene mapping demonstrates that red foxes in North America have been isolated from their Old World counterparts for over 400,000 years, thus raising the possibility that speciation has occurred, and that the previous binomial name of Vulpes fulva may be valid. They have a stride of 25–35 cm (9.8–13.8 in) when walking at a normal pace.
In the far north, red fox fossils have been found in Sangamonian deposits in the Fairbanks District and Medicine Hat. North American red foxes are generally lightly built, with comparatively long bodies for their mass and have a high degree of sexual dimorphism.
Domestication of the Red fox is also underway in Russia, and has resulted in the Domesticated red fox. caucasica, its fur is pale yellow or light grey, sometimes brownish-reddish and is fluffier and denser than that of other Caucasian subspecies. The hairs on the sole of the feet are copiously mixed with softer, woolly hairs. vulpes, it has smaller teeth and more widely spaced premolars.