172 Order RODENTIA 



eastern quarter of the United States, fig. 93. It is unknown on 

 the Eastern Piedmont Plateau. 



SYNAPTOMYS COOPERI Baird 

 Southern Bog Lemming Lemming Mouse 



Description. — The southern bog lemming is a mouse with an 

 exceptionally short tail and long, fine fur, which is rich brown 

 or almost chocolate brown on the back and silvery gray on the 

 belly. The short ears are almost concealed in the long fur. 



Length measurements: head and body 4-4^ inches (100-119 

 mm.); tail H'H inch (15-21 mm.); over-all M/iSYz inches 

 (115-140 mm.); hind foot about Ya inch (18-20 mm.). The 

 ratio of hind foot to tail varies from individual to individual, 

 but in most specimens it is 1:1. The foot may vary in length 

 from 3 mm. shorter to 3 mm. longer than the tail. 



The skull is 24.5-27.5 mm. (about 1 inch) long. It has 

 orange-colored, broad, and grooved upper incisors, fig. 74/. 

 Dental formula: I 1/1, C 0/0, Pm 0/0, M 3/3. 



The short tail, long fur, and broad, bright orange, grooved 

 upper incisors distinguish this mouse from all others in Illinois. 



Life History. — The bog lemming occurs in wet meadows and 

 bogs, usually where there is a thick stand of bluegrass or similar 

 ground cover. Within this cover, bog lemmings make a series 

 of interconnecting runways on the surface of the ground but en- 

 tirely hidden beneath the vegetation. This series of miniature 

 highways provides avenues to food supplies and to the several 

 holes leading to underground burrows. The lemmings construct 

 these highways by clipping away the grass with their incisor 

 teeth and keep them smooth by constant use. The runs are no 

 wider than the body of a lemming. 



This lemming is active during daylight hours as well as at 

 night. Most of the runs are quite dark during the daytime, for 

 each is rather effectively roofed by a mat of grass. 



The number of bog lemmings fluctuates markedly from year 

 to year. In one year these lemmings may be overrunning a bog 

 or meadow; in tlie next year few, if any, may be present. Ap- 

 parently they breed from March until late fall. For rearing 

 young, they build nests in chambers that join the undergroi^nd 

 burrows not far below the surface of the ground, or sometimes 

 in thick mats of dry bluegrass on the surface of the ground. 



