"Eastern Fox Squirrels have long, foxtail-like tails, which they flick when they are excited. They and Eastern Gray Squirrels are alike in many ways. They breed at the same time of year, nest in the same kind of places, and eat the same foods. However, Fox Squirrels prefer more open habitat, whereas Grays prefer good tree cover. Fox Squirrels spend more time foraging and running about on the ground than do the grays, and may be encountered in fields quite far from any trees, where gray squirrel would not stray. Both species feed on acorns, which are rich in tannins. Tannins are poisonous to many animals, including worms, and keep the squirrels free of roundworms and tapeworms. Fox Squirrels accumulate another chemical compound, porphyrin, in their bones and teeth, which makes their bones and teeth pink and bright red under ultraviolet light. Here's a mystery: Gray Squirrels eat the same foods and this does not happen to them—nor to any other healthy mammal."

Links:
Mammal Species of the World
Click here for The American Society of Mammalogists species account