MAMMALS OF MARYLAND 



89 



Marsh rice rats superficially resemble young Norway rats from 

 which they may be distinguished by the upper cheek teeth. These teeth 

 have two longitudinal rows of tubercles in rice rats; in Norway rats 

 there are three such rows. 



Measurements. — An average-sized female from West Ocean City, 

 Worcester County, has measurements as follows: Total length 247; 

 tail 120; hind foot 30; ear 10 ; greatest length of skull 30.4; zygomatic 

 breadth 16.4; interorbital breadth 4.9 ; length of upper mola r toothrow 

 4.4. 



An adult female from Nanjemoy Creek, Charles County, has the 

 following external measurements : Total length 262 ; tail 127 ; hind 

 foot 30. 



There is a peculiar size variation often encountered in this species. 

 Males and females generally average about the same size, but often 

 an apparently adult female may be strikingly smaller than the average. 



Habitat and habits. — ^This species is partially amphibious and shows 

 a great preference for wet meadows, marshy areas, watercourses, cane 

 breaks, and swamps, and is only rarely encountered in dry fields. 



Marsh rice rats are polyestrous and breed from March to November 

 in Maryland (Harris, 1953, p. 485). The gestation period is 25 days 

 and the female mates again immediately after parturition. Litter size 

 varies from one to five with the average being three. 



This species is an accomplished swimmer and does not hesitate to 

 dive and swim under water for great distances when alarmed. It makes 

 nests of grasses and weeds which may be placed under a mass of 

 tangled debris or woven into the rushes a foot or more above the high 

 water level. Its presence may usually be detected by the extensive and 

 well-defined runways it makes and by the mats of cut vegetation float- 

 ing at irregular intervals in the tidal waters. Sometimes, however, 

 there may be little evidence of rice rats in an area. Harris (1953, p. 

 481) says that in the Blackwater National Wildlife Refuge in Dor- 

 chester County 40 percent of the 86 rice rat captures were made at 

 trap stations showing no signs of small mammals, but some were taken 

 at muskrat houses. Also, he was never able to locate any rice rat nests 

 in this area, and noted that only a few of the runways found night 

 have been made by this species. Rice rats are primarily nocturnal, and 

 Harris states that only rarely was this species observ^ed in the daytime. 



In Maryland, rice rats are confined in distribution to the fresh and 

 salt water marshes of the Western Shore and Eastern Shore sections. 

 They seem to be particularly numerous in the fresh, brackish, and salt 

 water marshes of the lower Eastern Shore section and occur in great 

 numbers on Assateague Island, where they occupy the wetter portions 

 of these marshes. 



