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January 13, 2022

Male bighorn sheep have spectacular horns, making them popular among hikers, photographers and hunters. However, disease threatens the magnificence of these horns. Males that survive exposure to pneumonia outbreaks can experience severely stunted horn growth, according to a new study.

Male bighorn sheep
This image shows a male bighorn sheep with large horns.                         

“This research suggests that disease can pose hidden biological costs to bighorn sheep and may limit the number of animals available to be harvested in hunts after some disease outbreaks, depending on state regulations,” said Alynn Martin, a U.S. Geological Survey scientist and co-lead author of the paper.

The study can also inform wildlife managers about additional, hidden ways disease can influence bighorn sheep and help guide management strategies for the species.

Bighorn sheep are iconic animals of western North America, sometimes found moving among steep mountainous inclines. Mature males, called rams, are immediately recognizable by their large, curled horns, which are used in displays against other rams to secure dominance and mates. Large rams’ horns are valued by some hunters, and horn length is a main factor in determining when a ram can be legally hunted in some states.

In bighorn sheep herds that have not experienced pneumonia before, the disease can cause high rates of mortality followed by poor lamb survival for many years after an outbreak. Pneumonia can also have consequences for individuals who were exposed to the disease but survived. Specifically, disease exposure can reduce ram horn size, according to a recently published study lead by the USGS and the Montana Conservation Science Institute.

Researchers investigated the effects of pneumonia on horn size of male bighorn sheep in 12 herds across six states: Colorado, Idaho, Montana, Utah, Washington and Wyoming. Using the two herds for which they had annual horn growth data – one in Montana and one in Washington – they found that the horns of males exposed to pneumonia experienced a 12% to 35% decrease in yearly horn growth. That decrease in growth lasted several years after the initial outbreak as compared to males not living through disease outbreaks.

Using data from bighorn sheep harvested by hunters, the researchers also documented the total length of the horn at the time the animal was hunted. Of the 12 herds investigated, four herds experienced a negative effect of pneumonia on the total length of horns harvested. In six other herds, rams harvested after an outbreak were older, suggesting it took longer for those males to achieve horns that are of legal hunting size.  

“Both data types support the same conclusion, that disease can result in significantly shorter male horns,” Martin said. “It may take males exposed to pneumonia an additional one to two years for their horns to reach the size required for them to be eligible for legal harvest.”

In addition to large horns being valued by hunters, they help males compete for females, and it’s unclear if there will be an evolutionary consequence of shorter horns as a result of disease.

“Shorter horned, disease-exposed males may have less opportunities to breed, but we know they are capable of surviving pneumonia outbreaks,” said USGS scientist Paul Cross, a co-author on the paper. “Those are the types of genes you want to be passed on.”

Other factors such as forage availability and climate variables could act as stressors on bighorn sheep and slow horn growth. However, the effect of disease was much stronger than any environmental variables investigated in this study.

The study is published in The Journal of Wildlife Management.

Male and female bighorn sheep
Male (ram - left) and female (ewe) bighorn sheep outside of Yellowstone National Park.