REMOTE MONITORING OF OVULATION AND PREGNANCY OF YELLOWSTONE BISON

The physiological mechanisms that control reproductive success of wild bison (Bison bison) are not known, and the relatively small scattered herds prevent intensive study. Environmental, demographic, and physiological factors all play major interrelated roles in reproductive self-regulation. Thus, we validated the use of urinary and fecal steroid analysis as a means of detecting ovulation and pregnancy in uncaptured free-roaming ungulates, and identified the physiologic mechanisms that govern reproductive success in wild bison. Free-roaming bison of 2 subpopulations of the Yellowstone National Park herd were observed during 1989-91. Ovulation was detected by the measurement of urinary pregnanediol-3-glucuronide and fecal progesterone during the rutting season; pregnancy was detected by increased urinary estrone conjugates and fecal total estrogens during the third month of gestation. Among 54 sexually mature cows observed being tended (showed clinical signs of estrus and constantly attended by a bull) during the rutting season, 18.5% were lactating, and 81.4% were not lactating. The documented ovulation rate for 121 mature cows during the same period was only 14.5% among lactating cows. The estimated pregnancy rate over 2 years for 255 mature cows was 48.2%; 15.4% of the pregnancies were among lactating cows. Our data suggest that approximately 85% of all mature cows are pregnant on alternate years, approximately 15% of lactating cows are fertile, the cause of lowered fertility in lactating cows is lactational anovulation, and endocrine evidence of ovulation and pregnancy, based on urinary and fecal steroids is consistent with all other observed reproductive behaviors.