632 THE AMERICAN NATURALIST [Vol. XL V 



truth in all these theories, but I doubt if the principal 

 cause has yet been suggested. 



Cake of the Young 



Before the young are born, the female builds a nest as 

 elaborate as the means at hand will permit. Almost 

 any sheltered nook about buildings is a suitable nesting 

 place for the brown rat. On the farm, in addition to 

 breeding about buildings, it digs its burrows in the field 

 and nests in old straw stacks and grain still in the shock. 

 The nest consists for the most part of a coarse sub- 

 stance such as straw or corn husks, with a lining of a 

 softer material, especially feathers when obtainable. 

 Blue found that where rats use run-ways, they nest in 

 a branch leading off from the main course. This branch 

 is in the form of a Y with the nest in one arm and a 

 storehouse in the other. 



The young at birth are entirely helpless. The mother 

 gets them all together and then huddles over them for 

 hours at a time. She never lies on her side to let the 

 young suck; she always crouches over them while the 

 young lie on their sides or backs to get hold of the teats. 

 Usually all the young nurse at the same time, and a 

 litter of twelve, at about weaning time, almost holds the 

 female off her feet while they suck. 



The female can easily be induced to move her young 

 elsewhere by disturbing the nest. The young only a few 

 days old are caught around the body by the mother, but 

 if they are a week or so old, she takes hold of them by 

 the skin. In the latter case they are carried much as a 

 kitten is carried by its mother. 



In the laboratory the female spends most of her time 

 with the young; some even carry all their food into the 

 nests. Taking food into the nest is a common habit of 

 the rat and must not be considered as a trait peculiar to 



Growth 



The young at birth weigh on an average 6.4 grams. 

 The males are a little the larger, measuring in body 



