Reproductive synchrony in brucellosis-exposed bison in the southern greater Yellowstone ecosystem and in noninfected populations

Shooting of bison (Bison bison) in the Greater Yellowstone Ecosystem is a h ighly polarized, emotional issue because native ungulates exposed to brucel losis (Brucella abortus) may infect cattle when they disperse or migrate be yond protected reserves. Both bison and elk (Cervus elaphus) carry brucello sis, a disease that causes abortion in livestock and is often transmitted t hrough contact with expelled fetuses or birth membranes and fluids. If Bruc ella-infected bison experience a prolonged period of birth, cattle in areas of sympatry will have increased susceptibility to disease. We tested the h ypothesis that reproductive synchrony differs between Brucella-free and Bru cella-infected bison by contrasting patterns between the Brucella-exposed b ison population (in Jackson Hole, Wyoming) within the southern Greater Yell owstone Ecosystem with three non-exposed populations (National Bison Range in Montana, and Wind Cave and Badlands National Parks in South Dakota) that inhibit prairies. Populations averaged 42 births per site per year. Fifty percent of all the births occurred within 1 month, and 95% within 61 days a t three sites. It took 89 days for 95% of the births to occur at the noninf ected Badlands site. Multiple comparison of regression slopes of the relati onship between birth synchrony and the cumulative proportion of total birth s revealed interpopulation differences (p < 0.01), but the brucellosis-expo sed population did not exhibit a parturition period different from that of the nondiseased populations. These among-population similarities suggest th at reproductive synchrony is not protracted in Brucella-infected bison. Alt hough Brucella transmission from bison to cattle occurs experimentally, cap tive densities during testing exceeded the mean for the Greater Yellowstone Ecosystem by about 1500 times, and that in 10 noninfected populations by m ore than 100 times. Although mean density can be misleading as a surrogate for the potential transmission of disease from bison to cattle, bison densi ties have not approached those reported for disease transmission. Among the ecological factors that may affect the spread of brucellosis from bison to cattle in the southern Yellowstone region, two stand out: (1) access to hi gh-quality food, perhaps because animals in superior physiological conditio n tend to exhibit tighter reproductive synchrony and (2) incidence of disea se in elk. The timing of parturition in bison is neither a simple process o r is it likely to be based on responses to a single variable. A potentially fertile area for future investigation will concern relationships among foo d, reproductive synchrony, and spatial-temporal components of bison, elk, a nd cattle distribution.