MOSCOW, ID — A study from the University of Idaho and the Idaho Department of Fish and Game shows more than a century of population decline has reduced genetic diversity and connectivity among Idaho’s Rocky Mountain bighorn sheep.

Researchers found bighorn sheep once had greater genetic diversity, with frequent interbreeding between Rocky Mountain and desert subspecies. That connection has largely disappeared, with about 1% of modern sheep carrying desert lineage DNA compared to roughly 35% historically.

The study, published in Conservation Genetics, compared DNA from historic skull samples collected between 1897 and 1985 to samples gathered from 2000 to 2017 in the Salmon River region.

Researchers identified six genetic groups in the modern population, with limited movement between them. Some herds, including the East Fork population, show signs of isolation and reduced gene flow.

“Comparisons between historical and modern data reveal a consistent decline in genetic diversity,” said Frances Cassirer of Idaho Fish and Game.

Bighorn sheep populations declined sharply in the late 19th and early 20th centuries due to disease, overhunting and habitat competition. While numbers have improved in some areas, researchers say many herds remain small, fragmented and at risk.