New Research Brings Insight into Struggling Bighorn Sheep Populations in South Dakota


An ewe and her lamb search for food in the melting snow. Pneumonia may pose a bigger threat to bighorn populations than scientists originally thought (Credit: Phil Schillaci). 

There are many things one would expect to find in the back of a muddy pickup truck. Ultrasound equipment, forceps, and sterilization gear probably do not top that list.

Joshua Smith kept a truck stocked with these materials for four years while monitoring bighorn sheep in the eastern Black Hills. He wanted to understand why so many lambs in the region were not surviving until adulthood. The Ph.D. Candidate from South Dakota State University relied on the truck as a mobile research laboratory. He drove around the region, planting transmitters on wild ewes and later their newborn lambs. “As far as the herds that we have here in eastern Black Hills, I think we’ve now identified some of the major sources of mortality,” Smith said.

Understanding why so few lambs survive until adulthood could lead to discoveries that help preserve bighorn populations. Bighorn sheep flirted with extinction in the early 1900s. Hunting, predation, disease, and habitat loss decimated their numbers. Conservation efforts have significantly restored bighorn populations throughout the United States, but the sheep are still struggling. Less than 5% of the country’s original bighorn numbers roam today’s mountains and deserts.

Smith and coworkers identified pneumonia as the leading cause of death of bighorn lambs in the Black Hills region. Pneumonia is not a new disease to bighorn sheep. It contributed to the near extinction of the species in the 1900s, and continues to ravage reestablished populations today. However, biologists thought that these sweeping pneumonia deaths in lambs only occurred if a serious outbreak had threatened the entire herd. This new study suggests otherwise.


A herd of bighorn ewes graze in a field in early April (Credit: Phil Schillaci).

What’s surprising is that the herds Smith studied had no history of these aggressive pneumonia outbreaks. State officials previously detected the disease in a few Black Hills lambs, but no adults seemed to be suffering. Nobody really expected pneumonia was the driving factor behind these lamb deaths. “I think a lot of people when we initially started this thought that the increasing number of mountain lions pretty much coincided with this decline in bighorn sheep,” he recalled. “It [quickly] became more of this disease issue as opposed to predation seeming to drive this decline.”

A devastating 95% of the 74 lambs that Smith tracked with the research team died within their first year of life. Predation was the second leading cause of death. The research group also suspected pneumonia could have contributed to lamb vulnerability. The disease could make them slower and more susceptible to predation by mountain lions or coyotes.

Tom Besser, a professor in the College of Veterinary Medicine at Washington State University believes that Smith’s study could have implications for bighorn herds outside of the Black Hills. “I think that it is likely that many other bighorn sheep populations across the west experiencing [high offspring deaths] are also suffering from recurrent lamb pneumonia but haven’t documented the illness because of the effort and expense involved,” he said.

Both Smith and Besser agree that preventing the spread of pneumonia should be a priority for bighorn conservation efforts. Domestic goats and sheep grazing in bighorn territory are a likely source of the disease. Bighorn rams also have wider ranges than ewes, increasing chances of encountering infected livestock or other wildlife. Although rams remain on their own for most of the year, they could spread it to ewe populations during breeding season.


A ram pauses on a mountaintop.  Bighorn rams have wider ranges and may facilitate spread of pneumonia (Credit: Phil Schillaci).

Efforts to contain the disease have focused on trying to keep bighorn populations away from domestic animals. Developing antibiotics or vaccines would be ideal, but there are challenges to this research. Bighorn pneumonia is a complicated disease. Scientists need to have a better grasp of pneumonia before developing an effective treatment for wild populations.

Some researchers suspect that several bacteria may actually contribute to bighorn pneumonic deaths. Besser studies a nasty bacterium called Mycoplasma ovipneumoniae. He believes that domestic sheep and goats transmit this bacterium to bighorn sheep. The bacterium destroys important defense mechanisms against other diseases, setting the stage for pneumonia. The bighorn sheep become vulnerable to diseases and infections that collectively lead to the animals’ deaths.

This Mycoplasma bacterium would seem to represent a great target for antibiotics or vaccines. Unfortunately, drug development efforts have so far been unsuccessful. Besser says he also expects significant hurdles in vaccine development.

Even if scientists can successfully develop a drug or vaccine, there will be additional obstacles. “Delivering these vaccinations or antibiotics is problematic in any wild population,” Smith said. He knows first-hand that trapping bighorn sheep can be a challenge. Vaccinating an entire population may not be possible without extensive research. Nevertheless, Smith hopes drug development efforts will continue.

Smith’s adventures in the muddy pickup truck stocked with odd medical supplies may be over, but his research continues. The transmitters he planted on ewes and their lambs provided massive amounts of information. “Now I’m just a desk jockey,” he laughed. “I get to spend my time now behind my computer making sense of the data.” He hopes his field adventures and data analysis add another piece to the puzzle behind the disease that threatens bighorn sheep.

Smith JB et. al. (2014) “Disease and Predation: Sorting out Causes of a Bighorn Sheep” (Ovis canadensis) Decline.” PLoS One.

Besser TE et al. (2013) “Bighorn sheep pneumonia: sorting out the cause of a polymicrobial disease.” Prev Vet Med.

Photo credits: All photos used with permission of wildlife photographer, Phil Schillaci