. The Canadian field-naturalist. 248 The Canadian Field-Naturalist Vol. 8?. ^° 8 10 12 14 16 18 20 22 24 26 28 30 2 ""'' BREEDING DATES "'' Figure 1. Calculated breeding dates for 87 Richardson's ground squirrels in 1967 at Rochester, Alberta. males are less likely to survive to reproductive age than females (Fitch 1948:583 and McCarley 1966:307). Although there appears to be a progressive decrease in the proportion of males in my samples after 25 April (Table 1), this change is not statistically significant. Breeding Season Conception occurred from 8 April to 3 May, with the pe
. The Canadian field-naturalist. 248 The Canadian Field-Naturalist Vol. 8?. ^° 8 10 12 14 16 18 20 22 24 26 28 30 2 ""'' BREEDING DATES "'' Figure 1. Calculated breeding dates for 87 Richardson's ground squirrels in 1967 at Rochester, Alberta. males are less likely to survive to reproductive age than females (Fitch 1948:583 and McCarley 1966:307). Although there appears to be a progressive decrease in the proportion of males in my samples after 25 April (Table 1), this change is not statistically significant. Breeding Season Conception occurred from 8 April to 3 May, with the peak at 17-22 April (Fig. 1). Over three-fourths of the conceptions occurred in the two- week period of 14-27 April. Thus in 1967, breeding started about 2 weeks after and peaked about a month after the first squirrels were seen above ground on 22 March. Froductivity All 69 adult females taken after breeding ceased (3 May) were or had been pregnant; nonbreeding is apparently rare in this species. Eighty-four gravid females contained 735 live fetuses; an average of fetuses per female (Table 2). However, average litter size decreased signi- ficantly from early to late pregnancy so that litter size at birth was probably about This decrease occurred primarily during early pregnancy (Table 2). Litter size varied from 4 to 13. Seton (1929) gives an average of fetuses for 8 females, Howell (1938) gives fetuses per female, and Fuller (1948) gives fetuses per female but his sample contained only 3 females. Denniston (1957) gives an average of young at birth for 6 litters. A total of 42 fetuses ( percent) were being resorbed; but resorption rate increased significantly as gestation advanced (Table 2). Number of resorbing fetuses per female varied from 0 to 3; and resorption occurred in 38 percent of the females. Tomich (1962) found that 9 of 150 fetuses ( percent) were being resorbed and that 29 percent of 24 female Sper?jiophihis beecheyi were involved. Pl
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