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 physio logical factors all play major interrelated roles in reproductive self -regulation. Thus, we validated the use of urinary and fecal steroid a nalysis 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 su bpopulations of the Yellowstone National Park herd were observed durin g 1989-91. Ovulation was detected by the measurement of urinary pregna nediol-3-glucuronide and fecal progesterone during the rutting season; pregnancy was detected by increased urinary estrone conjugates and fe cal total estrogens during the third month of gestation. Among 54 sexu ally mature cows observed being tended (showed clinical signs of estru s 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 matu re cows was 48.2%; 15.4% of the pregnancies were among lactating cows. Our data suggest that approximately 85% of all mature cows are pregna nt on alternate years, approximately 15% of lactating cows are fertile , the cause of lowered fertility in lactating cows is lactational anov ulation, and endocrine evidence of ovulation and pregnancy, based on u rinary and fecal steroids is consistent with all other observed reprod uctive behaviors.