September, 1945 



Brown & Yeager: Squirrels in Illinois 



475 



a second time, that is, during the summer, 

 in their first 18 months after birth. Fe- 

 males from summer litters are usually too 

 immature to breed during the ensuing 

 winter season, at an age of 7 months or 

 less, but they breed during the following 

 summer, at an age of about a year, 

 'i'he gray squirrel has been found to 



1 he annual breeding cycle of males in 

 Illinois was found to coincide approximately 

 with that of females, but the two peaks 

 are less distinct in the males, which appear 

 to be in breeding condition continuously 

 from late fall to midsummer, although a 

 minor decline is apparent in February and 

 March, fig. 9 and table 14. The data 



Table 13. — Generalized peak mating dates of squirrels in three Illinois zones, based mainly 

 on 2 years of study. .July 1, 1940, to June ?>(). 1942. 



have breeding dates almost paralleling 

 those of the fox squirrel. Goodrum 

 ( 1940) in east Texas reported two breed- 

 ing seasons for the gray species, one be- 

 ginning about December, with winter 

 pregnancy reaching a peak in January or 

 early February, and the second beginning 

 in late May or early June, with summer 

 pregnancy reaching a peak in August. 

 In Kentucky, Hibbard (1935) also found 

 the gray squirrel to have two breeding 

 seasons. 



Data on the two breeding seasons for 

 fox and gray squirrels disclosed by the 

 Illinois investigation, table 13, are in 

 general agreement with the conclusions 

 of Allen for the fox squirrel. In Ohio, 

 Baumgartner (1940) indicates two fox 

 squirrel breeding seasons, one from De- 

 cember to April and another from late 

 May to early October. A logical ex- 

 planation of the two peaks in the female 

 cycle of Illinois squirrels, fig. 8 and table 

 15, is found in the fact that most old fe- 

 males, and females b(jrn the previous 

 spring, breed during December, January 

 and early February, and that little further 

 breeding ensues until females born the 

 pre\ious summer, and old females pro- 

 ducing second-season litters, reach the 

 oestrus condition, usually in May or June. 



indicate that males are in breeding con- 

 dition earlier than females. Cessation of 

 breeding occurs in midsummer. 



Our studies indicate that between the 

 northern and southern limits of Illinois 

 there is a difference of about 3 weeks in 

 the average breeding dates for both fox 

 and gray squirrels, the season being pro- 

 gressively later south to north, as would 

 be expected. The breeding peak for gray 

 squirrels, zone for zone, is about 10 days 

 to 2 weeks later than for fox squirrels. 

 These variations appear to hold roughly, 

 in old squirrels, for both the first and 

 second breeding seasons. In table 13 gen- 

 eralized breeding dates of Illinois squirrels 

 are given. 



It should be understood that dates 

 given in table 13 are based on only 2 years 

 of study, and that breeding periods are 

 subject to fluctuation from year to year. 

 Such fluctuation may include dates either 

 earlier or later than the dates given, and, 

 while the extent of fluctuation is not 

 known, it is believed not to exceed one 

 week. 



Several criteria were used in arriving at 

 peak breeding dates. As discussed below 

 and illustrated in figs. 10 and 11, these 

 criteria were average testes length, average 

 testes weight, average diameter of Cow- 



