September, 1945 



Brown & Yeager: Squirrels in Illinois 



487 



the litters had been born in wood duck 

 nest boxes, but a few of the fox squirrels 

 in leaf nests. The distribution of esti- 

 mated birth dates by 2-week periods is 

 shown in table 18. 



In addition to the fox squirrel births 

 included in table 18 were those involving 

 two nursing females taken on February 

 20 and another on February 21. In these 

 animals the uterus had returned to ap- 

 proximately normal size and all showed 

 placental scars. Scars do not appear until 

 the uterus has cleared, several days follow- 

 ing birth of the young. 



The earliest first-season birth date es- 

 timated for Illinois gray squirrels (only 

 four litters observed) was February 20 

 and the latest March 18. One pregnant 

 gray squirrel female, almost to term, was 

 taken March 29, indicating first-season 

 young are born at least through March. 



No birth records were obtained for the 

 second season in 1941, or for either the 

 first or second season in 1942. From the 

 numerous records of oestrus, pregnancy 

 and lactation in both species, however, the 

 birth peaks can be approximated. 



Birth Sites 



Because squirrel dens and leaf nests are 

 discussed later in this report, no details 

 on birth sites are given here. Although 

 \oung are born in both cavity and leaf 

 nests, it appears that the cavity type is 

 preferred. More litters were found in 

 natural cavities than in leaf nests, and 

 many litters were found in boxes construct- 

 ed and erected for wood ducks. The 

 senior author checked, on the Mason 

 County State Forest, a total of 89 leaf 

 nests during the first half of April and 

 found only 2 litters of partl_\' grown fox 

 squirrels. Only fox squirrels inhabited 

 this area. Goodrum (1940) states that 

 in east Texas gray squirrels are usually- 

 born in hollows of trees, but that they 

 may be transferred to leaf nests. Both 

 species move their young from nest to 

 nest when disturbed. On such occasions 

 the mother grasps the young in her mouth, 

 in the manner of a cat moving her kittens. 



Lactation 



The young of both fox and gray squir- 

 rels nurse for 8 weeks or more. The 



earliest and latest records of lactation 

 during the first season are for fox squir- 

 rels, February 20 and May 27 ; for gray 

 squirrels. March 1 and May 31, table 16. 

 Most fox squirrel females had finished 

 nursing by May 1 ; most gray squirrel 

 females by May 15. Lactation data for 

 both species and for both breeding seasons 

 are given by 2-week periods in table 15. 



As indicated in table 15, lactation in the 

 Illinois fox squirrels studied was most 

 prevalent in March and August, when 78 

 and 44 per cent, respectively, of the adult 

 females handled showed this condition. 

 In gray squirrels, lactation was most prev- 

 alent in April and September, when 75 

 and 60 per cent, respectively, of the adult 

 females handled were nursing. In some 

 females the cessation of lactation was 

 found to be delayed. In both species, 

 females showing second-season oestrus still 

 carried first-season milk, which could 

 readily be exuded by pinching the nipples. 

 The young observed during the study were 

 weaned when about 8 or 10 weeks old. 



Development of Young 



Fox squirrels handled shortly after birth 

 were 50 to 60 millimeters in length and 

 weighed 14 to 18 grams (about one-half 

 ounce). They were dark pinkish, naked, 

 except for vibrissae about the nose and 

 chin, and virtually helpless. The claws 

 were well developed. Both the eyes and 

 ears were closed, and neither the upper nor 

 lower incisors had erupted. The vibrissae 

 grew rapidly, and hair first appeared on 

 the upper dorsal surface at an age of 8 

 or 10 days. Further growth character- 

 istics are detailed in table 19, data for 

 which were obtained principally from 

 caged animals. 



Because of a growth differential be- 

 tween caged and wild specimens, the data 

 in table 19 may vary from corresponding 

 data for wild squirrels by 2 or 3 days. 

 Information relating to pelage is prob- 

 ably subject to wider variation than that 

 pertaining to eyes, ears, teeth or weight. 



There was no opportunity during the 

 study to weigh or measure gray squirrels 

 at birth. These animals are smaller and 

 somewhat lighter in color than fox squir- 

 rels. 



At weaning age, or during late April 

 and May and late August and September 



