216 NORTH AMERICAN FAUNA [No. 55 



water or along shores, building houses surrounded by water, or living 

 in bank burrows opening into the water. They are famous builders, 

 constructing conical or dome-shaped houses of plant stems, roots, 

 sods, and mud rising usually 3 or 4 and sometimes 5 feet above the 

 surface of the water, with broad bases resting on the bottom of shal- 

 low lakes or ponds. A single room occupies the center of the house 

 just above the water level with usually 2 or 3 doorways opening 

 downward through the water and out under the house into the lake 

 or pond. The heavy walls, often a foot thick, keep out the winter 

 cold and many of their enemies, and the room within often accom- 

 modates a whole family of 6 or 8 animals in the moist bed of water 

 plants close to the water level. Even in the coldest weather the in- 

 side of the house is kept warm and the water is prevented from 

 freezing by the body warmth of the muskrats, while the porous walls 

 admit sufficient ventilation to afford them healthy existence. In case 

 of danger or alarm the muskrats dive quickly through the water 

 holes and swim long distances under water before coming to the 

 surface, or swim under the winter ice to other houses or to bank 

 burrows. 



In deep streams or lakes the muskrats usually live in bank bur- 

 rows, or tunnels leading from well under water back into the banks 

 and upward until a nest chamber is formed above the water level. 

 In high banks these bank dens are usually well hidden and even 

 safer from enemies than are the house nests, and in many places both 

 bank dens and houses are used by the same animals. There is evi- 

 dently much visiting back and forth among the houses and dens, but 

 to what extent the sociability reaches beyond the family circle is not 

 known. 



Breeding habits. Muskrats normally have 4 pairs of mammae 2 

 inguinal and 2 pectoral and the usual numbers of embryos recorded 

 are 6 to 8. On large old female containing 13 embryos evidently 

 was abnormally fertile. The first young of the season are born in 

 May and June, but later litters born during July and August may 

 be second litters, or the first litters of young females. The number of 

 litters produced in a season by one mother has not been satisfactorily 

 determined. 



The young are born blind, naked, and helpless and do not come 

 out of the house until well furred and old enough to swim and dive 

 and take care of themselves. They mature rather slowly and appar- 

 ently do not reach full size during the first season, or until a full 

 year old. 



Food habits. The food of muskrats consists principally of roots, 

 tubers, bulbs, and the tender basal portion of tules, sedges, cattails, 

 grasses, and other marsh plants. The long rhizomes of cattails, rich 

 in starch and gluten, furnish much food, while the blanched tender 

 basal portion of the stems of both cattails and tules are extensively 

 eaten. Waterlily roots and leaves are a favorite food. The tender 

 young shoots of grasses, sedges, wildrice, and numerous other green 

 plants are eaten, while clover and alfalfa are always acceptable food. 

 Rolled oats are eagerly eaten as are carrots and many cultivated 

 crops, but in wild lands, grains and seeds, except wildrice, are not 

 often obtained. Small turtles, muscles, and crawfish are sometimes 



