September, 1945 



Brown & YEAGtR: Squirrels in Illinois 



485 



Fig. 13. — Lactating gray squirrels, Coles County, September 15, 19+4. Note Histended ap- 

 pearance of mammary glands and nipples. 



gestation period in squirrels. Allen 

 (1942, 1943) and Lyon (1936) indicated 

 that gestation in fox squirrels is probabh' 

 about 45 days. Since the oestrual peak in 

 this species in Illinois, as determined by 

 the present study, was late in December 

 and early in January, a 45-day term would 

 place the birth peak in February. The 

 oestrus records in table 15 and the birth 

 records in table 18 indicate that the ges- 

 tation period in Illinois fox squirrels is 

 about 45 days. Seton (1928, 4:44), 

 quoting Powers, gives the gestation period 

 in gray, squirrels as 44 days. Goodrum 

 (1940) and Baumgartner' (1940) also 

 considered that 44 days was the probable 

 term. The oestrus-mating peak in the 

 small number of gray squirrels observed 

 in the Illinois study was early or mid 

 January, table 15, and the birth peak was 

 early March, table 18, indicating an av- 

 erage gestation period of about that sug- 



gested by Goodrum, Baumgartner and 

 other authors mentioned above. 



Mammae. — In both fox and gray 

 squirrels, the mammae are eight in num- 

 ber and are arranged in four pairs, oc- 

 cupying the ventral body surface behind 

 the forelegs, fig. 13. Allen (1942) des- 

 ignated the mammae positions as pectoral 

 1, pectoral 2, abdominal and inguinal. He 

 stated that, in the fox squirrel, the abdomi- 

 nal pair develops first following insemina- 

 tion, and that the usual order of develop- 

 ment is abdominal, inguinal, pectoral 2 

 and pectoral 1. In some cases, however, 

 Allen found that the pectoral 2 and ab- 

 dominal pairs developed equally, or the 

 former preceded the latter in development. 

 The pectoral 1 pair may not be functional 

 in some cases. Milk can be squeezed 

 from the nipples a few days before the 

 young are born and, in some cases at least, 

 for 2 or 3 weeks after the young are 



