We investigated the impacts of brucellosis (Brucella abortus) on elk (Cervus canadensis) productivity using serological data from over 6000 captures since 1990 in the Greater Yellowstone Ecosystem, USA. Over 1000 of these records included known age and pregnancy status. Using Bayesian multilevel models, we estimated the age-specific pregnancy probabilities of exposed and native elk. We then used repeat-capture data to investigate the full effects of the disease on life history. Brucellosis exposure reduced pregnancy rates of elk captured in mid- and late-winter. In an average year, we found 60% of exposed 2-year-old elk were pregnant compared to 91% of their native counterparts (a 31 percentage point reduction, 89% HPDI = 20-42%), whereas exposed 3- to 9-year-olds were 7 percentage points less likely to be pregnant than native elk of their same age (89% HPDI = 2-11%). We found these reduced rates of pregnancy to be independent from disease-induced abortions, which afflict a portion of exposed elk. We estimate that the combination of reduced pregnancy by mid-winter and the abortions following mid-winter reduces the reproductive output of exposed female elk by 24%, which affects population dynamics to a similar extent as severe winters or droughts. Exposing hidden reproductive costs of disease is essential to avoid conflating them with the effects of climate and predation. Such reproductive costs cause complex population dynamics, and the magnitude of the effect we found should drive a strong selection gradient if there is heritable resistance.
Author ORCID Identifier
Johan du Toit https://orcid.org/0000-0003-0705-7117
.csv, .R, .txt
Wyoming Game and Fish Department; U.S. Geological Survey; Rocky Mountain Elk Foundation; Utah State University
Utah State University
Data were collected in western Wyoming, south of Yellowstone National Park, USA, where supplementary winter feedgrounds are used by approximately 80% of the region's elk. The National Elk Refuge in Jackson, WY, is operated by the US Fish and Wildlife Service (USFWS) and 22 additional feedgrounds are operated by the Wyoming Game and Fish Department (WGFD; see Cotterill et al. 2018). Our data were collected by the WGFD for research and management purposes across all 23 feedgrounds and 2 nearby unfed wintering locations between 1995 and 2017. All captured elk receive permanent ear tags, which enable identification at subsequent recaptures and this allowed us to track a subset of individual elk over time. Age, pregnancy, and brucellosis serostatus were known for 1236 records of female elk. All serology and pregnancy data were collected between January 3 and April 15, with 90% of data collection occurring prior to March 1, of any year.
Cotterill GG, Cross PC, Middleton AD, Rogerson JD, Scurlock BM, du Toit JT. Hidden cost of disease in a free‐ranging ungulate: brucellosis reduces mid‐winter pregnancy in elk. Ecol Evol. 2018;00:1–10. https://doi.org/10.1002/ece3.4521
Cotterill, G.G, Cross, P.C., Cole, E.K., Fuda, R.F., Rogerson, J.D., Scurlock, B.M. & du Toit, J.T. 2018. Winter feeding of elk in the Greater Yellowstone Ecosystem and its effects on disease dynamics. Philosophical Transactions of the Royal Society B 373:20170093. http://dx.doi.org/10.1098/rstb.2017.0093
age-prev.csv "Age" denotes the age of individual elk during mid-winter. "prev" denotes a modeled estimate for age-specific seroprevalence of brucellosis across 20+ years of data collection at 23 feedgrounds. These are the point estimates from a generalized additive model.
calf-data.csv "Capture_Year" corresponds to the year in which the feeding season ended. "Capture_Location" corresponds to a specific feedground. "Calf_Ratio" is an extrapolated number of calves per 100 female elk based on ground counts performed at feedgrounds during peak winter.
preg-data.csv "UID" is a unique identifier which enables tracking individuals over time. "FPA" is the fluorescent polarization assay score in millipolarization units. "Capture_Date" is the day when an animal was processed for serology or pregnancy testing. "Capture_Year", self-explanatory. "pregnant": 1 = pregnant, 0 = not pregnant "s": a categorical interpretation of serostatus where 1 = seropositive, 0 = seronegative. "m": method used for determining pregnancy where 1 = PSPB, 0 = transrectal ultrasound. "year_id" is an integer value corresponding to the capture year for use in STAN programming language. "age_id" is an integer value where 1-19 correspond to actual ages, 20 corresponds to unknown age. "age_class2" is an integer value where 1-4 correspond to age bins used in analyses (yearling, 2 years old, 3-9 years old, 10+ years old) and 5 corresponds to unknown age.
prev-data.csv "Capture_Location", self-explanatory. "Capture_Year", self-explanatory. "prev": herd-level seroprevalence for a given feedground in a given year. These are point estimates from generalized additive models fit to each of the sites which had the most robust longitudinal data.
Annotated code files: main_models.R: Includes main pregnancy models, the creation of figures 1 and 2, Appendix C, figures C2 and D2. fpa_tsi_and_jd.R: Includes FP, time-since-infection, and Julian day analyses, appendices A and B. calf_cow.R: Includes calf:cow analysis and figure 1 in appendix D.
This work is licensed under a Creative Commons Attribution 4.0 License.
du Toit, J. T., & Cotterill, G. (2018). Elk female age, brucellosis status, and pregnancy status collected over >20 years at winter feedgrounds in Wyoming. Utah State University. https://doi.org/10.15142/T39M0B
Additional FilesREADME_elkdata.txt (5 kB)
metadata.txt (2 kB)
age-prev.csv (1 kB)
calf-data.csv (1 kB)
preg-data.csv (70 kB)
prev-data.csv (2 kB)
calf_cow.R (9 kB)
fpa_tsi_and_jd.R (13 kB)
main_models.R (20 kB)