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We captured and radio collared 10 female bison in 1995 and an additional 12 female bison in 1996 in the Lamar Valley of Yellowstone National Park during a study of the epidemiology and pathogenicity of brucellosis in bison. Bison were regularly radio-tracked between October 1995 and May 1997. Pregnancy was determined using ultrasound in the October captures and by rectal palpation in February captures. Bison calving behavior was recorded during eight observation sessions. Calving success was determined by inserting vaginal transmitters in collared females 1-3 months prior to parturition and by daily radio tracking females to observe newborn calves. In 1996-97, we also determined late winter pregnancy status in 19 female bison killed and necropsied during winter control measures and 111 adult females captured in handling facilities within YNP. The average female bison home range was 54,126 ha. (Range=28,590-91,770). Bison are elevational migrants occupying significantly lower elevation habitats during winter and spring than during summer and fall. In winter 1995-96, three female bison did not leave the Lamar Valley. Seven female bison moved to the Gardiner Basin in March and remained until May when they migrated to the Madison Junction area. One of these bison continued moving to West Yellowstone and eventually summered in Hayden Valley. All 20 marked female bison migrated from the Lamar Valley to the Gardiner Basin in January 1996 and returned to summer range during April. All marked female bison were pregnant in October 1995, and 9 of 10 (90%) produced calves between April 28 and May 13, 1996. In 1996, 20 of 22 marked female bison were pregnant in October and 8 of 16 (50%) surviving mothers calved between April 19 and May 31, 1997. Thirteen of 19 female bison necropsied during winter control measures, and 89 of 111 captured bison, palpated during capture, were pregnant for a total pregnancy rate of 78.5%. Pregnancy rates were significantly higher in prime aged females (4-8) but lower in young aged (2-3) and old aged (>8) female bison. Eleven of 13 (84.6%) and 10 of 14 (71.4%) ovary pairs examined macroscopically in 1995 and 1996 respectively had corpora lutea. Corpora albicans were present in 53.8 and 64.3% of the ovary pairs from 1995 and 1996 respectively. Female bison in the Lamar are capable of becoming pregnant with the previous year's calf by their side. Intraspecific interactions are frequent and physical contact with birth products common during birthing events. Calving was rapid, usually less than 3 hours, and calves were precocial. Fourteen birthsites were found to be relatively clean with limited tissue and fluids remaining following birth events. We discuss the implications of our findings toward understanding bison population dynamics and the management of brucellosis in Yellowstone National Park.

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Found at Bison LibraryNatureBib ID: 602526

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