460 



Illinois Natural History Survey Bulletin 



Vol. 23. Art. 5 



The actual sex ratio of Illinois squirrels 

 may be more nearly equal in both species 

 than shown in table 4. It seems that 

 males, because of somewhat more active 

 habits, offered hunters more opportunities 



to writers) placed some value on compar- 

 ative growth stages, skin texture, and de- 

 velopment of pigment and hair on various 

 parts of the body, particularly on the scro- 

 tal sac of the males and on the ventral 



Fig. 6. — Juvenile (left) and adult female fox squirrels, Mason County, August 15, 1944. 

 Juvenile was first-season j-oung; adult had weaned a first-season litter, but was neither pregnant 

 nor lactating. Note the inconspicuousness of the mammary glands of the juvenile, and the 

 prominent, blackened nipples of the adult. 



for shots than did females, and probably 

 for the same reason proportionately more 

 males were taken in steel traps. 



Age Classes 



No simple, yet infallible, method of de- 

 termining age classes in fox and gray 

 squirrels has been reported. Such tech- 

 niques are much needed, not only for squir- 

 rels but for many other animal groups. 

 The nearest approach to a workable 

 method for aging squirrels requires the 

 use of a number of criteria introduced by 

 several workers, and validity of the method 

 is somewhat conditioned by the experience 

 of the user. For fox squirrels, Allen 

 (1942, 1943) and Baumgartner (in letter 



region of females. Chapman (1938(7, b) 

 working with gray squirrels, found a cor- 

 relation of age and weight classes. 



In both species males were the more 

 difficult of the sexes to age. Juvenile 

 males born in late winter or early spring 

 were easily confused wnth adult males by 

 the October and November following ; and 

 by December, when weights and measure- 

 ments of the two groups were approxi- 

 mately equal, they could scarcely be differ- 

 entiated. In adult males the scrotal sac 

 is more pendent, and is blackened and less 

 nearly clothed by hair on the ventral sur- 

 face, fig. 5. In December, appearance of 

 the teeth, and of hair and skin on the soles 

 of the feet, is very similar in the two age 

 groups. Some juvenile males born in the 



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