JACKSON, Wyo. — A special bighorn sheep study between the Wyoming Game and Fish Department (WGFD) and University of Wyoming (UW) is showing a pattern for the Jackson bighorn sheep herd where, when the population reaches about 500 sheep, an all ages pneumonia die-off follows.
The Jackson herd is separate from the smaller, more isolated Teton bighorn sheep herd. According to Aly Courtemanch, wildlife biologist at the WGFD, baseline monitoring of bighorn sheep by helicopter surveys puts the current minimum count of the Jackson herd population at about 500.
“This is what’s worrying to us,” Courtemanch says. “It seems that whenever we get to 500 sheep, it’s followed by these [population] crashes.”
The WGFD/UW study has been looking at the Jackson herd, in addition to the Whiskey herd and some sheep in the Absaroka herd near Cody, Wyoming, since 2015. The project is the first time this type of study has been done anywhere with Rocky Mountain bighorn sheep.
Between 2015 and 2019, Courtemanch says body fat levels were consistently high for bighorn sheep, which suggests the Jackson herd had good summer nutrition.
“Coming out of summer, you would expect them to have their highest body fat,” says Mark Gocke, public information specialist at the WGFD. “They spend winter living off those fat reserves.”
However, data shows that in the past three years there has been a very noticeable drop in the Jackson herd’s body condition, particularly coming out of summer.
“If history repeats itself, we’re setting ourselves up for another pneumonia crash.”
Aly Courtemanch, WGFD wildlife biologist
“As this herd is growing, once we get around 500 sheep, we’re seeing this corresponding drop in body fat,” Courtemanch says. “That’s what we would expect for a population that’s reaching or exceeding its carrying capacity on the landscape. If history repeats itself, we’re setting ourselves up for another pneumonia crash.”
Pregnancy rates for these bighorn sheep is expected to be at least 80 percent, closer to 90 percent, but the capture this March revealed only a 40 percent pregnancy rate.
“That’s shockingly low for an ungulate species,” Courtemanch says. “That’s another sign something is going on.”
The hypothesis from the study is that as a bighorn sheep population exceeds the habitat’s carrying capacity, a reduction of available nutrition and corresponding weakened body conditions make the herd more susceptible to pneumonia. Pneumonia was originally transmitted to Jackson bighorn sheep from domestic sheep and is constantly circulating through the Jackson herd population.
Enhancing habitat carrying capacity can be done through prescribed burns, mowing sagebrush and invasive plant species control, but these steps can take a long time to implement and are not allowed in wilderness areas, which is where Courtemanch says the herd spends a majority of their time.
While it may seem counterintuitive, hunting bighorn sheep when data shows the sheep are in very poor condition is an effective way to support the herd by keeping numbers at a more stable, sustainable place.
“Hunting is one of our main tools for managing populations, so we’re constantly trying to avoid these big increases in population so that we don’t have the big crash,” Gocke says.
The Jackson herd is showing a slightly elevated mortality this winter, but nothing near the mule deer and pronghorn winter mortality rates. Even with the low levels of body fat, the WGFD expects most of the bighorn sheep will survive this winter.
