The largest squirrels living today are the marmots (Marmota)
of North America and Asia, including the well-known
yellow-bellied marmot of the Rocky Mountains and the
woodchuck of the eastern USA and Canada (the “ground-
hog” of February 2). One of the largest is the gray marmot,
found in the mountains of Kazakhstan. Marmots put on
weight before they enter hibernation, and may even double
their weight, so the animals are heaviest at the end of the
summer. At this time, the largest gray marmots may weigh
more than 8 kg (18 lb). The largest tree squirrels, the giant
tree squirrels of Southeast Asia (Ratufa), are not nearly as big
as marmots, but they are still quite large—ranging from 2 kg
(4.4 lb) up to 3 kg (6.6 lb). With their beautiful long tails and
striking coloration, these squirrels are an impressive sight
bounding through the trees.
In contrast, the smallest squirrels are the pygmy tree
squirrels of western Africa (Myosciurus) and Southeast Asia
(Exilisciurus), which are smaller than some mice. The small-
est adults of both genera average approximately 14 or 15 g
(roughly 0.5 oz).
Rodents have jaws that function in two diff erent ways,
for gnawing and for chewing. In gnawing, the jaw is moved
forward so that the incisors meet. In chewing, the jaw is
moved backward so that the upper and lower molars come
in contact with each other, and the upper and lower incisors
do not. The muscles that pull the jaw forward are arranged
diff erently in various groups of rodents. However, all squir-
rels have one of the jaw muscles (the anterior deep masseter)
originating on the side of the face in front of the eye and
then passing beneath the eye to insert onto the lower jaw.
This muscle appears both to move the jaw forward and to
strengthen the gnawing capabilities of squirrels, for which
they are well known.
The incisor teeth are similar in all squirrels, although in
some they are stouter and in others they are more gracile.
There are also diff erences in how far they protrude (per-
forming an almost forcepslike action) or in how vertically
they are oriented (for effi cient gnawing). All incisors con-
tinue to grow throughout the life of the squirrel, unlike the
remaining teeth. There is much greater variance in the
cheek teeth, depending on the diet of the species. Those
squirrels that feed on animal matter and soft vegetable mat-
ter have the simplest teeth, with few cones and few ridges
on the premolar and molar surfaces. Those that feed on
leaves and other items that need a substantial amount of
chewing have more complex ridges and cones on their tooth
surfaces.Anatomy: Form and Function
X-ray of a Rheithrosciurus skull, showing the incisors and
molars.