486 



Illinois Natural History Survey Bulletin 



Vol. 23, Art. 5 



weaned. Thus the glands may contain 

 milk for 3 months or more subsequent to 

 parturition. 



The teats and mammary glands of 

 squirrels develop rapidly after pregnancy 

 begins. In both species, the teats swell 

 and become black pigmented. The glan- 

 dular tissue becomes thickened and spreads 

 to cover the ventral body surface. Hair 

 on the mammae of nursing females be- 

 comes worn and matted, and the nipples 

 show^ an obviously distended and stretched 

 condition, fig. 13. On cessation of lac- 

 tation the teats become greatly reduced 

 and, in fox squirrels at least, the black 



pigmented condition of the nipples remains 

 apparent, fig. 6. 



YOUNG REARING 



In the term young rearing, as it is 

 usually employed in this paper, are in- 

 cluded the biological phenomena associated 

 with young squirrels. 



Birth Dates 



In the spring of 1941 birth dates were 

 estimated for 35 litters of fox squirrels 

 and 4 litters of gray squirrels. Most of 



Table 18. — Birth dates by 2-week periods for 35 fox squirrel litters and 4 gray squirrel 

 litters in Illinois, all zones, spring, 1941. 



Table 19. — Aging criteria for juvenile fox squirrels in Illinois. Data obtained principally 

 from caged animals, which were weaned at the age of about 10 weeks. 



*AveraEe of 7 individuals weighed at weekly intervals. 

 fSquirrels 9 and 10 weeks old similar in appearance to these in eighth week but larger. 



