MAMMALS OF MINNESOTA. 215 



space about the lodge is cleared of vegetation. The rats dive 

 to the bottom and pull up the rushes and water- lillies by the 

 roots and build them into the structure with large masses of 

 earth adhering. The bottom is deepened and thus the danger 

 of freezing solid is obviated. The earth is used in plastering 

 the outside of the hut, though this is not systematically done. 

 A curious habit we have occasionally observed is the thatching 

 of the hut with the large water-lily leaves so overlapped and 

 cemented with mud as to form an impervious roof. By this 

 time the ice is beginning to form and the hut has settled as 

 much as it will, being buoyed up by the ice. The rats now 

 burrow into the house from near the bottom forming a passage 

 in the form of a letter U inverted, the uppermost part being 

 above the water level and here a small chamber is excavated. 

 Run-ways are excavated beneath the ice to various parts of the 

 pond. In the long excursions undertaken beneath the ice the 

 rats are said to ascend to the ice and exhaust the lungs, per- 

 mitting the expired air to absorb oxygen and then reinspire it. 

 This we have never been able to observe. The roots of NupJiar 

 are built into the house or are stored conveniently near for 

 winter use. The great mass of vegetable matter soon begins 

 to "heat," generating warmth enough not only to add to the 

 comfort of the occupants of this curious home, but to cause 

 new sprouts to spring from the roots. The chamber is en- 

 larged during the winter and the part removed serves to supply 

 food in case of outside famine. The outer layer of mud freezes 

 solid and forms an adequate defense against the wolves which 



Fig. 13. Muskrat houses. 



