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Five parameters of reproductive biology (number of live calves, rates of tending lactating and nonlactating cows, and age-specific calving and pregnancy rates) were measured for the Mary Mountain (≈2500) and Northern Range (≈600) herds of bison (Bison bison) in Yellowstone National Park between 1990 and 1993. Three physiological events (ovulation, pregnancy, fetal loss) affecting reproductive success were measured using urinary or fecal steroid analysis. Adult Mary Mountain cows had significantly fewer pregnancies and calves than the Northern Range cows. Approximately 85% of cows tended by bulls, 76% of detected ovulations, and 78% of detected fall pregnancies were among nonlactating cows. Greater than 80% of all pregnancies occurred in cows ≥4 years old in both herds. However, 100% of all pregnancies among lactating cows occurred in cows ≥5 years old in both herds. In the Mary Mountain subpopulation, only 5% of all pregnancies were among cows <4 years old, while 14.2% of all pregnancies in the Northern Range herd occurred in cows in the same age group. These data indicate that (i) the number of live calves and the pregnancy rate are significantly reduced among lactating cows, (ii) reproductive failure among lactating cows is primarily caused by ovulation failure, (iii) lactating cows that successfully reproduce are exclusively animals ≥5 years old, and (iv) the results of urinary or fecal steroid analysis are consistent with behavioral and demographic data and can be used to evaluate specific aspects of reproductive physiology among free-ranging ungulates.

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