Reproductive Synchrony in Brucellosis-Exposed Bison in the Southern Greater Yellowstone Ecosystem and in Noninfected Populations

Abstract

Shooting of bison (Bison bison) in the Greater Yellowstone Ecosystem is a highly polarized, emo- tional issue because native ungulates exposed to brucellosis (Brucella abortus) may infect cattle when they dis- perse or migrate beyond protected reserves. Both bison and elk (Cervus elaphus) carry brucellosis, a disease that causes abortion in livestock and is often transmitted through contact with expelled fetuses or birth mem- branes andfluids. If Brucella-infected bison experience a prolongedperiod of birth, cattle in areas of sympatry will have increased susceptibility to disease. We tested the hypothesis that reproductive synchrony differs be- tween Brucella-free and Brucella-infected bison by contrasting patterns between the Brucella-exposed bison population (in Jackson Hole, Wyoming) within the southern Greater Yellowstone Ecosystem with three non- exposed populations (National Bison Range in Montana, and Wind Cave and Badlands National Parks in South Dakota) that inhabit prairies. Populations averaged 42 births per site per year. Fifty percent of all the births occurred within 1 month, and 95% within 61 days at three sites. It took 89 days for 95% of the births to occur at the noninfected Badlands site. Multiple comparison of regression slopes of the relationship between birth synchrony and the cumulative proportion of total births revealed interpopulation differences (p < 0.01), but the brucellosis-exposed population did not exhibit a parturition period different from that of the nondiseased populations. These among-population similarities suggest that reproductive synchrony is not protracted in Brucella-infected bison. Although Brucella transmission from bison to cattle occurs experimen- tally, captive densities during testing exceeded the mean for the Greater Yellowstone Ecosystem by about 1500 times, and that in 10 noninfected populations by more than 100 times. Although mean density can be mis- leading as a surrogate for the potential transmission of disease from bison to cattle, bison densities have not approached those reportedfor disease transmission. Among the ecologicalfactors that may affect the spread of brucellosisfrom bison to cattle in the southern Yellowstone region, two stand out: (1) access to high-quality food, perhaps because animals in superior physiological condition tend to exhibit tighter reproductive syn- chrony and (2) incidence of disease in elk. The timing of parturition in bison is neither a simnple process nor is it likely to be based on responses to a single variable. A potentially fertile area for future investigation will concern relationships among food, reproductive synchrony, and spatial-temporal components of bison, elk, and cattle distribution.