8 AnatomyThe body proportions of squirrels vary in several ways,
most notably in limb lengths and tail length relative to body
size. The shortest legs and tails are found in ground squir-
rels. Tree squirrels have longer legs and tails, and the longest
of both are found in large fl ying squirrels. The greatest elon-
gation of the limbs is in the forearm of fl ying squirrels,
which supports the wing tip. Presumably, shorter limbs and
tails are more functional in burrowing animals, and the tail
may even be used for sensing the way when the animal is
moving backward. Longer limbs allow tree squirrels to have
a more extensive reach around a tree and a greater jumping
ability. The longer tail is obviously used for balance when
jumping, among other functions. The limb elongation of fl y-
ing squirrels permits these animals to have a larger “wing.”
The total wing is almost square (or even wider than long),
with an aspect ratio (width/length) commonly between 1.
and 2.0, which gives it good stability and enables the squirrel
to land at low speeds. Flying squirrels clearly use their lon-
ger tails for balance and for stability while gliding. Among
arboreal squirrels there is a great deal of fl exion at the base
of the tail, which allows the tail to the moved as a unit for
balance. The short vertebrae at the base of the tail result in
more intervertebral joints, which is where the fl exion oc-
curs. The fl exibility at the base of the tail enables squirrels
to hold their tails in their very distinctive way.
On many squirrels the hairs of the tail are arranged in a
distichous fashion, with the lateral hairs being longer than
the hairs on the top and the bottom. This featherlike ar-
rangement is presumably excellent for steering and balanc-
ing in small tree squirrels and fl ying squirrels. Large fl ying
squirrels (> 1000 g) and giant tree squirrels have long tails
with hairs that are the same length on the top, the sides,
and the bottom, which suggests that distichous tails are not
eff ective on larger animals. In these animals, their tails ap-
pear to be used less for steering and more for stability, like
a kite tail.
The ankle morphology of squirrels is also very distinc-
tive, and it enables tree squirrels to rotate their hind feet 180
degrees, so that a squirrel coming down a tree trunk head-
fi rst can have its hind feet pointed back up the tree. Most of
this rotation occurs between the two major bones, the as-
tragalus and the calcaneus, with their broad joint facets.
Flying squirrels evolved from tree squirrels, and they
have developed a number of anatomical specializations
associated with their gliding fl ight. The most obvious is
the extension of the skin to form a “wing” (the patagium),
stretching from the forelimb to the hind limb. The normal
skin musculature of tree squirrels is incorporated into this
membrane and enables the fl ying squirrel to gather the pa-
tagium against its body when it is not gliding. Another skin
muscle extends from the wrist to the ankle and stabilizes
the outer edge of the patagium when the animal is gliding,
like the leech line on the sail of a sailboat. On large fl ying
squirrels there is usually an extension of the wing (the
uropatagium) between the hind limb and the tail. One of the
hamstring muscles of the thigh supports this, and it also
serves as a leech line, becoming disassociated from the
other thigh muscles when the uropatagium is large.
Flying squirrels also have a specialized wrist. One of the
sesamoid bones of the wrist in tree squirrels becomes elon-
gated, forming the styliform process at the heel of the hand
in fl ying squirrels. It is hinged so that it can be held against
the forearm when the squirrel is not gliding, but it can be
extended at 90 degrees from the wrist, supporting the wing
tip of the patagium when the animal is gliding. When glid-
ing, the squirrel holds its wrist in such a way that the wing
tip is elevated above the rest of the patagium, which pre-
sumably serves one or two functions: reducing the drag of
the wing and/or increasing stability. The musculature that
enables the styliform process to be extended is remarkable.
The styliform process is on the little-fi nger side of the wrist,A Congo rope squirrel (Funisciurus congicus), showing ankle
rotation. Photo courtesy Jon Hall, http://www.mammalwatching
.com.