Factors affecting elk (Cervus canadensis) reproductive physiology in the Greater Yellowstone Ecosystem (GYE) are the subject of much interest and debate. Drought, aspen (Populus tremuloides) decline, elk density, as well as stress and behavioral responses associated with local predator density have all been investigated as mechanisms contributing to lower observed pregnancy rates in elk. Effects of brucellosis (Brucella abortus) were thought to be limited to the disease-induced abortions that occur among some infected animals, and have received less attention. Based on ultrasound tests performed during the course of previous research efforts, we expected a lower probability of pregnancy in elk that were seropositive for brucellosis versus those that were seronegative during mid- and late-winter, prior to the abortion period. Banked blood samples were tested for pregnancy-specific protein B (PSPB) and the results analyzed in combination with serologic, ultrasound, and age data for female elk across 20+ years of data from the feedgrounds in the southern GYE. We built Bayesian multilevel models to estimate the age-specific pregnancy probabilities of seronegative and seropositive elk and to assess relative support for some possible biological mechanisms underlying our results.
Results/Conclusions
Probability of pregnancy was significantly reduced in seropositive elk prior to the onset of the disease-induced abortion period. Two-year-old seropositive elk were 30% less likely to be pregnant than seronegatives in their cohort independent of location and year (89% HPDI = 20-42%), whereas seropositive 3- to 9-year-old elk were only 7% less likely to be pregnant than seronegatives (89% HPDI = 2-11%). Further testing revealed that these reductions in pregnancy are additive to the previously established reductions attributed to disease-induced abortions. Through the full reproductive cycle, seropositive elk have an approximately 25% lower reproductive output than seronegative elk. This is likely a significant selective force acting on elk in this ecosystem. We did not detect any change in calf:cow ratios (another indicator of productivity), which may be due to compensatory survival of young individuals on the supplementary feedgrounds. Failure to account for disease could confound research into the effects of climate, predation, and habitat on elk population dynamics in the GYE, a study system of unique importance.