476 



Illinois Natural History Survey Bulletin 



Vol. 23, Art. 5 



per's glands and evidence of oestrus. In 

 the first three characteristics, all peculiar 

 to males, there was close similarity but 

 not exact uniformity in the two species. 

 In both fox and gray squirrels, testes 

 length and weight peaks were reached 

 before or by the time of peak mating. It 

 is evident that none of the criteria based 

 on male organs are fully satisfactory for 

 determining the breeding seasons, and for 

 this reason we have placed greater reliance 



in the percentage of females showing oes- 

 trus, fig. 8, assuming that peak oestrus and 

 mating occur simultaneously, or nearly 

 so. Dates given in table 13 are in general 

 agreement with findings reported by other 

 workers. 



The chronology of squirrel breeding 

 data in Illinois, including the earliest, 

 latest and peak dates for various repro- 

 ductive activities or conditions, are given 

 in table 16. Peak dates are applicable 



OESTRUS 



dJ 80 



10 



3 60 



tO 

 LJ 



O40 



z 



to 

 H 20 



u 



I L. 



_l L. 



LACTATION 



u 80 



JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV 

 Fig. 8. — Breeding seasons in female squirrels in Illinois, 1940—1942. 



DEC 



