268 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1949 
America possess long, prehensile tails which they use in many 
ways, especially to hang by or to use as a supplementary hand 
in steadying themselves. In the picture (pl. 11) it will be seen that 
one of the animals is using its tail to hold itself while it leans far 
forward. On one occasion I saw seven spider monkeys in this group 
leaning forward, held by their tails, watching for the door to open so 
they could go into the house for their afternoon meal. 
Some rodents have large, powerful incisor teeth that protrude 
much more than those of the majority of rodents, and one naturally 
surmises that this trait is associated with some habit peculiar to them, 
which indeed it is. The animals possessing such teeth use them in 
burrowing; that is, they cut away the earth, remove small stones, 
and cut roots, thus performing most of the functions that the powerful 
hands of the mole perform and, in addition, do the cutting work 
which the mole’s hands cannot do. Some other rodents, and many 
of the insectivores also, have projecting incisor teeth, but these are 
relatively small and slender and are obiously not suitable for such 
heavy work as digging, moving small stones, or cutting. These are 
used as forceps for the picking up of food, which consists mainly of 
small fish, insects, or worms. 
Such animals as the kangaroos (Macropus), the kangaroo rats 
(Dipodomys), the African spring hare or spring hass (Pedetes), the 
jerboas (Alactaga, Scarturus, Salpingotus) and others have large, 
long, powerful hind legs, relatively small, weak, short forelegs, and 
long tails which are either tufted with long hairs near the tip or are 
rather heavy, often very thick near the base (pl. 5, fig. 1). These 
animals are leapers and use the hind legs almost exclusively in propul- 
sion. The tail is a balancing member; it also acts as a tripod leg 
in some forms. 
The types of leaps of different animals vary greatly. One interest- 
ing difference is the contrast between the leap of the flying squirrel, 
an animal that leaps directly toward its objective with a very flat 
trajectory, often planning to land below its target and then climb 
up to it, and the leap of the tarsier (Tarsisus) which has a very high 
trajectory and lands the leaper at its objective. After becoming 
accustomed to photographing the leaps of flying squirrels, I tried 
photographing tarsiers leaping and aimed my camera as I would 
for the squirrels. With the pronounced leap upward of the tarsier, 
I at first caught only the tail and hind portion of the animal in the 
upper portion of the picture. After a few such experiences I learned 
the difference in their leaps and corrected my aim accordingly. 
The list of such highly specialized forms could be expanded indefi- 
nitely to include representatives of all groups of animal life, for 
every animal is specialized to lead a certain type of life. 
