September, 1945 



Brown & Yeager: Squirrels in Illinois 



461 



summer remain distinguishable from adult 

 males until about May of the following 

 jear. 



In females, the teats offer the best means 

 of determining age classes. In individuals 

 that have never bred, these structures are 

 small, light in color, and with the entire 

 gland more or less hidden in the growth 

 of hair. Mature females, after breeding 

 and nursing, have much larger and more 

 conspicuous mammary glands, the nipples 

 of which, in both species, are dark in color, 



fig- 6- . . 



The criteria found useful in aging 



squirrels in this study are listed in table 



5. Collectively, and when used by an 



experienced observer, they are believed to 



be reliable for most individuals of both 



species. No single criterion w'as found 



applicable to all individuals of a given 

 species or sex, and in some individuals age 

 could not be determined with complete 

 satisfaction when all criteria were used. 



Hunters distinguish young from adult 

 squirrels by several criteria other than ap- 

 pearance. The skinning test is perhaps 

 most common, young squirrels being "easy" 

 to skin and old squirrels "tough." Young 

 squirrels remain "easy" to skin well into 

 the fall or into the following spring, ac- 

 cording to time of birth. The ease with 

 which the leg bones may be snapped when 

 the animals are being dressed is another 

 criterion sometimes used by hunters. 



Squirrels may properly be classed as 

 adults after they reach breeding condition, 

 and on this basis first-season young, born 

 in late winter or early spring, become 



Table 5. — Criteria for distinguishing between adult and Juvenile fox and gray squirrels. 



Adults 



Juveniles 



Males 



Females 



Mammary glands large and noticeable, not 

 hidden by hair growth; teats (in bred fox 

 squirrel females) black tipped. 



Uterus contracted in posterior position of 

 coelom, horns about 2 mm. wide and flattened. 



1. Teats inconspicuous, more or less hidden in 

 growth of hair. 



2. Uterus threadlike, extending forward toward 

 kidneys. 



Males and Females 



