ANIMAL BEHAVIOR—ERNEST P. WALKER 265 
they can get away, they throw themselves on their backs incredibly 
quickly and are prepared to fight savagely with their sharp, fairly 
strong teeth and tiny claws. If they are in their nests, they pay little 
attention to outside disturbances, at most sniffing from just inside the 
doorway. 
Baby ‘‘hammies” arrive in litters of as many as 17, tiny, blind, 
naked, pink, helpless little fellows that lie on their backs and nurse 
while mother “‘hammy” hovers over them to keep them warm in the 
snug cup-shaped nest of fibers she has constructed in a remote portion 
of the burrow or den. The little ones grow very rapidly, and by the 
thirteenth day they have a good coat of fur, are eating solid food, and 
begin wandering about in the burrow, den, or cage. Even though 
their eyes are not yet open, they find their way back to the nest if 
mother has not detected their absence and carried them home. By 
the fifteenth day their eyes are open, and by the seventy-third day they 
produce babies of their own. ‘They bear large families in rapid suc- 
cession but cease to breed regularly when about 1 year old, and the 
life span is short (probably a maximum of about 3 years). There is 
a high mortality rate, probably throughout life. 
The structure and lives of the flying squirrels (Glaucomys volans) of 
North America are in marked contrast to those of the hamster. 
Flying squirrels weigh about 3 ounces, have slender bodies, long, 
slender, strong arms, fingers, legs, and toes with fairly long, curved 
claws, and a rather long tail. Theskin of the body is much larger than 
is necessary to enclose the delicate little form and is extended outward 
along the side as hair-covered membrane indistinguishable from the 
skin of the body. This skin reaches down to the wrists and ankles, 
so their outline is almost square when the little creatures put their 
arms forward and out and their legs back and out as they do while 
gliding through the air. This adaptation more than doubles the area 
of the upper and lower surfaces as it appears when the animals are at 
rest. In keeping with this form, the hairs of the sides of the tail are 
long, and closely set, and project at almost right angles, whereas those 
of the upper and lower surfaces are very short and lie close to the bone 
of the tail, so that the tail is, in cross section, almost like a feather. 
All these are modifications for life in an entirely different habitat from 
that of the hamster. 
Flying squirrels live among the trees, generally making their nests 
in hollow limbs, old woodpecker holes, or other shelters high above 
the ground. They are strictly nocturnal, having very large eyes 
adapted to admitting the maximum light available. They are not 
limited in their movements to climbing up and down the trees or to 
mere leaping between objects, for their remarkable form gives them 
power to glide long distances. ‘To do this they seek an elevated point, 
leap out into space and spread their arms and legs so that they take 
