Sierra Nevada Bighorn Sheep Program - The Risk of Disease

Bighorn sheep are extremely susceptible to respiratory disease caused by pathogens carried by, yet harmless to domestic sheep. The introduction of domestic sheep grazing in the Sierra during John Muir’s time, the 1800s, contributed to the decline of bighorn throughout their range. A similar pattern of disease and decline swept across the West as settlers brought their domestic sheep and goats with them. Today herds of bighorn throughout the west suffer catastrophic losses after contact with domestic sheep or goats. Some herds fail to raise young to adulthood; other herds fail entirely and become extinct. Luckily, bighorn in the Sierra have escaped such a catastrophe in recent decades, but the risk still looms large and threatens to wipe out bighorn sheep and the decades of restoration efforts that have grown their numbers.

Being in the same genus, bighorn sheep (Ovis canadensis) and domestic sheep (Ovis aries) are able to cross-breed. During the rut, rams travel large distances looking for female sheep to mate. Two different rams from the Mt. Warren herd in Yosemite have been documented by GPS tracking collars making long forays (up to 33 miles) out of bighorn habitat during the late fall mating season. One ram journeyed into a seasonal domestic sheep grazing allotment (see Fig 1 at left) narrowly missing contact with domestic sheep which had been moved off the allotment only two weeks prior.

The blue dots represent locations recorded from the ram’s GPS collar. The green line shows the long distance movement into domestic sheep allotments outlined in red.

The Sierra Nevada Bighorn Sheep Recovery Program at California Department of Fish and Wildlife has worked closely with Yosemite National Park, the Bureau of Land Management, the U.S. Forest Service, and local ranchers to mitigate the risk of bighorn contacting domestic sheep. Since 1999, the time of emergency listing as an endangered species, several domestic sheep grazing allotments near the Yosemite herds have been vacated or closed (see Fig 2 above and at left). While this has certainly reduced the threat of disease caused by domestic sheep, there is still concern that bighorn sheep in the northern most herds (the herds that currently populate Yosemite National Park) could make contact with domestic sheep. As the number of bighorn sheep in the Sierra increases and the populations expand, this risk will only increase, making real-time data on the locations of bighorn obtained with GPS collars critically important to reducing this risk.

Bighorn ram with domestic sheep
A bighorn ram intermingling with domestic sheep and their guard dog

Movements of bighorn ram
Movements of a Bighorn ram across domestic sheep allotments

Bighorn Sheep Herds and Domestic Sheep Allotments
Bighorn Sheep Herd Units and Domestic Sheep Allotments

Ram migration A bighorn ram in the midst of a long distance migration through agriculutral lands (Bridgeport, CA)