ECONOMIC IMPORTANCE OF MAMMALS 135 
injure man’s land and his crops. Rats and mice consume 
much food and destroy much that they do not eat, besides 
carrying diseases. Red 
houses, where they dam¬ 
age beds and furniture, 
and out of doors they 
destroy the eggs and 
young of birds. 
Rats , 1 mice, and guinea 
pigs have proved of great 
use in laboratories where 
the causes and effects 
of diseases are studied. 
Their reaction to diph¬ 
theria, tuberculosis, can¬ 
cer, and other diseases is 
similar to that of man. 
As a result of these 
studies the skillful physi¬ 
cian is more successful in 
relieving the sufferings of 
man. 
Report on Mammals to be filled out first from general knowledge, 
later extended by trips to fields, woods, or parks. 
Kinds 
Where 
’ Found 
Food 
Kind of 
Food 
Life in 
Winter 
Life in 
Summer 
Beneficial 
Harmful 
1 The Brown Rat causes damage to cereals and grains estimated at 
more than $200,000,000 annually in the United States. Read “The 
Brown Rat in the United States/' by David Lantz. Bulletin No. 33, 
Biological Survey, United States Department of Agriculture. 
squirrels gnaw their way into 
Figure 126. — Camel. The Ship of 
the Desert. 
In making long trips across the desert, 
the camel is able to go without drinking. 
During these journeys, the hump grows 
smaller as the fat in it is used as food. 
This food is gradually changed until part 
of it becomes water. We might say that 
the fat in the camel’s hump is a special 
water reservoir. 
