18 AN ECONOMIC STUDY OF FIELD MICE. 



provinces and as the cultivated areas in the West are extended under 

 the stimulus of the United States Reclamation Service, the danger 

 of serious ravages by meadow mice will increase rather than diminish. 



II. THE PRAIRIE MOUSE. 

 Microtus ochroga.strr Wiisinev. 



The prairie mouse is probably second onW to the common meadow 

 mouse in the extent of its injury to crops. Outwardly it differs but 

 slightly from the latter species. Its tail is shorter (less than twice 

 the length of the hind foot) and its fur is coarser. In winter the 

 pelage is grayer. The color of the underparts shades into a buff 

 or cinnamon. The contrast between the upper and lower parts of 

 the tail is much sharper than in the common meadoAv mouse. The 

 foot pads are 5, and the number of mammae 6(2 pectoral and 4 ingui- 

 nal). Its average measurements are about as follows: Length, 155 

 mm. (0 inches) : tail vertebraN 38 mm. (1.5 inches) ; hind foot, 21 

 mm. (0.82 inch). 



The prairie mouse occurs in southern Wisconsin, in Indiana, Illi- 

 nois, Iowa, Missouri, Nebraska, Kansas, and a part of Oklahoma. 

 It lives in the open prairie countiy, mainly in the Upper Austral zone. 

 Thus it is much more likely to invade crops than if its natural 

 habitat were in swamps. I have found it on the borders of corn and 

 cane fields and in native meadows, as well as in cultivated clover and 

 alfalfa fields. It seems especially partial to fields that have been 

 allowed to lie fallow for several seasons. The soft mixed annual 

 grasses and weeds that partly replace the original prairie cover 

 seem to furnish it congenial surroundings. Close grazing of the open 

 ranges tends to drive out voles, but when ranges are not closely pas- 

 tured, so that an abundance of old grass is left, prairie mice soon 

 become numerous and appreciably reduce the amount of forage. 



In the same manner the prairie mouse invades pastures and neg- 

 lected orchards whenever dry grass is permitted to accumulate and 

 remain over winter. If no crops are near, the animals subsist on 

 wild herbage, roots, and seeds: but when cultivated crops are acces- 

 sible their trails soon extend far into the tilled fields. 



Nests of prairie mice usually are less bulky than those of the com- 

 mon meadow mouse, but are built in a similar way and in like situa- 

 tions. The number of young at a birth is usually three or four, 

 rarely five or six. In ordinary seasons the first litter is born in April, 

 but in dry, warm springs the time may be fully a month earlier. The 

 number of litters in a season varies with climatic and other condi- 

 tions. On the whole, prairie mice multiply less rapidly than meadow 

 mice, since the number of young at a l)irth is smaller, and the long 

 summer droughts and extreme Avinters of the interior prairies of the 

 West often limit reproduction. 



