The coat of dark kangaroo mouse is long, silky, and soft with its back being brownish to greyish black while its belly having a greyish or whitish hue. Its tail is swollen in the middle (fat deposits). The fat deposits vary in size as season changes because it is used as a source of energy during dormancy. The tails are thickest before entering winter hibernation and thin in the spring, when they come out of hibernation and assume normal activity. This is unique among North American small mammals.