Anatomy 9but the muscle (the abductor pollicis) that extends it is con-
nected to the thumb, which is minuscule in fl ying squirrels.
There is a ligament that reaches across the hand from the
thumb to the base of the styliform process in such a way
that when the squirrel contracts the abductor pollicis and dor-
sifl exes the wrist, the styliform process and the wing tip are
extended and elevated.
The forearm of fl ying squirrels is notable in one other
way. It appears that there is decreased mobility between
three carpal bones of the wrist (the triquetrum, the scaph-
olunate, and the pisiform), probably to provide greater sta-
bility for the wing tip. In addition, a number of fl ying
squirrels have the two bones of the forearm (the radius and
the ulna) tightly bound together with ligaments, presum-
ably for the same reason. In most mammals this would
preclude rotation of the hand (pronation and supination),
but fl ying squirrels are able to manipulate their food easily,
pronating and supinating their hands with ease. The rota-
tion does not occur at the wrist, however, but rather at the
elbow, with the ulna rocking on its articulation with the
humerus, which is an extraordinary way to maintain man-
ual dexterity.
Modern ground squirrels probably evolved from tree
squirrels, and the anatomy of a tree squirrel seems to re-
quire little change for the animal to adapt to terrestrial
habitats. Cheek pouches are one distinctive feature of the
tribe Marmotini—including marmots, chipmunks, antelope
ground squirrels, and other ground squirrels of this group.
These enable the squirrels to carry more food in their
mouths than they would be able to otherwise. Various mus-
cles are associated with the cheek pouches, such as the one
that helps them empty the pouch. This originates behind
the upper incisors, where there is a fairly distinct muscle
scar, which has enabled paleontologists to determine when
cheek pouches evolved in the fossil record of ground
squirrels.
Another interesting change that occurred in only some
of the Marmotini—marmots, ground squirrels, and prairie
dogs—is the development of the deltoid muscle, which nor-
mally extends from the clavicle to the upper half of the hu-a b(a) A Japanese giant flying squirrel (Petaurista leucogenys) in flight, showing both the patagium and the uropatagium. Photo
courtesy Takeo Kawamichi. (b) Red giant flying squirrel (Petaurista petaurista) flight sequence. Photo courtesy David Bakewell.
An eastern chipmunk (Tamias striatus) with full cheek
pouches. Photo courtesy Jen Goellnitz.