Family CRICETIDAE 177 



The prairie vole may be active day and night, winter and 

 summer, but is probably most active at night. In winter it makes 

 burrows through the snow at the surface of the ground. Under 

 the protective mantle of snow, it may make runs to the trunks 

 of trees, where it feeds on the bark. 



When conditions are ideal, prairie voles may increase rap- 

 idly, and a population peak of 100 or more per acre may result. 

 Usually within a year or two after the peak has been reached, 

 there will be a rather rapid and marked decrease in the number 

 of voles. Later, the population may build up to another peak. 

 Increases and decreases in population follow a rather definite 

 time pattern or cycle. Biologists believe that the cycle is com- 

 pleted about every 4 years. 



Signs. — The runways and nests described above are signs of 

 the presence of the prairie vole. Other signs are tooth marks, 

 beginning immediately above ground, on the trunks of trees, 

 fig. 37. Tracks of the prairie vole are like those of the meadow 

 vole. 



Distribution. — The prairie vole is most abundant in central 

 and southern Illinois but is known to occur in suitable habitats 

 throughout the state. The Illinois subspecies is Microtus ochro- 

 gaster ochrogaster (Wagner). The range of the species covers 

 an area from western West Virginia to southeastern Alberta 

 and southward to central Oklahoma and western Tennessee. 



PITYMYS PINETORUM (LeConte) 

 Pine Vole Pine Mouse 



Description. — The pine vole, fig. 97, is a short-tailed mouse 

 with small eyes. The fur on the back and sides is chestnut or 

 bright brown, with a gloss or sheen, and is dense, almost like 

 that of a mole. The under parts are gray and may be washed 

 with buff. The tail is brown, lighter on the under than on the 

 upper side. The hind foot is usually 1 to 3 mm. shorter than the 

 tail. There are four (two pairs of) mammary glands. 



Length measurements: head and body 3^4-4}4 inches (95-108 

 mm.) ; tail about Y\ inch (15-22 mm.) ; over-all 414-518 inches 

 (110-130 mm.); hind foot about % inch (16-18 mm.). 



The skull is much like that of other voles but is broader 

 through the interorbital region (more than 4 mm., or more 

 than ' j< inch) and the posterior border of the palate has a 



