AN ASSESSMENT OF THE REPRODUCTIVE-BIOLOGY OF YELLOWSTONE BISON (BISON-BISON) SUBPOPULATIONS USING NONCAPTURE METHODS

Five parameters of reproductive biology (number of live calves, rates of tending lactating and nonlactating cows, and age-specific calving a nd pregnancy rates) were measured for the Mary Mountain (approximate t o 2500) and Northern Range (approximate to 600) herds of bison (Bison bison) in Yellowstone National Park between 1990 and 1993. Three physi ological events (ovulation, pregnancy, fetal loss) affecting reproduct ive success were measured using urinary or fecal steroid analysis. Adu lt Mary Mountain cows had significantly fewer pregnancies and calves t han the Northern Range cows. Approximately 85% of cows tended by bulls , 76% of detected ovulations, and 78% of detected fall pregnancies wer e among nonlactating cows. Greater than 80% of all pregnancies occurre d in cows greater than or equal to 4 years old in both herds. However, 100% of all pregnancies among lactating cows occurred in cows greater than or equal to 5 years old in both herds. In the Mary Mountain subp opulation, only 5% of all pregnancies were among cows <4 years old, wh ile 14.2% of all pregnancies in the Northern Range herd occurred in co ws in the same age group. These data indicate that (i) the number of l ive calves and the pregnancy rate are significantly reduced among lact ating cows, (ii) reproductive failure among lactating cows is primaril y caused by ovulation failure, (iii) lactating cows that successfully reproduce are exclusively animals greater than or equal to 5 years old , and (iv) the results of urinary or fecal steroid analysis are consis tent with behavioral and demographic data and can be used to evaluate specific aspects of reproductive physiology among free-ranging ungulat es.