ANIMAL BEHAVIOR—ERNEST P. WALKER 267 
slowly, are not well clothed in fur until about the twenty-eighth day, 
and continue nursing until about the fifty-sixth day. In recognition 
of the dangers of an active life in trees, they, like young tree squirrels 
(Sciurus), start out exploring near the nest with great caution, and 
venture farther and take more chances only as they gain strength, 
experience, and confidence. Their gestation period is about 60 days 
and young are born when the parents are about 1 year old. 
These brief descriptions of the widely different hamsters and flying 
squirrels, which inhabit very different ecological niches, give a glimpse 
of the specialization among animals which enables them to survive 
and to lead their lives under diverse conditions. 
Giraffes (Giraffa) have developed such a high degree of specializa- 
tion that they are able to fill an ecological niche without competition 
with other mammals except in those portions of their range which 
overlap that of the elephants. Much of the food of the giraffe is 
taken from the upper portions of shrubs or from the lower branches 
of trees far above the reach of most mammals. In addition to its long 
legs and very long neck, the giraffe has elongate, mobile lips and can 
extend its tongue several inches to help it gather in leaves and twigs. 
The moles (Scalopus and other genera), which burrow in the earth, 
are blind, have exceedingly short forelimbs, with nose and tail very 
sensitive to touch, large, powerful hands armed with long, strong 
claws, and very strong muscles in the fore part of the body, particularly 
those associated with the arms. The fur is very soft and short and 
will lie in any direction; thus it does not impede the mole when it 
desires to run backward in its burrow, which it often does. Moles 
feed on earthworms, grubs, and other small animal life they catch in 
the earth. (See pl. 5, fig. 2.) 
In marked contrast are the gibbons (Hylobates and Symphalangus), 
highly specialized members of the primate or monkey group. Their 
arms are exceedingly long and powerful, with elongate hands and four 
strong fingers, the thumb being small and situated far back on the 
hand. The legs are rather long, and the feet are better fitted for 
grasping than are the hands. The body is small. These adaptations 
enable gibbons to be the most expert of all mammals in swinging by 
their arms through the forest from branch to branch and even from 
tree to tree. In this process the hands are used as hooks, the thumb 
taking no part in grasping. Since the hands are usually occupied 
when traveling through the trees, gibbons carry objects such as food 
or branches by grasping them with the feet. The same method of 
carrying objects is used by chimpanzees (Pan) and orangutangs 
(Pongo). 
Whereas the gibbons, chimpanzees, gorillas (Gorilla), and orang- 
utangs lack tails, the spider monkeys (Ateles) of Central and South 
