Rocky Mountain National Park's Bighorn Sheep: Herds and Lamb Survival

The question

How many bighorn sheep live in Rocky Mountain National Park, where are they, and how many lambs survive?

Biologists blamed the fall to 1970 on too little low-elevation winter range inside the park and disease from nearby domestic livestock. Anecdotal reports suggested another big drop in the mid-1990s. One suspect was pneumonia, which can kill more than 50 percent of a herd quickly.

The study

Gary C. White, Brett McClintock and a crew from Colorado State University caught 59 adult ewes with net guns in the winter of 2002–2003 and fitted each with a radio collar. Over two years they relocated the ewes from the air and on the ground, counted population with the mark-resight method, and counted lambs during the relocation surveys. The data were analyzed with Bowden's estimator.

Results

About 360 bighorn sheep lived in the park in 2004.

• The western herds' ranges overlap at The Crater, a known mineral lick. Every collared ewe east of the Divide used Sheep Lakes in Horseshoe Park as a mineral lick.
• Collared ewes bore at least 37 lambs — a fertility estimate of 86 percent.
• 89 percent of lambs survived three months, but only 24 percent survived a year.

Outlook: unless reproduction and survival improve, biologists expect the population to decline.

What came next

Because the study pointed to a possible decline, new research began:

• tracking population trends without handling sheep, using DNA from collected droppings;
• analyzing what the mineral licks contain;
• measuring mineral levels in the sheep themselves.

Sources

Based on "Bighorn Sheep Population," by Travis Guy, Rocky Mountain National Park, National Park Service (2006, updated 2009); a work of the United States government in the public domain. The summary is not a statement of park policy. A photo caption, "Bighorn sheep congregating near Milner Pass," had been run into the results text in the original and is left out.