MAMMALS OF MARYLANID 



123 



feed. From the sewers it will readily pass into buildings where it may 

 cause considerable damage. 



The species is an efficient burrower, and out-of-doors its bank bur- 

 rows consist of winding galleries furnished with several escape holes. 

 On farms, it frequently makes burrows in manure piles, rubbish 

 mounds, wheat stacks, and hay ricks. Many Norway rats spend the 

 summer months in fields and meadows, and at the approach of cold 

 weather migrate into towns and villages where they seek the warmth 

 of commercial buildings and other habitations. 



The gestation period in the Norway rat is 21 days. Studies in 

 England (Hinton, 1931, p. 13) indicate that the average number of 

 young per litter is eight or nine, but that there are records of as many 

 as 23. Usually the number ranges between 6 and 19, and the females 

 may produce five or six litters annually. 



It is generally believed that the Norway rat and the black rat are 

 incompatible, and that the larger, more aggressive Norway rat will 

 drive out or kill the smaller, weaker black species wherever they are 

 occupying the same area. There is no proof of this, however, and there 

 are even cases known where the two species have lived together in 

 the confines of a small ship (John Jones, U.S. Fish and Wildlife 

 Service, in verbis) . The fact that they only infrequently occur together 

 is probably the result of their preference for different climatic situa- 

 tions. The Norway rat is essentially a northern, cool climate animal 

 and prospers in the temperate regions of northern Europe and North 

 America. The black rat originated in warm, semitropical areas and 

 find its optimum conditions in the warm Mediterranean regions and 

 in the southern portions of the United States. It seems probable that 

 the black rat, although repeatedly introduced, has never been firmly 

 established in the northeastern United States and that the Norway 

 rat has been the common house rat in Maryland since early colonial 

 tim^. 



Speciments examined. — AnTie Arundel County: Fort Meade, 1. Bal- 

 timore City: 1. Calvert County: Solomons Island, 1. Montgomery 

 County: Silver Spring, 1 mile N, 3. District of Colvmibia: 77. 



HOUSE MOUSE 



Mus musculus Linnaeus 



Mu^ musculus Linnaeus, Syst. nat., ed. 10, Vol. 1, p. 62, 1768. 

 Type locality. — Uppsala, Sweden. 



General distribution. — This is an old world species that has been introduced 

 into the United States and is now found in a commensal and feral state through- 

 out the country. 



