The summer pelage differs from the winter in that it is more brightly colored. The skull has a short rostrum and is ovate. The postorbital processes are slender and long. The subgenus Neotamias , to which the red-tailed chipmunk belongs, is differentiated from the subgenus Tamias in that Neotamias has two premolars on each side of the upper jaw and Tamias only has one. The red-tailed chipmunk is sexually dimorphic.
Males also have narrower skulls than females.
They feed at discrete times during daylight hours. These animals become torpid in the winter and feed from time to time upon their larder. Their diet includes the seeds of fir trees, honeysuckles, locusts, and cranberries. In Idaho, they have also been observed feeding upon the seeds of Douglas knotweed. In Montana, the chipmunk eats the fruits and seeds of nine-bark, wild rose, Ponderosa pine, snow brush, serviceberry, big whortleberry, buckbrush, knotweed, grass, huckleberry, mountain maple, and bull thistle.