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 toobserve. The roots of Nuphar 
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. Theouter layer of mud freezes 
solid and forms an adequate defense against the wolves which 



Fig. 13. Muskrat houses. 
