6 



farmers' BULLETIN S(>9. 



until the domelike top rises 2 or 3 feet above the water. The mud on 

 the outside and in the walls of muskrat houses seems to be collected 

 accidentally with the roots. Within the part of the structure above 

 the water a chamber is excavated, from which two or three passages 

 load downward through the mass into the water, reaching it at points 

 well below the frost line. If the water is shallow, the animals exca- 

 vate deeper channels leading from the house to various parts of the 

 pond. 



The houses are mostly for winter shelter and food and are seldom 

 used as receptacles for the young. Occasionally, when driven from 

 other houses or when excluded from underground burrows by bar- 

 riers of ice or frozen ground, more than one family may occupy a 

 single house temporarily. 



When banks of streams or ponds are high enough for the purpose, 

 muskrats burrow into them. Entrances to the tunnels are almost 

 always under water, and the approach to them is, if possible, by 

 channels of sufficient depth to prevent ice from closing the passage. 

 The tunnels extend upward into the bank above the level of the 

 water. They often rise to within a few inches of the surface of the 

 ground and are frequently protected above by roots, by trees and 

 shrubs, or by thickly matted turf. These tunnels extend 10 to 50 

 feet into the bank and terminate in a roomy chamber which sometimes 

 contains a bulky nest composed of dried vegetation. Usually two 

 tunnels lead from the nest to the water, and often a tunnel has two 

 branches or outlets. 



When burrows can be made, muskrats occupy them in winter and 

 summer; but in shallow ponds and marshes, and especially in north- 

 ern latitudes, the entrances are often closed by ice in winter. In 

 such situations and when banks are not suitable for burrows, houses 

 become a necessity, but they are seldom seen along the borders of 

 deep ponds and canals, and, except in extensive swamps unbroken by 

 hillocks, they are not found in the southern parts of the muskrat's 

 range. 



As cold weather approaches, the animals become very active, adding 

 to their old winter houses, building new ones, and deepening channels 

 that lead to houses and burrows. They do not hibernate, and, aside 

 from the vegetation of which their houses are made, seem to make 

 little provision for winter. However, some of the surplus food col- 

 lected may be found in their burrows at almost any time. 



BREEDING. 



Early writers about the muskrat gave widely divergent accounts 

 of its breeding habits. It is now well established that the animals 

 breed from three to five times in a year and that the litters average 

 from 0 to 8 young. The early spring litters are usually less in number, 



