ANIMAL BEHAVIOR—ERNEST P. WALKER ”263 
and its food is definitely limited by the products that it can obtain in 
the habitat which it occupies. Animals that do not use the same food 
or are not antagonistic, because one does not prey upon the other, 
can occupy the same areas or overlapping areas, so that ecological 
niches do not have definite, clearly defined boundaries, but are rather 
areas or space spheres of activity having also a time component. The 
structure of the animal to a large extent governs the type of ecological 
niche that it may occupy, since through evolution the animal’s form 
has been modified to adapt it to the type of ecological niche that has 
been available to its long line of ancestors down to itself. Thus we 
can to some extent explain and understand the great variety of forms 
of animal life. 
We might consider two little creatures of approximately the same 
size which occupy different ecological niches and are very different 
in structure, and see how they fit into their respective spheres. These 
are golden hamsters (Mesocricetus auratus) and “‘flying’’ squirrels 
(Glaucomys volans). 
The golden hamster of Syria averages about 4 ounces in weight, 
with a maximum of about 8 ounces. It is a stout-bodied, short-necked, 
short-armed, and short-legged little mammal with short fingers and 
toes and a tail only about half an inch in length. It has very large 
cheek pouches that open inside the lips and extend far back of the 
shoulders, in which it can carry surprising loads of food or nest mate- 
rial. It is rich golden-reddish brown above and white beneath, with 
white hands and feet. The little creature is an inhabitant of an arid, 
rocky region where it lives mainly among rocks or in burrows in the 
ground around the rocks. Syria’s climatic conditions result in a 
scarcity of food material for such little creatures for long periods of 
time; therefore, the storage of food is necessary and the cheek pouches 
are of great value in carrying food to its burrow or den. It works 
industriously at this, apparently the only limit to the amount of food 
it carries being determined by the amount that is available. 
They are relatively slow-moving, clumsy little creatures in com- 
parison with many other animals that we know, but are obviously 
well adapted to leading their lives in an efficient manner in their 
habitat. Their movements remind one of military tanks in that they 
are slow, ponderous, and persistent. There is nothing sprightly or 
agile about them, but if they want to go in a certain direction or up a 
certain surface they keep at it so persistently that frequently they 
succeed, and they can wedge themselves through surprisingly small 
crevices. They display remarkable persistence and determination to 
get into crevices that would seem to be so small and uncomfortable as 
not to be attractive to them. 
The mouths of rodents are definitely on the underpart of the head 
and back of the nose, making it difficult for them to cut upward when 
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