. Bulletin. Natural history; Natural history. 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
. Bulletin. Natural history; Natural history. 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 t
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