Figure 26. — Female reproductive tract of the Virginia muskrat (Ondatra s. macrodon) . 

 Left: Early-spring-litter subadult showing small, glistening, thin-walled uterine 

 horns. Center : First-breeding-season adult showing pigmented placental scars of 

 two litters, of three young and five young, as indicated by the relative size of con- 

 gested areas. Right : Older adult almost ready for spring mating ; horns slightly 

 swollen, cream-colored, and with only an indication of pigmentation remaining. 



groups. Kits have thin, papery 

 pelts of a soft, downy quality. 

 Tough, thick pelts unquestionably 

 denote adults. 



Internal examination of the re- 

 productive tract will dispel any 

 doubt about the age group to which 

 a muskrat belongs. The condition 

 of their reproductive tracts distin- 

 guish the productive females from 

 the nonproductive (fig. 26). In 

 muskrats that have recently pro- 

 duced young the mesenteries ad- 

 joining the horns of the uterus con- 

 tain small areas of minute, con- 

 gested blood vessels. Pigmented 

 dark spots that occur on the uterine 

 forks, or horns, represent placental 

 scars, or sites of embryonic attach- 

 ment. These scars remain distin- 

 guishable from the birth of the lit- 

 ter until the latter part of the fol- 

 lowing February when they start 

 to disappear. By early March 



these scars usually have entirely 

 cleared and the uterine horns have 

 become cream-colored and slightly 

 swollen in direct contrast with those 

 of the subadult female, which are 

 small, glistening, and thin-walled. 

 Since placental scars do not per- 

 sist into the new breeding season, 

 the productivity of the females can 

 be determined only during the late 

 fall and winter. Early in the trap- 

 ping season the scars are distinct 

 and can be counted, and the average 

 number of kits to a litter deter- 

 mined. In many cases, the actual 

 number of litters produced that 

 breeding season can be checked by 

 the comparative size and the dis- 

 tinctness of the pigmented spots. 

 The spots vary for each of the lit- 

 ters, the smallest and the faintest 

 denoting the oldest litter. Such a 

 count can be a relatively accurate 

 index for races that produce but 



23 



