408 MICHIGAN SURVEY, 1908. 



to 20. In British Columbia the females are said to hav'e three litters 

 each season and to successively diminish in fertility with each 

 brood. The species thus appears to be more fertile in the northern part 

 of its range. 



The food of muskrats consists of grasses and water plants, and in- 

 cludes a wide range of vegetable food. In addition to this vegetable 

 food, occasionally dead fish and mussels (Unionidae) are eaten in 

 quantities, as is shown by the abundance of shells along the banks of 

 streams and the margins of ponds and lakes. Such heaps of shells are 

 generally credited to the Muskrat, but the Mink, as has been suggested, 

 may share in this. Direct observations bearing upon this point are 

 very desirable, but the usual nocturnal habits of the Muskrat doubtless 

 account for the limited information on this subject. There is a sur- 

 prisingly small amount of direct information, in the accessable litera- 

 ture, on the relation of Muskrats to the mussels. Thus Kennicott ('57 

 p. 106) states that "Collecting them [mussels] from the bottom, it car^ 

 ries them in its teeth to a log or stone, where, sitting, upon its haunches, 

 and grasping them in the fore-paws, it opens the shells with the in- 

 cisors as skillfully as it could be done with an oyster-knife." * * * 

 "I have observed that those species with thin shells are more sought 

 for, and have often found large specimens of Unio pJicatiis unopened 

 among the piles of empty shells, the muskrat apparently considering 

 them not worth the trouble of gnawing apart the valves at the back, in 

 which manner the heavy shells are sometimes opened." The Muskrat, 

 like the Beaver, does not hibernate in winter but leads an active life, 

 which means that they require a food supply throughout the winter. 

 This they are usually able to secure under the ice, where they may store 

 a, supply, or by eating their lodge (Merriam, '81, p. 277), but in ex- 

 ceptionally cold winters or during a dry season, their winter sunnly 

 may be frozen up; under which circumstances they may be frozen in 

 their winter quarters, or must search for food above ground. 



Extensive burrows are made in the banks of streams or In the shores 

 of the bodies of water which they frequent, and in these they usually 

 rear their young, although the houses or lodges may also be so used, 

 especially in swampy areas. These lodges are generally built in the fall 

 for winter use, and are constructed of grass, roots, mud and sticks; 

 within this is a chamber, reached by a submerged passageway, lead- 

 ing under the ice. 



Drouth, disease, large owls, Mink and perhaps the Otter are the 

 most prominent native enemies of muskrats. 



The conditions which cause migration are of interest on account 

 of their bearings upon the geographic range and isolation of muskrats. 

 Severe cold may shut off their supply of submerged food and 

 necessitate a migration during the winter; also during the summer 

 a change of residence may be necessitated by drouth, especially of 

 those species which inhabit shallow ponds. This no doubt in part ex- 

 plains the occurrence of those animals which are occasionally taken far 

 from water. Such migrations will not only explain in part the trans- 

 ference of these animals from one drainage system to another, but 

 also the populating of isolated bodies of water. 



Geographic Favf/c. An examination of the ranges of the five species 



