10 Anatomymerus, as it does in humans. In these burrowing squirrels
there is an extra part of the deltoid (the clavobrachialis) that
extends onto the forearm and assists the other forearm fl ex-
ors (the biceps and the brachialis). It would be interesting to
know exactly when it is used—during walking and running,
or more specifi cally during digging activities. In other ways,
these ground squirrels seem just like short-tailed tree squir-
rels, with short, stout legs.
Squirrels have very good eyesight and exceptional focus-
ing ability. They have sharp vision across the entire retina,
like the vision humans have in their fovea centralis, the small
area of the retina where the cones are most densely packed
and thus where a person’s eyesight is most acute. This al-
lows a motionless squirrel, without moving its head, to see
clearly what is next to it and above it at the same time. Many
squirrels have yellow-tinted eye lenses: ground squirrels
have dark yellow lenses, and tree squirrels have paler yel-
low ones. These yellow lenses, much like sunglasses, reduce
glare from bright light and increase the contrast between
colors, giving the squirrel sharper vision. Flying squirrels,
however, have clear lenses. Since they are nocturnal and
seldom encounter bright light, they have no need for a
tinted lens.
As in most mammals, a squirrel’s retina contains both
rods and cones. Rods are specially adapted cells that allow
vision in low light, and cones are specifi cally designed cells
for daylight vision, color vision, and the discrimination of
detail. The retinas of diurnal squirrels contain rods, because
the animals need to be able to see at dusk, dawn, and in
shaded areas. Ground squirrels, such as prairie dogs, are su-
perbly adapted to bright light and have many more cones
than rods. In fact, it was once thought that they had no rods
at all. The retinas of nocturnal fl ying squirrels, on the other
hand, have mostly rods and only a few cones, which give
them excellent night vision.