112 



NORTH AMEHICAN, FAUNA 66 



In the Maryland marshes, muskrats are active at all hours, in the 

 spring and throughout the summer. 



Muskrat homes are of two general types depending on the topog- 

 raphy of the area in which they live. In the uplands they dig burrows 

 into the banks of streams and other bodies of water, while in the 

 marshes they build dome-shaped structures. The entrance hole to a 

 bank burrow is always below the normal level of water. The burrow 

 turns upward above water level and ends in a nest of grass. The dome- 

 shaped structures that they build in the marshes may be as much as 7 or 

 8 feet in diameter and more than 4 feet high. They are constructed of 

 stalks, roots, and peaty remains of plants and are built on or around a 

 firm foundation such as a stump or the base of a tree. Each house con- 

 tains one or more nests from which passages lead to plunge holes in the 

 floor. These holes in turn lead to underground timnels that connect 

 with the surface several feet from the house. 



Muskrats are primarily aquatic and construct elaborate canals, 6 

 inches to a foot wide, and sometimes a foot or more deep, which are not 

 visible when the water is high. Those canals which are used as main 

 arteries of travel are always wider and deeper than those used only 

 as temporary leads made in search of food. In dry areas the muskrats 

 use surface trails concealed in the grass that lead in all directions, and 

 except for size, resemble those made by the meadow mouse. 



In addition to surface canals and trails, muskrats construct elaborate 

 systems of underground burrows and tunnels that spread out in all 

 directions and are connected with the surface and the canals by plunge 

 holes scattered at convenient intervals. 



Smith (1938, p. 16) found that in Maryland the muskrat may breed 

 in any month with the possible exception of November and December. 

 Most of the young are bom from mid- April to mid-September. Most 

 Maryland trappers report that there are three litters a year, but Smith 

 (1938, p. 16) was only able to obtain two a year in pen-raised animals. 

 The number of young is variable ; in the Maryland investigations con- 

 ducted by Smith, the number averaged 4.4, seven being the most found 

 in any one uterus. The gestation period appears to be about 29 or 30 

 days, and muskrats probably first breed at the age of 1 year. 



The population of muskrats in Maryland marshes apparently has 

 been decreasing since 1939. The number of muskrats trapped in the 

 marshes from year to year may not reflect the actual muskrat popula- 

 tion since many factors such as food, predation, salinity, and breeding, 

 which are not readily observable, together with the value of pelts, act 

 together to affect muskrat abundance, and may be different from year 

 to year. Furthermore, these factors may cause different reactions on the 

 part of the muskrat populations at different levels of abundance. 

 Nevertheless, trapping records probably give an adequate index of the 



