Table 6. Pregnancy results for bison harvested in 1986-89 by 

 brucellosis test results. 



Includes 5 pregnant and 1 not pregnant animal that tested suspicious for 

 brucel losis 



^ An additional 6 females were tested for pregnancy but not tested for 

 brucellosis. All 6 were pregnant. Therefore, out of 102 females that 

 were tested for pregnancy, 75 were pregnant. 



Fetal Sex Ratios 



A total of 52 fetuses was collected. A significantly 

 higher proportion of ferr.ales (58%, n=30) to males (n=22) were 

 found in utero (p<.05). Previous studies showed higher 

 percentages of male fetuses in utero (Meagher 1986, Reynolds 

 et al. 1982). Rutberg (1986) found a significantly higher 

 percentage of non-lactating compared to lactating cows 

 carrying male fetuses. He concluded that females in good 

 condition produce more male offspring. However, Wolff (1990) 

 and Shaw and Carter (1989) found all females equally likely to 

 produce male and female calves. Lactation and body condition 

 were not determined in this study but would be of interest in 

 future collections since fetal sex ratios are significantly 

 different than previously documented. 



Conception/Calving Dates 



Fetal weights varied from 1.3 to 35.0 pounds. Crown - 

 rump measurements varied from 23.9 to 93.9 cm with similar 

 means for female (54.0 cm) and male (53.8 cm) fetuses. 

 (Appendix I) . 



Bison and cattle fetal weight and crown-rump measurements 

 are comparable (L. Stackhouse D.V.M. personal communication). 

 Size and characteristics of bovine fetuses (Roberts 1956) were 

 extrapolated to estimate bison fetal conception and calving 

 dates (Appendix I) . Meagher (1973) reported most calving 

 occurred in early May with completion by late May. However, 

 she also indicated that as the population increases, the 

 calving season is extended. Our data support this 

 observation, suggesting that the calving season is even more 



12 



