September, 1945 



Brown & Yeager: Squirrels in Illinois 



481 



The writers believe that most of the 

 females not breeding during the first or 

 winter season fail to do so because of im- 

 maturity or senility. Of the two causes, 

 immaturity is probably the more common. 

 Probably poor physical condition is the 

 most important cause of the failure of 

 some old females to produce the second- 

 season litter. 



The first and last dates of reproductive 

 condition or activity observed in Illinois 

 during this study and the apparent season 

 duration are given in table 17. Season 

 length appears to be about the same in 

 the two species. 



into two or more separate chases. The 

 chases involving nine and six gray squir- 

 rels, respectively, occurred late in the fore- 

 noon. Observations similar to the fore- 

 going w^ere made by Baker (1944) in 

 Texas. 



In both species, barking or "chatting" 

 was noticed more in December than in 

 any other month, but it was more or less 

 common throughout the summer, fall and 

 early winter. Intermittent barking may 

 occur during any month. Although bark- 

 ing is characteristic of mating behavior, 

 it may, of course, denote alarm, surprise, 

 anger and probabl\' other reactions. 



Mating Chase 



The most obvious indication of breeding 

 seasons in squirrels is the mating chase. 

 Fox squirrels appear to be shier than 

 gray squirrels in their mating activities. 

 During the present study, the number of 

 mating chases observed, in proportion to 

 populations, was less in fox squirrels ; like- 

 wise, the average number of fox squirrels 

 per chase was less. Seldom w^re more 

 than three fox squirrels observed in a 

 single chase, whereas, in gray squirrels, 

 observations were made on chases of nine 

 and of six animals, and many of five or 

 less. The average mating group, how- 

 ever, even in gray squirrels, involved only 

 three or four animals. Chases were en- 

 acted indiscriminately in trees, on logs 

 and on the ground. 



In the chase in which nine gray squirrels 

 participated, two of the animals were fe- 

 males, and it is believed that the remaining 

 seven were males, one a juvenile. The 

 two females barked continuously. One 

 female, despite her calling, turned and 

 fought the nearest male when pursued too 

 closely; the other, when chased into 

 isolated positions, such as the smaller 

 branches of a tree, was mounted by several 

 males, but copulation was not observed. 

 One of the females was mounted as she 

 raced up a tree, but she fell upon reaching 

 the top, whereupon the male lost his 

 position. The nearest pursuing male 

 often turned to drive back his rivals. 

 This chase covered an acre of dense un- 

 dergrowth, lasted about 20 minutes, and 

 was well advertised by continuous barking 

 and noisy quarreling. Chases involving a 

 large number of squirrels sometimes split 



Physiological Aspects 



In studying the physiological aspects of 

 breeding, it was necessary, due to limita- 

 tions of personnel and to field conditions, 

 to rely upon gross laboratory examination 

 rather than the more precise technique of 

 vaginal smears and gonad sectioning with 

 microscopical study. 



Mossman, Lawlah k Bradley (1932), 

 who studied the male squirrel reproductive 

 tract, including the Cowper's glands, re- 

 ported the male reproductive organs in fox 

 and gray squirrels to be similar. 



Testes. — In both species the testes, fig. 

 10 and table 14, began to enlarge in Sep- 

 tember. In mature males, enlargement 

 was from a flaccid to a turgid condition. 

 For the first breeding season the maximum 

 average testis length, December 1-15, was 

 30.4 millimeters in fox squirrels, and, De- 

 cember 16-January 15, 27.7 millimeters 

 in gray squirrels. Testis length during 

 the second breeding season was slightly 

 greater than in the first or winter season 

 in fox squirrels, but slightly less in gray 

 squirrels. Following the second season, 

 the testes became much smaller and ap- 

 parently inactive; the minimum average 

 length in mature males for any 15-day 

 period was 20.4 millimeters in fox squir- 

 rels and 19.0 millimeters in gray squirrels. 

 These measurements were reached in late 

 August and early September, respectively-. 

 At this time, the testes may have been 

 drawn into the abdominal cavity, and the 

 dormant-like condition have continued un- 

 til some time in September, when the 

 organs began an increase in size that con- 

 tinued until the winter maximum was 

 reached. Testes measurements for juve- 



